Water Supply: the Present Practice of Sinking and Boring Wells
by Ernest Spon · English · Gutenberg #45703 · 472,898 chars · public domain (US)
The Project Gutenberg eBook of Water Supply: the Present Practice of Sinking and Boring Wells
This eBook is for the use of anyone anywhere in the United States and
most other parts of the world at no cost and with almost no restrictions
whatsoever. You may copy it, give it away or re-use it under the terms
of the Project Gutenberg License included with this eBook or online
at www.gutenberg.org. If you are not located in the United States,
you will have to check the laws of the country where you are located
before using this eBook.
Title: Water Supply: the Present Practice of Sinking and Boring Wells
Author: Ernest Spon
Release date: May 20, 2014 [eBook #45703]
Most recently updated: October 24, 2024
Language: English
Other information and formats: www.gutenberg.org/ebooks/45703
Credits: E-text prepared by Chris Curnow, Brian Wilcox, and the Online Distributed Proofreading Team (http://www.pgdp.net) from page images generously made available by Internet Archive (https://archive.org)
*** START OF THE PROJECT GUTENBERG EBOOK WATER SUPPLY: THE PRESENT PRACTICE OF SINKING AND BORING WELLS ***
Note: Project Gutenberg also has an HTML version of this
file which includes the numerous original illustrations.
See 45703-h.htm or 45703-h.zip:
(https://www.gutenberg.org/cache/epub/45703/pg45703-images.html)
or
(https://www.gutenberg.org/files/45703/45703-h.zip)
Images of the original pages are available through
Internet Archive. See
https://archive.org/details/presentpracticeo00sponuoft
Transcriber's note:
Text enclosed by underscores is in italics (_italics_).
Text enclosed by equal signs is in bold face (=bold=).
Small capitals have been changed to ALL CAPITALS.
[V] and [T] symbols represent sans-serif characters in
the original text.
SINKING AND BORING WELLS.
[Illustration: BORING SHEAR FRAME.]
WATER SUPPLY.
THE PRESENT PRACTICE OF SINKING AND BORING WELLS;
With Geological Considerations and Examples of Wells Executed.
by
ERNEST SPON,
Member of the Society of Engineers; of the Franklin Institute;
of the Iron and Steel Institute; and of the Geologists' Association.
[Illustration: Ornate Monogram.]
London:
E. & F. N. Spon, 48, Charing Cross.
New York: 446, Broome Street.
1875.
CONTENTS.
CHAP. PAGE
PREFACE. v.
I. GEOLOGICAL CONSIDERATIONS. 1
II. THE NEW RED SANDSTONE. 35
III. WELL SINKING. 40
IV. WELL BORING. 60
V. AMERICAN TUBE WELL. 81
VI. WELL BORING AT GREAT DEPTHS. 85
VII. EXAMPLES OF WELLS EXECUTED, AND OF DISTRICTS
SUPPLIED BY WELLS. 155
VIII. TABLES AND MISCELLANEOUS INFORMATION. 202
INDEX. 211
E. & F. N. SPON'S NEW BOOKS. Advertisements
PREFACE.
In modern times the tendency of the inhabitants of a country to dwell
together in large communities, and the consequent need for accumulating
in a particular locality a sufficient supply of water for household,
social, and industrial purposes, have rendered necessary the
construction of such engineering works as impounding reservoirs and
wells, by means of which the abundant measure of sparsely populated
districts may be utilized, and water obtained not only free from those
impurities which it collects in densely populated districts, but also in
greater quantity than the natural sources of the district are capable of
supplying.
Of the works mentioned, wells have fairly a primary claim upon the
notice of the sanitary engineer, for, without undervaluing other sources
of supply, the water from them certainly possesses the advantage over
that from rivers and surface drainage, of being without organic
admixture and unimpregnated with those deadly spores which find their
way into surface waters and are so fatal in seasons of epidemic
visitation. A great deal of the irregularity in the action of wells, and
the consequent distrust with which they are regarded by many, is
attributable either to improper situation or to the haphazard manner in
which the search for underground water is frequently conducted. As
regards the first cause, it cannot be too strongly stated that extreme
caution is necessary in the choice of situations for wells, and that a
sound geological knowledge of the country in which the attempt is to be
made should precede any sinking or boring for this purpose, otherwise
much useless expense may be incurred without a chance of success.
Indeed, the power of indicating those points where wells may, in all
probability, be successfully established, is one of the chief practical
applications of geology to the useful purposes of life.
Two cases in point are before me as I write; in the one 15,000_l._ has
been spent in sinking a shaft and driving headings which yield but
little water, found abundantly at the same depth in a mine adjoining;
and in the other a town would be, but for its surface wells, entirely
without water, the waterworks having been idle for weeks, and the
sinkers are feebly endeavouring to obtain water by deep sinkings, in a
position where its occurrence in any quantity is physically impossible.
Ample supplies could be obtained in both these cases by shifting the
situation a few hundred yards.
The subject-matter of the following pages is divided into chapters which
treat of geological considerations, the new red sandstone, well sinking,
well boring, the American tube well, well boring at great depths, and
examples of wells executed and of localities supplied respectively, with
tables and miscellaneous information. Each system with its adjuncts has
been kept complete in itself, instead of separating the various tools
and appliances into classes, the plan adopted in the most approved
French and German technical works. This, however, when too rigidly
adhered to, as is the case with German works in particular, renders it
troublesome for even a practised engineer to grasp a strange system in
its entirety, while the pupil is wearied and retarded in his reading by
an over-elaborate classification.
It may, perhaps, be remarked that undue prominence has been given to the
tertiary and cretaceous formations, but it is urged in extenuation that
they happen to underlie two of the most important cities in Europe, and
that they have, in consequence, received a more thorough investigation
than has been accorded to other districts. The records of wells in many
formations are singularly scanty and unreliable, but it is hoped that
the time is not far distant when the water-bearing characteristics of
strata, such as the new red sandstone and permian, will receive proper
attention, and that correct official records of well-work will be found
in every locality, as this alone can rescue an important branch of
hydraulic engineering from the charge of empiricism.
In the course of the work the writings of G. R. Burnell, C.E., Baldwin
Latham, C.E., M. Dru, Emerson Bainbridge, C.E., G. C. Greenwell, and
other well known authorities, have been freely referred to, particular
recourse having been had to the works of Professor Prestwich, F.G.S.
I am indebted to Geo. G. André, C.E., F.G.S., Messrs. S. Baker and Son,
and Messrs. T. Docwra and Son, for many suggestions and much valuable
information; to Messrs. Docwra special thanks are due for some of the
important sections illustrating chapter vii.
Any claim to attention the book may deserve is based upon its being an
attempt to embody, in a collected form, facts and information derived
from practice, or from various sources not accessible to the majority of
those engaged in the superintendence, or otherwise interested in the
construction of wells.
ERNEST SPON.
16, CRAVEN STREET, CHARING CROSS,
_June, 1875_.
SINKING AND BORING WELLS.
CHAPTER I.
GEOLOGICAL CONSIDERATIONS.
Nearly every civil engineer is familiar with the fact that certain
porous soils, such as sand or gravel, absorb water with rapidity, and
that the ground composed of them soon dries up after showers. If a well
be sunk in such soils, we often penetrate to considerable depths before
we meet with water; but this is usually found on our approaching some
lower part of the porous formation where it rests on an impervious bed;
for here the water, unable to make its way downwards in a direct line,
accumulates as in a reservoir, and is ready to ooze out into any opening
which may be made, in the same manner as we see the salt water filtrate
into and fill any hollow which we dig in the sands of the shore at low
tide. A spring, then, is the lowest point or lip of an underground
reservoir of water in the stratification. A well, therefore, sunk in
such strata will most probably furnish, besides the volume of the
spring, an additional supply of water.
The transmission of water through a porous medium being so rapid, we may
easily understand why springs are thrown out on the side of a hill,
where the upper set of strata consist of chalk, sand, and other
permeable substances, whilst those lying beneath are composed of clay or
other retentive soils. The only difficulty, indeed, is to explain why
the water does not ooze out everywhere along the line of junction of the
two formations, so as to form one continuous land-soak, instead of a
few springs only, and these oftentimes far distant from each other. The
principal cause of such a concentration of the waters at a few points
is, first, the existence of inequalities in the upper surface of the
impermeable stratum, which lead the water, as valleys do on the external
surface of a country, into certain low levels and channels; and
secondly, the frequency of rents and fissures, which act as natural
drains. That the generality of springs owe their supply to the
atmosphere is evident from this, that they vary in the different seasons
of the year, becoming languid or entirely ceasing to flow after long
droughts, and being again replenished after a continuance of rain. Many
of them are probably indebted for the constancy and uniformity of their
volume to the great extent of the subterranean reservoirs with which
they communicate, and the time required for these to empty themselves by
percolation. Such a gradual and regulated discharge is exhibited, though
in a less perfect degree, in all great lakes, for these are not sensibly
affected in their levels by a sudden shower, but are only slightly
raised, and their channels of efflux, instead of being swollen suddenly
like the bed of a torrent, carry off the surplus water gradually.
An Artesian well, so called from the province of Artois, in France, is a
shaft sunk or bored through impermeable strata, until a water-bearing
stratum is tapped, when the water is forced upwards by the hydrostatic
pressure due to the superior level at which the rain-water was received.
Among the causes of the failure of Artesian wells, we may mention those
numerous rents and faults which abound in some rocks, and the deep
ravines and valleys by which many countries are traversed; for when
these natural lines of drainage exist, there remains only a small
quantity of water to escape by artificial issues. We are also liable to
be baffled by the great thickness either of porous or impervious strata,
or by the dip of the beds, which may carry off the waters from adjoining
high lands to some trough in an opposite direction,--as when the borings
are made at the foot of an escarpment where the strata incline inwards,
or in a direction opposite to the face of the cliffs.
[Illustration: Effect of Strata on Artesian Well.
Fig. 1.]
[Illustration: Effect of a Fault.
Fig. 2.]
As instances of the way in which the character of the strata may
influence the water-bearing capacity of any given locality, we give the
following examples, taken from Baldwin Latham's papers on 'The Supply of
Water to Towns.' Fig. 1 illustrates the causes which sometimes conduce
to a limited supply of water in Artesian wells. Rain descending on the
outcrop E F of the porous stratum A, which lies between the impervious
stratum B B, will make its appearance in the form of a spring at S; but
such spring will not yield any great quantity of water, as the area E F,
which receives the rainfall, is limited in its extent. A well sunk at W,
in a stratum of the above description, would not be likely to furnish a
large supply of water, if any. The effect of a fault is shown in Fig. 2.
A spring will in all probability make its appearance at the point S, and
give large quantities of water, as the whole body of water flowing
through the porous strata A is intercepted by being thrown against the
impermeable stratum B. Permeable rock intersected by a dyke and
overlying an impermeable stratum is seen in Fig. 3. The water flowing
through A, if intersected by a dyke D, will appear at S in the form of a
spring, and if the area of A is of large extent, then the spring S will
be very copious. As to the depth necessary to bore certain wells, in a
case similar to Fig. 4, owing to the fault, a well sunk at A would
require to be sunk deeper than the well B, although both wells derive
their supply from the same description of strata. If there is any
inclination in the water-bearing strata, or if there is a current of
water only in one direction, then one of the wells would prove a failure
owing to the proximity of the fault, while the other would furnish an
abundant supply of water.
[Illustration: Permeable Rock Intersected by a Dyke.
Fig. 3.]
It should be borne in mind that there are two primary geological
conditions upon which the quantity of water that may be supplied to the
water-bearing strata depends; they are, the extent of superficial area
presented by these deposits, by which the quantity of rain-water
received on their surface in any given time is determined; and the
character and thickness of the strata, as by this the proportion of
water that can be absorbed, and the quantity which the whole volume of
the permeable strata can transmit, is regulated. The operation of these
general principles will constantly vary in accordance with local
phenomena, all of which must, in each separate case, be taken into
consideration.
[Illustration: Well Depth either side of a Fault.
Fig. 4.]
The mere distance of hills or mountains need not discourage us from
making trials; for the waters which fall on these higher lands readily
penetrate to great depths through highly-inclined or vertical strata, or
through the fissures of shattered rocks; and after flowing for a great
distance, must often reascend and be brought up again by other fissures,
so as to approach the surface in the lower country. Here they may be
concealed beneath a covering of undisturbed horizontal beds, which it
may be necessary to pierce in order to reach them. The course of water
flowing underground is not strictly analogous to that of rivers on the
surface, there being, in the one case, a constant descent from a higher
to a lower level from the source of the stream to the sea; whereas, in
the other, the water may at one time sink far below the level of the
ocean, and afterwards rise again high above it.
For the purposes under consideration, we may range the various strata of
which the outer crust of the earth is composed under four heads, namely:
1, drift; 2, alluvion; 3, the tertiary and secondary beds, composed of
loose, arenaceous and permeable strata, impervious, argillaceous and
marly strata, and thick strata of compact rock, more or less broken up
by fissures, as the Norwich red and coralline crag, the Molasse
sandstones, the Bagshot sands, the London clay, and the Woolwich beds,
in the tertiary division; and the chalk, chalk marl, gault, the
greensands, the Wealden clay, and the Hastings sand; the oolites, the
has, the Rhætic beds, and Keuper, and the new red sandstone, in the
secondary division; and 4, the primary beds, as the magnesian limestone,
the lower red sand, and the coal measures, which consist mainly of
alternating beds of sandstones and shales with coal.
The first of these divisions, the drift, consisting mainly of sand and
gravel, having been formed by the action of flowing water, is very
irregular in thickness, and exists frequently in detached masses. This
irregularity is due to the inequalities of the surface at the period
when the drift was brought down. Hollows then existing would often be
filled up, while either none was deposited on level surfaces, or, if
deposited, was subsequently removed by denudation. Hence we cannot infer
when boring through deposits of this character that the same, or nearly
the same, thickness will be found at even a few yards' distance. In
valleys this deposit may exist to a great depth, the slopes of hills are
frequently covered with drift, which has either been arrested by the
elevated surface or brought down from the upper portions of that surface
by the action of rain. In the former case the deposits will probably
consist of gravel, and in the latter, of the same elements as the hill
itself.
The permeability of such beds will, of course, depend wholly upon the
nature of the deposit. Some rocks produce deposits through which water
percolates readily, while others allow a passage only through such
fissures as may exist. Sand and gravel constitute an extremely absorbent
medium, while an argillaceous deposit may be wholly impervious. In
mountainous districts springs may often be found in the drift; their
existence in such formations will, however, depend upon the position and
character of the rock strata; thus, if the drift cover an elevated and
extensive slope of a nature similar to that of the rocks by which it is
formed, springs due to infiltration through this covering will certainly
exist near the foot of the slope. Upon the opposite slope, the small
spaces which exist between the different beds of rock receive these
infiltrations directly, and serve to completely drain the deposit which,
in the former case, is, on the contrary, saturated with water. If,
however, the foliations or the joints of the rocks afford no issue to
the water, whether such a circumstance be due to the character of their
formation, or to the stopping up of the issues by the drift itself,
these results will not be produced.
It will be obvious how, in this way, by passing under a mass of drift
the water descending from the top of hill slopes reappears at their
foot in the form of springs. If now we suppose these issues stopped, or
covered by an impervious stratum of great thickness, and this stratum
pierced by a boring, the water will ascend through this new outlet to a
level above that of its original issue, in virtue of the head of water
measured from the points at which the infiltration takes place to the
point in which it is struck by the boring.
Alluvion, like drift, consists of fragments of various strata carried
away and deposited by flowing water; it differs from the latter only
in being more extensive and regular, and, generally, in being composed
of elements brought from a great distance, and having no analogy with
the strata with which it is in contact. Usually it consists of sand,
gravel, rolled pebbles, marls or clays. The older deposits often occupy
very elevated districts, which they overlie throughout a large extent
of surface. At the period when the large rivers were formed, the
valleys were filled up with alluvial deposits, which at the present day
are covered by vegetable soil, and a rich growth of plants, through
which the water percolates more slowly than formerly. The permeability
of these deposits allows the water to flow away subterraneously to
a great distance from the points at which it enters. Springs are
common in the alluvion, and more frequently than in the case of drift,
they can be found by boring. As the surface, which is covered by the
deposit, is extensive, the water circulates from a distance through
permeable strata often overlaid by others that are impervious. If at a
considerable distance from the points of infiltration, and at a lower
level, a boring be put down, the water will ascend in the bore-hole
in virtue of its tendency to place itself in equilibrium. Where the
country is open and uninhabited, the water from shallow wells sunk
in alluvion is generally found to be good enough and in sufficient
quantity for domestic purposes.
The strata of the tertiary and secondary beds, especially the latter,
are far more extensive than the preceding, and yield much larger
quantities of water. The chalk is the great water-bearing stratum for
the larger portion of the south of England. The water in it can be
obtained either by means of ordinary shafts, or by Artesian wells bored
sometimes to great depths, from which the water will frequently rise
to the surface. It should be observed that water does not circulate
through the chalk by general permeation of the mass, but through
fissures. A rule given by some for the level at which water may be
found in this stratum is, "Take the level of the highest source of
supply, and that of the lowest to be found. The mean level will be the
depth at which water will be found at any intermediate point, after
allowing an inclination of at least 10 feet a mile." This rule will
also apply to the greensand. This formation contains large quantities
of water, which is more evenly distributed than in the chalk. The gault
clay is interposed between the upper and the lower greensand, the
latter of which also furnishes good supplies. In boring into the upper
greensand, caution should be observed so as not to pierce the gault
clay, because water which permeates through that system becomes either
ferruginous, or contaminated by salts and other impurities.
The next strata in which water is found are the upper and inferior
oolites, between which are the Kimmeridge and Oxford clays, which are
separated by the coral rag. There are instances in which the Oxford clay
is met with immediately below the Kimmeridge, rendering any attempt at
boring useless, because the water in the Oxford clay is generally so
impure as to be unfit for use. And with regard to finding water in the
oolitic limestone, it is impossible to determine with any amount of
precision the depth at which it may be reached, owing to the numerous
faults which occur in the formation. It will therefore be necessary to
employ the greatest care before proceeding with any borings. Lower down
in the order are the upper has, the marlstone, the lower has, and the
new red sandstone. In the marlstone, between the upper and lower beds of
the has, there may be found a large supply of water, but the level of
this is as a rule too low to rise to the surface through a boring. It
will be necessary to sink shafts in the ordinary way to reach it. In the
new red sandstone, also, to find the water, borings must be made to a
considerable depth, but when this formation exists a copious supply may
be confidently anticipated, and when found the water is of excellent
quality.
Every permeable stratum may yield water, and its ability to do this, and
the quantity it can yield, depend upon its position and extent. When
underlaid by an impervious stratum, it constitutes a reservoir of water
from which a supply may be drawn by means of a sinking or a bore-hole.
If the permeable stratum be also overlaid by an impervious stratum, the
water will be under pressure and will ascend the bore-hole to a height
that will depend on the height of the points of infiltration above the
bottom of the bore-hole. The quantity to be obtained in such a case as
we have already pointed out, will depend upon the extent of surface
possessed by the outcrop of the permeable stratum. In searching for
water under such conditions a careful examination of the geological
features of the district must be made. Frequently an extended view of
the surface of the district, such as may be obtained from an eminence,
and a consideration of the particular configuration of that surface,
will be sufficient to enable the practical eye to discover the various
routes which are followed by the subterranean water, and to predicate
with some degree of certainty that at a given point water will be found
in abundance, or that no water at all exists at that point. To do this,
it is sufficient to note the dip and the surfaces of the strata which
are exposed to the rains. When these strata are nearly horizontal, water
can penetrate them only through their fissures or pores; when, on the
contrary, they lie at right-angles, they absorb the larger portion of
the water that falls upon their outcrop. When such strata are
intercepted by valleys, numerous springs will exist. But if, instead of
being intercepted, the strata rise around a common point, they form a
kind of irregular basin, in the centre of which the water will
accumulate. In this case the surface springs will be less numerous than
when the strata are broken. But it is possible to obtain water under
pressure in the lower portions of the basin, if the point at which the
trial is made is situate below the outcrop.
The primary rocks afford generally but little water. Having been
subjected to violent convulsions, they are thrown into every possible
position and broken by numerous fissures; and as no permeable stratum is
interposed, as in the more recent formations, no reservoir of water
exists. In the unstratified rocks, the water circulates in all
directions through the fissures that traverse them, and thus occupies no
fixed level. It is also impossible to discover by a surface examination
where the fissures may be struck by a boring. For purposes of water
supply, therefore, these rocks are of little importance. It must be
remarked here, however, that large quantities of water are frequently
met with in the magnesian limestone and the lower red sand, which form
the upper portion of the primary series.
Joseph Prestwich, jun., in his 'Geological Inquiry respecting the
Water-bearing Strata round London,' gives the following valuable epitome
of the geological conditions affecting the value of water-bearing
deposits; and although the illustrations are confined to the Tertiary
deposits, the same mode of inquiry will apply with but little
modification to any other formation.
The main points are--
The extent of the superficial area occupied by the water-bearing
deposit.
The lithological character and thickness of the water-bearing deposit,
and the extent of its underground range.
The position of the outcrop of the deposit, whether in valleys or hills,
and whether its outcrop is denuded, or covered with any description of
drift.
The general elevation of the country occupied by this outcrop above the
levels of the district in which it is proposed to sink wells.
The quantity of rain which falls in the district under consideration,
and whether, in addition, it receives any portion of the drainage from
adjoining tracts, when the strata are impermeable.
The disturbances which may affect the water-bearing strata, and break
their continuous character, as by this the subterranean flow of water
would be impeded or prevented.
EXTENT OF SUPERFICIAL AREA.
To proceed to the application of the questions in the particular
instance of the lower tertiary strata. With regard to the first
question, it is evident that a series of permeable strata encased
between two impermeable formations can receive a supply of water at
those points only where they crop out and are exposed on the surface of
the land. The primary conditions affecting the result depend upon the
fall of rain in the district where the outcrop takes place; the quantity
of rain-water which any permeable strata can gather being in the same
ratio as their respective areas. If the mean annual fall in any district
amounts to 24 inches, then each square mile will receive a daily average
of 950,947 gallons of rain-water. It is therefore a matter of essential
importance to ascertain, with as much accuracy as possible, the extent
of exposed surface of any water-bearing deposit, so as to determine the
maximum quantity of rain-water it is capable of receiving.
The surface formed by the outcropping of any deposit in a country of
hill and valley is necessarily extremely limited, and it would be
difficult to measure in the ordinary way. Prestwich therefore used
another method, which seems to give results sufficiently accurate for
the purpose. It is a plan borrowed from geographers, that of cutting out
from a map on paper of uniform thickness and on a large scale, say one
inch to the mile, and weighing the superficial area of each deposit.
Knowing the weight of a square of 100 miles cut out of the same paper,
it is easy to estimate roughly the area in square miles of any other
surface, whatever may be its figure.
MINERAL CHARACTER OF THE FORMATION.
The second question relates to the mineral character of the formation,
and the effect it will have upon the quantity of water which it may hold
or transmit.
If the strata consist of sand, water will pass through them with
facility, and they will also hold a considerable quantity between the
interstices of their component grains; whereas a bed of pure clay will
not allow of the passage of water. These are the two extremes of the
case; the intermixture of these materials in the same bed will of
course, according to their relative proportions, modify the transmission
of water. Prestwich found by experiment that a silicious sand of
ordinary character will hold on an average rather more than one-third of
its bulk of water, or from two to two and a half gallons in one cubic
foot. In strata so composed the water may be termed free, as it passes
easily in all directions, and under the pressure of a column of water is
comparatively but little impeded by capillary attraction. These are the
conditions of a true permeable stratum. Where the strata are more
compact and solid, as in sandstone, limestone, and oolite, although all
such rocks imbibe more or less water, yet the water so absorbed does not
pass freely through the mass, but is held in the pores of the rock by
capillary attraction, and parted with very slowly; so that in such
deposits water can be freely transmitted only in the planes of bedding
and in fissures. If the water-bearing deposit is of uniform lithological
character over a large area, then the proposition is reduced to its
simplest form; but when, as in the deposit between the London clay and
the chalk, the strata consist of variable mineral ingredients, it
becomes essential to estimate the extent of these variations; for very
different conclusions might be drawn from an inspection of the Lower
Tertiary strata at different localities.
[Illustration: _a_ London clay, _b_ Sands and clay, _c_ Chalk.
Fig. 5.]
In the fine section exposed in the cliffs between Herne Bay and the
Reculvers, in England, a considerable mass of fossiliferous sands is
seen to rise from beneath the London clay. Fig. 5 represents a view of a
portion of this cliff a mile and a half east of Herne Bay and continued
downwards, by estimation below the surface of the ground to the chalk.
In this section there is evidently a very large proportion of sand, and
consequently a large capacity for water. Again, at Upnor, near
Rochester, the sands marked 3 are as much as 60 to 80 feet thick, and
continue so to Gravesend, Purfleet, and Erith. In the first of these
places they may be seen capping Windmill Hill; in the second, forming
the hill, now removed, on which the lighthouse is built; and in the
third, in the large ballast pits on the banks of the river Thames. The
average thickness of these sands in this district may be about 50 to 60
feet. In their range from east to west, the beds 2 become more clayey
and less permeable, and 1, very thin. As we approach London the
thickness of 3 also diminishes. In the ballast pits at the west end of
Woolwich, this sand-bed is not more than 35 feet thick, and as it passes
under London becomes still thinner.
[Illustration: Fig. 6.
General Section of Strata below London.]
Fig. 6 is a general or average section of the strata on which London
stands. The increase in the proportion of the argillaceous strata, and
the decrease of the beds of sand, in the Lower Tertiary strata is here
very apparent, and from this point westward to Hungerford, clays
decidedly predominate; while at the same time the series presents such
rapid variations, even on the same level and at short distances, that no
two sections are alike. On the southern boundary of the Tertiary
district, from Croydon to Leatherhead, the sands 3 maintain a thickness
of 20 to 40 feet, whilst the associated beds of clay are of inferior
importance. We will take another section, Fig. 7, representing the
usual features of the deposit in the northern part of the Tertiary
district. It is from a cutting at a brickfield west of the small village
of Hedgerley, 6 miles northward of Windsor.
[Illustration: Unusual Deposit Features 6 miles North of Windsor.
Fig. 7.]
Here we see a large development of the mottled clays, and but little
sand. A somewhat similar section is exhibited at Oak End, near Chalfont
St. Giles. But to show how rapidly this series changes its character,
the section of a pit only a third of a mile westward of the one at
Hedgerley is given in Fig. 8.
[Illustration: Section of a Pit.
Fig. 8.]
In this latter section the mottled clays have nearly disappeared, and
are replaced by beds of sand with thin seams of mottled clays. At
Twyford, near Reading, and at Old Basing, near Basingstoke, the mottled
clays again occupy, as at Hedgerley, nearly the whole space between the
London clays and the chalk. Near Reading a good section of these beds
was exhibited in the Sonning cutting of the Great Western Railway; they
consisted chiefly of mottled clays. At the Katsgrove pits, Reading, the
beds are more sandy. Referring back to Fig. 6, it may be noticed that
there is generally a small quantity of water found in the bed marked 1,
in parts of the neighbourhood of London. Owing, however, to the constant
presence of green and ferruginous sands, traces of vegetable matters and
remains of fossil shells, the water is usually indifferent and
chalybeate. The well-diggers term this a slow spring. They well express
the difference by saying that the water creeps up from this stratum,
whereas that it bursts up from the lower sands 3, which is the great
water-bearing stratum. In the irregular sand-beds interstratified with
the mottled clays between these two strata water is also found, but not
in any large quantity.
[Illustration: Section at Pebble Hill.
Fig. 9.]
Fig. 9 is a section at the western extremity of the Tertiary district at
Pebble Hill, near Hungerford. Here again the mottled clays are in
considerable force, sands forming the smaller part of the series.
The following lists exhibit the aggregate thickness of all the beds of
sand occurring between the London clay and the chalk at various
localities in the Tertiary district. It will appear from them that the
mean results of the whole is very different from any of those obtained
in separate divisions of the country. The mean thickness of the deposit
throughout the whole Tertiary area may be taken at 62 feet, of which 36
feet consist of sands and 26 feet of clays; but as only a portion of
this district contributes to the water supply of London, it will
facilitate our inquiry if we divide it into two parts, the one westward
of and including London, and the other eastward of it, introducing also
some further subdivisions into each.
MEASUREMENT OF SECTIONS EASTWARD OF LONDON.
+---------------------+-----+-----+
|Southern Boundary. |Sand.|Clay.|
+---------------------+-----+-----+
| | ft. | ft. |
|Lewisham | 65 | 26 |
|Woolwich | 66 | 18 |
|Upnor | 80? | 8 |
|Herne Bay | 70? | 50 |
| | | |
| | | |
| | | |
| | | |
| +-----+-----+
| Average | 70 | 25 |
+---------------------+-----+-----+
+---------------------+-----+-----+
|Northern Boundary. |Sand.|Clay.|
+---------------------+-----+-----+
| | ft.| ft. |
|Hertford | 26 | 3 |
|Beaumont Green, | | |
| near Hoddesdon | 16 | 10 |
|Broxbourne | 28 | 2 |
|Gestingthorpe, | | |
| near Sudbury | 50?| ? |
|Whitton, | | |
| near Ipswich | 60?| 5 |
| +-----+-----+
| Average | 36 | 5 |
+---------------------+-----+-----+
The mean of the three columns in two western sections gives a thickness
to this formation of 57 feet, of which only 19 feet are sand and
permeable to water, and the remaining 38 feet consist of impermeable
clays, affording no supply of water.
The area, both at the surface and underground, over which they extend
is about 1086 square miles.
MEASUREMENT OF SECTIONS WESTWARD OF LONDON.
+--------------------------------------------+
| On or near the Southern Boundary |
| of the Tertiary District. |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| | ft.| ft.|
|Streatham | 30 | 25 |
|Mitcham | 47 | 34 |
|Croydon | 35?| 20?|
|Epsom | 31 | 23 |
|Fetcham | 35 | 20 |
|Guildford | 10?| 40 |
|Chinham, near Basingstoke | 20?| 30 |
|Itchingswell, near Kingsclere | 22 | 34 |
|Highclere | 24 | 27 |
|Pebble Hill, near Hungerford | 9 | 39 |
| | | |
| | | |
| | | |
| +-----+-----+
| Average | 26 | 29 |
+--------------------------------+-----+-----+
+--------------------------------------------+
| On a Central Line in the |
| Tertiary District. |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| Sand.Clay.| ft.| ft.|
| ft. ft. | | |
|London: | | |
| Millbank 49 40} | | |
| Trafalgar Square 49 30} | | |
| Tottenham Court Road 35 30} | | |
| Pentonville 34 44} | 46 | 39 |
| Barclay's Brewery 55 42} | | |
| Lombard Street 53 35} | | |
| The Mint 49 38} | | |
| Whitechapel 45 50} | | |
|Garrett, near Wandsworth | 20 | 52 |
|Isleworth | 17 | 70 |
|Twickenham | 7 | 50 |
|Chobham | 3 | 45 |
| +-----+-----+
| Average | 18 | 51 |
+--------------------------------+-----+-----+
+--------------------------------------------+
| On or near the Northern Boundary |
| of the Tertiary District |
+--------------------------------+-----+-----+
| |Sand.|Clay.|
| +-----+-----+
| | ft.| ft.|
|Hatfield | 23 | 2 |
|Watford | 25 | 10 |
|Pinner | 12 | 32 |
|Oak End, Chalfont St. Giles | 3 | 40 |
|Hedgerley, near Slough | 5 | 45 |
|Starveall, " " | 13 | 20 |
|Twyford | 5 | 60 |
|Sonning, near Reading | 12 | 54 |
|Reading | 16 | 33 |
|Newbury | 20 | 36 |
|Pebble Hill | 9 | 39 |
| | | |
| | | |
| +-----+-----+
| Average | 13 | 34 |
+--------------------------------+-----+-----+
The average total thickness of the eastern district deduced from the
nine sections we have taken gives 68 feet, of which 53 feet are sands
and 15 feet clays. The larger area, 1849 square miles, over which the
eastern portion of the Tertiary series extends, and the greater volume
of the water-bearing beds, constitute important differences in favour of
this district; and if there had been no geological disturbances to
interfere with the continuous character of the strata, we might have
looked to this quarter for a large supply of water to the Artesian wells
of London.
[Illustration: A Geological Section.
Fig. 10.]
From these tables it will be readily perceived that the strata of which
the water-bearing deposits are composed are very variable in their
relative thickness. They consist, in fact, of alternating beds of clay
and sand, in proportions constantly changing. In one place, as at
Hedgerley, the aggregate beds of sand may be 5 feet thick, and the clays
45 feet; whilst at another, as at Leatherhead, the sands may be 35, and
the clays 20 feet thick, and some such variation is observable in every
locality. But although we may thus in some measure judge of the capacity
of these beds for water, this method fails to show whether the
communication from one part of the area to another is free, or impeded
by causes connected with mineral character. Now as we know that these
beds not only vary in their thickness, but that they also frequently
thin out, and sometimes pass one into another, it may happen that a very
large development of clay at any one place may altogether stop the
transit of the water in that locality. Thus in Fig. 10 the beds of sand
at y allow of the free passage of water, but at x, where clays occupy
the whole thickness, it cannot pass; the obstruction which this cause
may offer to the underground flow of water can only be determined by
experience. It must not, however, be supposed that such a variation in
the strata is permanent or general along any given line. It is always
local, some of the beds of clay commonly thinning out after a certain
horizontal range, so that, although the water may be impeded or
retarded in a direct course, it most probably can, in part or
altogether, pass round by some point where the strata have not undergone
the same alteration.
POSITION AND GENERAL CONDITIONS OF THE OUTCROP.
This involves some considerations to which an exact value cannot at
present be given, yet which require notice, as they to a great extent
determine the proportion of water which can pass from the surface into
the mass of the water-bearing strata. In the first place, when the
outcrop of these strata occurs in a valley, as represented in Fig. 11,
it is evident that _b_ may not only retain all the water which might
fall on its surface, but also would receive a proportion of that
draining off from the strata of _a_ and _c_. This form of the surface
generally prevails wherever the water-bearing strata are softer and less
coherent than the strata above and below them.
[Illustration: Water-bearing Strata in a Valley.
Fig. 11.]
It may be observed in the Lower Tertiary series at Sutton, Carshalton,
and Croydon, where a small and shallow valley, excavated in these sands
and mottled clays, ranges parallel with the chalk hills.
It is apparent again between Epsom and Leatherhead, and also in some
places between Guildford and Farnham, as well as between Odiham and
Kingsclere. The Southampton Railway crosses this small valley on an
embankment at Old Basing.
This may be considered as the prevailing, but not exclusive, form of
structure from Croydon to near Hungerford. The advantage, however, to be
gained from it in point of water supply is much limited by the rather
high angle at which the strata are inclined, as well as by their small
development, which greatly restrict the breadth of the surface occupied
by the outcrop. It rarely exceeds a quarter of a mile, and is generally
very much less, often not more than 100 to 200 feet. The next
modification of outcrop, represented in Fig. 12, is one not uncommon on
the south side of the Tertiary district. The strata _b_ here crop out on
the slope of the chalk hills, and the rain falling upon them, unless
rapidly absorbed, tends to drain at once from their surface into the
adjacent valleys. V, L, shows the line of valley level.
[Illustration: Strata on Slope of Chalk Hills.
Fig. 12.]
This arrangement is not unfrequent between Kingsclere and Inkpen, and
also between Guildford and Leatherhead. Eastward of London it is
exhibited on a larger scale at the base of the chalk hills, in places
between Chatham and Faversham, a line along which the sands of the Lower
Tertiary strata, _b_, are more fully developed than elsewhere. As,
however, the surface of _b_ is there usually more coincident with the
valley level, V, L, of the district, it is in a better position for
retaining more of the rainfall.
[Illustration: Chalk Stratum at Base of a Hill.
Fig. 13.]
A third position of outcrop, much more unfavourable for the
water-bearing strata, prevails generally along the greater part of the
northern boundary of the Tertiary strata. Instead of forming a valley,
or outcropping at the base of the chalk hills, almost the whole length
of this outcrop lies on the slope of the hills, as in Fig. 13, where the
chalk _c_ forms the base of the hill and the lower ground at its foot,
whilst the London clay, _a_, caps the summit, thus restricting the
outcrop of _b_ to a very narrow zone and a sloping surface. This form
of structure is exhibited in the hills round Sonning, Reading,
Hedgerley, Rickmansworth, and Watford; thence by Shenley Hill, Hatfield,
Hertford, Sudbury; and also at Hadleigh this position of outcrop is
continued. If, as on the southern side of the Tertiary district, the
outcrop were continued in a nearly unbroken line, then these
unfavourable conditions would prevail uninterruptedly; but the hills are
in broken groups, and intersected at short distances by transverse
valleys, as that of the Kennet at Reading, of the Loddon at Twyford, of
the Colne at Uxbridge, and so on. Between Watford and Hatfield there is
a constant succession of small valleys running back for short distances
from the Lower district of the chalk, through the hills of the Tertiary
district. The Valley of the Lea at Roydon and Hoddesdon is a similar and
stronger case in point. The effect of these transverse valleys is to
open out a larger surface of the strata _b_ than would otherwise be
exposed, for if the horizontal line, V, L, Fig. 13, were carried back
beyond the point _x_, to meet the prolongation of _b_, then these Lower
Tertiary strata would not only be intersected by the line of valley
level, but would form a much smaller angle with the plane V, L, and
therefore spread over a larger area than where they crop out on the side
of the hills.
The foregoing are the three most general forms of outcrop, but
occasionally the outcrop takes place wholly or partly on the summit of a
hill, as, near the Reculvers in the neighbourhood of Canterbury, of
Sittingbourne, and at the Addington Hills, near Croydon, in which cases
the area of the Lower Tertiary is expanded. When the dip is very slight,
and the beds nearly horizontal, the Lower Tertiary sands occasionally
spread over a still larger extent of surface, as between Stoke Pogis,
Burnham Common, and Beaconsfield, and in the case of the flat-topped
hill, forming Blackheath and Bexley Heath, as in Fig. 14. Favourable as
such districts might at first appear to be from the extent of their
exposed surface, nevertheless they rarely contribute to the water supply
of the wells sunk into the Lower Tertiary sands under London, the
continuity of the strata being broken by intersecting valleys; thus the
district last mentioned is bounded on the north by the valley of the
Thames, on the west by that of Ravensbourne, and on the east by the
valley of the Cray; consequently the rain-water, which has been absorbed
by the very permeable strata on the intermediate higher ground, passes
out on the sides of the hills, into the surface channels in the valleys,
or into the chalk. Almost all the wells at Bexley Heath, for their
supply of water, have, in fact, to be sunk into the chalk through the
overlying 100 to 133 feet of sand and pebble beds, _b_.
[Illustration: Section at Blackheath and Bexley Heath.
Fig. 14.]
Thus far we have considered this question, as if, in each instance, the
outcropping edges of the water-bearing strata, _b_, were laid bare, and
presented no impediment to the absorption of the rain-water falling
immediately upon their surface, or passing on to it from some more
impermeable deposits. But there is another consideration which
influences materially the extent of the water supply.
If the strata _b_ were always bare, we should have to consider their
outcrop as an absorbent surface, of power varying according to the
lithological character and dip of the strata only. But the outcropping
edges of the strata do not commonly present bare and denuded surfaces.
Thus a large extent of the country round London is more or less covered
by beds of drift, which protect the outcropping beds of _b_, and turn
off a portion of the water falling upon them.
The drift differs considerably in its power of interference with the
passage of the rain-water into the strata beneath. The ochreous sandy
flint gravel, forming so generally the subsoil of London, admits of the
passage of water. All the shallow surface springs, from 10 to 20 feet
deep, are produced by water which has fallen on, and passed through,
this gravel, _g_, Fig. 15, down to the top of the London clay, _a_, on
the irregular surface of which it is held up.
[Illustration: Section through London Subsoil.
Fig. 15.]
When the London clay is wanting, this gravel lies immediately upon the
Lower Tertiary strata, as in the valley between Windsor and Maidenhead,
and in that of the Kennet between Newbury and Thatcham, transmitting to
the underlying strata part of the surface water. Where beds of brick
earth occur in the drift, as between West Drayton and Uxbridge, the
passage of the surface water into the underlying strata is intercepted.
Sometimes the drift is composed of gravel mixed very irregularly with
broken up London clay, and although commonly not more than 3 to 8 feet
thick, it is generally impermeable.
Over a considerable portion of Suffolk and part of Essex, a drift,
composed of coarse and usually light-coloured sand with fine gravel,
occurs. Water percolates through it with extreme facility, but it is
generally covered by a thick mass of stiff tenacious bluish grey clay,
perfectly impervious. This clay drift, or boulder clay, caps, to a depth
of from 10 to 50 feet or more, almost all the hills in the northern
division of Essex, and a large portion of Suffolk and Norfolk. It so
conceals the underlying strata that it is difficult to trace the course
of the outcrop of the Lower Tertiary sands between Ware and Ipswich; and
often, as in Fig. 16, notwithstanding the breadth, apart from this cause
of the outcrop of the Tertiary sands, _b_, and of the drift of sand and
gravel, 2, they are both so covered by the boulder clay, 1, that the
small surface exposed can be of comparatively little value.
[Illustration: Outcrop covered by Boulder Clay.
Fig. 16.]
There are also, in some valleys, river deposits of silt, mud, and
gravel. These are, however, of little importance to the subject before
us. Under ordinary conditions they are generally sufficiently impervious
to prevent the water from passing through the beds beneath.
HEIGHT OF WATER-BEARING STRATA ABOVE SURFACE OF COUNTRY.
The height of the districts, wherein the water-bearing strata crop out,
above that of the surface of the country in which the wells are placed,
should be made the subject of careful consideration, as upon this point
depends the level to which the water in Artesian wells may ascend.
Again, taking the London district as an example, Prestwich remarks that,
as the country rises on both sides of the Thames to the edge of the
chalk escarpments, and as the outcrop of the Lower Tertiary strata is
intermediate between these escarpments and the Thames, it follows that
the outcrop of these lower beds must, in all cases, be on a higher level
than the Thames itself, where it flows through the centre of the
Tertiary district. Its altitude is, of course, very variable, as shown
in the following list of its approximate height above Trinity high
water-mark at London. These heights are taken where the Tertiaries are
at their lowest level in the several localities mentioned.
--------------------------------+-------------------------
South of London. | North of London.
--------------------------------+-------------------------
Croydon about 130 feet. | Thetford about 200 feet.
Leatherhead " 90 " | Watford " 170 "
Guildford " 96 " | Slough " 60 "
Old Basing " 250 " | Reading " 120 "
Near Hungerford " 360 " | Newbury " 236 "
--------------------------------+-------------------------
Eastward of London these strata crop out at a gradually decreasing
level. In consequence, therefore, of the outcrop of the water-bearing
strata being thus much above the surface of the central Tertiary
district bordering the Thames, the water in these strata beneath London
tended originally to rise above that surface.
As, however, these beds crop out on a level with the Thames immediately
east of the city between Deptford, Blackwall, and Bow, the water, having
this natural issue so near, could never have risen in London much above
the level of the river.
RAINFALL IN THE DISTRICT WHERE THE WATER-BEARING STRATA CROP OUT.
When inquiring into the probable relative value of any water-bearing
strata, it is necessary to compare the rainfall in their respective
districts.
Rain is of all meteorological phenomena the most capricious, both as
regards its frequency and the amount which falls in a given time. In
some places it rarely or never falls, whilst in others it rains almost
every day; and there does not yet exist any theory from which a probable
estimate of the rainfall in a given district can be deduced
independently of direct observation. But although dealing with one of
the most capricious of the elements, we nevertheless find a workable
average in the quantity of rain to be expected in any particular place,
if careful and continued observations are made with the rain-gauge. G.
J. Symons, the meteorologist, to whose continued investigations we are
indebted for our most reliable data upon the subject of rainfall, gives
the following practical instructions for using a rain-gauge;--
"The mouth of the gauge must be set quite level, and so fixed that it
will remain so; it should never be less than 6 inches above the ground,
nor more than 1 foot except when a greater elevation is absolutely
necessary to obtain a proper exposure.
"It must be set on a level piece of ground, at a distance from shrubs,
trees, walls, and buildings, at the very least as many feet from their
base as they are in height.
"If a thoroughly clear site cannot be obtained, shelter is most
endurable from N.W., N., and E., less so from S., S.E., and W., and not
at all from S.W. or N.E.
"Special prohibition must issue as to keeping all tall-growing flowers
away from the gauges.
"In order to prevent rust, it will be desirable to give the japanned
gauges a coat of paint every two or three years.
"The gauge should, if possible, be emptied daily at 9 A.M., and the
amount entered against the previous day.
"When making an observation, care should be taken to hold the glass
upright.
"It can hardly be necessary to give here a treatise on decimal
arithmetic; suffice it therefore to say that rain-gauge glasses usually
hold half an inch of rain (0·50) and that each 1/100 (0·01) is marked;
if the fall is less than half an inch, the number of hundredths is read
off at once, if it is over half an inch, the glass must be filled up to
the half inch (0·50), and the remainder (say 0·22) measured afterwards,
the total (0·50 + 0·22) = 0·72 being entered. If less than 1/10 (0·10)
has fallen, the cipher must always be prefixed; thus if the measure is
full up to the seventh line, it must be entered as 0·07, that is, no
inches, no tenths, and seven hundredths. For the sake of clearness it
has been found necessary to lay down an invariable rule that there shall
always be two figures to the right of the decimal point. If there be
only one figure, as in the case of one-tenth of an inch, usually written
0·1, a cipher must be added, making it 0·10. Neglect of this rule causes
much inconvenience.
"In snow three methods may be adopted--it is well to try them all. 1.
Melt what is caught in the funnel, and measure that as rain. 2. Select a
place where the snow has not drifted, invert the funnel, and turning it
round, lift and melt what is enclosed. 3. Measure with a rule the
average depth of snow, and take one-twelfth as the equivalent of water.
Some observers use in snowy weather a cylinder of the same diameter as
the rain-gauge, and of considerable depth. If the wind is at all rough,
all the snow is blown out of a flat-funnelled rain-gauge."
A drainage area is almost always a district of country enclosed by a
ridge or watershed line, continuous except at the place where the waters
of the basin find an outlet. It may be, and generally is, divided by
branch ridge-lines into a number of smaller basins, each drained by its
own stream into the main stream. In order to measure the area of a
catchment basin a plan of the country is required, which either shows
the ridge-lines or gives data for finding their positions by means of
detached levels, or of contour lines.
When a catchment basin is very extensive it is advisable to measure the
smaller basins of which it consists, as the depths of rainfall in them
may be different; and sometimes, also, for the same reason, to divide
those basins into portions at different distances from the mountain
chains, where rain-clouds are chiefly formed.
The exceptional cases, in which the boundary of a drainage area is not a
ridge-line on the surface of the country, are those in which the
rain-water sinks into a porous stratum until its descent is stopped by
an impervious stratum, and in which, consequently, one boundary at least
of the drainage area depends on the figure of the impervious stratum,
being, in fact, a ridge-line on the upper surface of that stratum,
instead of on the ground, and very often marking the upper edge of the
outcrop of that stratum. If the porous stratum is partly covered by a
second impervious stratum, the nearest ridge-line on the latter stratum
to the point where the porous stratum crops out will be another boundary
of the drainage area. In order to determine a drainage area under these
circumstances it is necessary to have a geological map and sections of
the district.
The depth of rainfall in a given time varies to a great extent at
different seasons, in different years, and in different places. The
extreme limits of annual depth of rainfall in different parts of the
world may be held to be respectively nothing and 150 inches. The average
annual depth of rainfall in different parts of Britain ranges from 22
inches to 140 inches, and the least annual depth recorded in Britain is
about 15 inches.
The rainfall in different parts of a given country is, in general,
greatest in those districts which lie towards the quarter from which the
prevailing winds blow; in Great Britain, for instance, the western
districts have the most rain. Upon a given mountain ridge, however, the
reverse is the case, the greatest rainfall taking place on that side
which lies to leeward, as regards the prevailing winds. To the same
cause may be ascribed the fact that the rainfall is greater in
mountainous than in flat districts, and greater at points near high
mountain summits than at points farther from them; and the difference
due to elevation is often greater by far than that due to 100 miles
geographical distance.
The most important data respecting the depth of rainfall in a given
district, for practical purposes, are, the least annual rainfall; mean
annual rainfall; greatest annual rainfall; distribution of the rainfall
at different seasons, and especially, the longest continuous drought;
greatest flood rainfall, or continuous fall of rain in a short period.
The available rainfall of a district is that part of the total rainfall
which remains to be stored in reservoirs, or carried away by streams,
after deducting the loss through evaporation, through permanent
absorption by plants and by the ground, and other causes.
The proportion borne by the available to the total rainfall varies very
much, being affected by the rapidity of the rainfall and the compactness
or porosity of the soil, the steepness or flatness of the ground, the
nature and quantity of the vegetation upon it, the temperature and
moisture of the air, which will affect the rate of evaporation, the
existence of artificial drains, and other circumstances. The following
are examples:
Available Rainfall.
Ground. ÷
Total Rainfall.
Steep surfaces of granite, gneiss, and slate, nearly 1
Moorland and hilly pasture from ·8 to ·6
Flat cultivated country from ·5 to ·4
Chalk 0
Deep-seated springs and wells give from ·3 to ·4 of the total rainfall.
Stephenson found that for the chalk district round Watford the
evaporation was about 34 per cent., the quantity carried off by streams
23·2 per cent., leaving 42·8 per cent., which sank below the surface to
form springs. In formations less absorbent than the chalk it can be
calculated roughly, that streams carry off one-third, that another
third evaporates, and that the remaining third of the total rainfall
sinks into the earth.
Such data as the above may be used in approximately estimating the
probable available rainfall of a district; but a much more accurate and
satisfactory method is to measure the actual discharge of the streams,
and the quantity lost by evaporation, at the same time that the
rain-gauge observations are made, and so to find the actual proportion
of available to total rainfall.
The following Table gives the mean annual rainfall in various parts of
the world;--
TABLE OF RAINFALL. Collected by G. J. Symons.
-----------------------------+--------------+----------+-------
| Period | | Mean
Country and Station. | of | Latitude.| Annual
| Observations.| | Fall.
-----------------------------+--------------+----------+-------
EUROPE. | years | ° ´ | ins.
AUSTRIA--Cracow | 5 | 50 4N | 33·1
Prague | 47 | 50 5 | 15·1
Vienna | 10 | 48 12 | 19·6
BELGIUM--Brussels | 20 | 50 51 | 28·6
Ghent | 13 | 51 4 | 30·6
Louvain | 12 | 50 33 | 28·6
DENMARK--Copenhagen | 12 | 55 41 | 22·3
FRANCE--Bayonne | 10 | 43 29 | 56·2
Bordeaux | 32 | 44 50 | 32·4
Brest | 30 | 48 23 | 38·8
Dijon | 20 | 47 14 | 31·1
FRANCE--Lyons | .. | 45 46 | 37·0
Marseilles | 60 | 43 17 | 19·0
Montpelier | 51 | 43 36 | 30·3
Nice | 20 | 43 43 | 55·2
Paris | 44 | 48 50 | 22·9
Pau | 12 | 43 19 | 37·1
Rouen | 10 | 49 27 | 33·7
Toulon | .. | 43 4 | 19·7
Toulouse | 52 | 43 36 | 24·9
GREAT BRITAIN-- | | |
England, London | 40 | 51 31 | 24·0
" Manchester | 40 | 53 29 | 36·0
" Exeter | 40 | 50 44 | 33·0
" Lincoln | 40 | 53 15 | 20·0
Wales, Cardiff | 40 | 51 28 | 43·0
" Llandudno | 40 | 53 19 | 30·0
Scotland, Edinburgh | 40 | 55 57 | 24·0
" Glasgow | 40 | 55 52 | 39·0
" Aberdeen | 40 | 57 8 | 31·0
Ireland, Cork | 40 | 51 54 | 40·0
" Dublin | 40 | 53 23 | 30·0
" Galway | 40 | 53 15 | 50·0
HOLLAND--Rotterdam | .. | 51 55 | 22·0
ICELAND--Reikiavik | 5 | 64 8 | 28·0
IONIAN ISLES--Corfu | 22 | 39 37 | 42·4
ITALY--Florence | 8 | 43 46 | 35·9
Milan | 68 | 45 29 | 38·0
Naples | 8 | 40 52 | 39·3
Rome | 40 | 41 53 | 30·9
Turin | 4 | 45 5 | 38·6
Venice | 19 | 45 25 | 34·1
MALTA | .. | 35 54 | 15·0
NORWAY--Bergen | 10 | 60 24 | 84·8
Christiania | .. | 59 54 | 26·7
PORTUGAL--Coimbra | | |
(in Vale of Mondego) | 2 | 40 13 | 224·0?
Lisbon | 20 | 38 42 | 23·0
PRUSSIA--Berlin | 6 | 52 30 | 23·6
Cologne | 10 | 50 55 | 24·0
Hanover | 3 | 52 24 | 22·4
Potsdam | 10 | 52 24 | 20·3
RUSSIA--St. Petersburg | 14 | 59 56 | 16·2
Archangel | 1 | 64 32 | 14·5
Astrakhan | 4 | 46 24 | 6·1
Finland, Uleaborg | .. | 65 0 | 13·5
SICILY--Palermo | 24 | 38 8 | 22·8
SPAIN--Madrid | .. | 40 24 | 9·0
Oviedo | 1 | 43 22 | 111·1
SWEDEN--Stockholm | 8 | 59 20 | 19·7
SWITZERLAND--Geneva | 72 | 46 12 | 31·8
Great St. Bernard | 43 | 45 50 | 58·5
Lausanne | 8 | 46 30 | 38·5
| | |
ASIA. | | |
CHINA--Canton | 14 | 23 6 | 69·3
Macao | .. | 22 24 | 68·3
Pekin | 7 | 39 54 | 26·9
INDIA-- | | |
Ceylon, Colombo | .. | 6 56 | 91·7
" Kandy | .. | 7 18 | 84·0
" Adam's Peak | .. | 6 50 | 100·0
Bombay | 33 | 18 56 | 84·7
Calcutta | 20 | 22 35 | 66·9
Cherrapongee | .. | 25 16 | 610·3?
Darjeeling | .. | 27 3 | 127·3
Madras | 22 | 13 4 | 44·6
Mahabuleshwur | 15 | 17 56 | 254·0
Malabar, Tellicherry | .. | 11 44 | 116·0
Palamcotta | 5 | 8 30 | 21·1
Patna | .. | 25 40 | 36·7
Poonah | 4 | 18 30 | 23·4
MALAY--Pulo Penang | .. | 5 25 | 100·5
Singapore | .. | 1 17 | 190·0
PERSIA--Lencoran | 3 | 38 44 | 42·8
Ooroomiah | 1 | 37 28 | 21·5
RUSSIA--Barnaoul | 15 | 53 20 | 11·8
Nertchinsk | 12 | 51 18 | 17·5
Okhotsk | 2 | 59 13 | 35·2
Tiflis | 6 | 41 42 | 19·3
Tobolsk | 2 | 58 12 | 23·0
TURKEY--Palestine, Jerusalem | { 14 | 31 47 | 65·0?
| { 3 | 31 47 | 16·3
Smyrna | .. | 38 26 | 27·6
| | |
AFRICA. | | |
ABYSSINIA--Gondar | .. | 12 36 | 37·3
ALGERIA--Algiers | 10 | 36 47 | 37·0
Constantina | .. | 36 24 | 30·8
Mostaganem | 1 | 35 50 | 22·0
Oran | 2 | 35 50 | 22·1
ASCENSION | 2 | 8 8S | 11·5
CAPE COLONY--Cape Town | 20 | 33 52 | 24·3
GUINEA--Christiansborg | .. | 5 30N | 19·2
MADEIRA | 4 | 33 30 | 30·9
MAURITIUS--Port Louis | .. | 20 3S | 35·2
NATAL--Maritzburgh | .. | 29 36 | 27·6
ST. HELENA | 3 | 15 55N | 18·8
SIERRA LEONE | .. | 8 30 | 86·0
TENERIFFE | 2 | 28 28 | 22·3
| | |
NORTH AMERICA. | | |
BRITISH COLUMBIA-- | | |
New Westminster | 3 | 49 12 | 54·1
CANADA--Montreal, | | |
St. Martin's | 2 | 45 31 | 47·3
Toronto | 16 | 43 39 | 31·4
HONDURAS--Belize | 1 | 17 29 | 153·0
MEXICO--Vera Cruz | .. | 19 12 | 66·1
RUSSIAN AMERICA--Sitka | 7 | 57 3 | 89·9
UNITED STATES--Arkansas, | | |
Fort Smith | 15 | 35 23 | 42·1
California, San Francisco | 9 | 37 48 | 23·4
Nebraska, Fort Kearny | 6 | 40 38 | 28·8
New Mexico, Socorro | 2 | 34 10 | 7·9
New York, West Point | 12 | 41 23 | 46·5
Ohio, Cincinnati | 20 | 39 6 | 46·9
Pennsylvania, Philadelphia | 19 | 39 57 | 43·6
South Carolina, Charlestown| 15 | 32 46 | 48·3
Texas, Matamoras | 6 | 25 54 | 35·2
WEST INDIES--Antigua | .. | 17 3 | 39·5
Barbadoes | 10 | 13 12 | 75·0
" St. Philip | 20 | 13 13 | 56·1
Cuba, Havannah | 2 | 23 9 | 50·2
Matanzas | 1 | 23 2 | 55·3
Grenada | .. | 12 8 | 126·0
Guadaloupe, Basseterre | .. | 16 5 | 126·9
" Matonba | .. | 16 5 | 285·8
Jamaica, Caraib | .. | 18 3 | 97·0
" Kingstown | .. | 17 58 | 83·0
St. Domingo, Cape Haitien | .. | 19 43 | 127·9
" Tivoli | .. | 19 0 | 106·7
Trinidad | .. | 10 40 | 62·9
Virgin Isles, St. Thomas' | .. | 18 17 | 60·6
" Tortola | .. | 18 27 | 65·1
| | |
SOUTH AMERICA. | | |
BRAZIL--Rio Janeiro | .. | 22 54S | 58·7
S. Luis de Maranhao | .. | 3 0 | 276·0
GUYANA--Cayenne | 6 | 4 56 | 138·3
Demerara, George Town | 5 | 6 50 | 87·9
Paramaribo | .. | 6 0 | 229·2
NEW GRANADA--La Baja | 6 | 7 22 | 54·1
Marmato | 15 | 5 29 | 90·0
Santa Fé de Bogota | 6 | 4 36 | 43·8
VENEZUELA--Cumana | .. | 10 27 | 7·5
Curaçoa | .. | 12 15N | 26·6
| | |
AUSTRALIA. | | |
NEW SOUTH WALES--Bathurst | 3 | 33 24S | 22·7
Deniliquin | 2 | 35 32 | 13·8
Newcastle | 3 | 32 57 | 55·3
Port Macquarie | 12 | 31 29 | 70·8
Sydney | 6 | 33 52 | 46·2
NEW ZEALAND--Auckland | 2 | 36 50 | 31·2
Christchurch | 3 | 43 45 | 31·7
Nelson | 2 | 41 18 | 38·4
Taranaki | 2 | 39 3 | 52·7
Wellington | 2 | 41 17 | 37·8
SOUTH AUSTRALIA--Adelaide | 6 | 34 55 | 19·2
TASMANIA--Hobart Town | 12 | 42 54 | 20·3
VICTORIA--Melbourne | 6 | 37 49 | 30·9
Port Phillip | 11 | 38 30 | 29·2
WEST AUSTRALIA--Albany | .. | 35 0 | 32·1
York | 1 | 31 55 | 25·4
| | |
POLYNESIA. | | |
SOCIETY ISLANDS-- | | |
Tahiti, Papiete | 5 | 17 32 | 45·7
-----------------------------+--------------+----------+--------
DISTURBANCES OF THE STRATA.
The last question to be considered relates to the disturbances which may
have affected the strata; for whatever may be the absorbent power of the
strata, the yield of water will be more or less diminished whenever the
channels of communication have suffered break or fracture.
If the strata remained continuous and unbroken, we should merely have to
ascertain the dimensions and lithological character of the strata in
order to determine their water value. But if the strata is broken, the
interference with the subterranean transmission of water will be
proportionate to the extent of the disturbance.
Although the Tertiary formations around London have probably suffered
less from the action of disturbing forces than the strata of any other
district of the same extent in England, yet they nevertheless now
exhibit considerable alterations from their original position.
The principal change has been that which, by elevation of the sides or
depression of the centre of the district, gave the Tertiary deposits
their present trough-shaped form, assuming it not to be the result of
original deposition. If no further change had taken place we might have
expected to find an uninterrupted communication in the Lower Tertiary
strata from their northern outcrop at Hertford to their southern outcrop
at Croydon, as well as from Newbury on the west to the sea on the east;
and the entire length of 260 miles of outcrop would have contributed to
the general supply of water at the centre.
But this is far from being the case; several disturbing causes have
deranged the regularity of original structure. The principal one has
caused a low axis of elevation, or rather a line of flexure running east
and west, following nearly the course of the Thames from the Nore to
Deptford, and apparently continued thence beyond Windsor. It brings up
the chalk at Cliff, Purfleet, Woolwich, and Loampit Hill to varied but
moderate elevations above the river level. Between Lewisham and Deptford
the chalk disappears below the Tertiary series, and does not come to the
surface till we reach the neighbourhood of Windsor and Maidenhead.
There is also, probably, another line of disturbance running between
some points north and south and intersecting the first line at Deptford.
It passes apparently near Beckenham and Lewisham, and then, crossing the
Thames near Deptford, continues up a part, if not along the whole length
of the valley of the Lea towards Hoddesdon. This disturbance appears in
some places to have resulted in a fracture or a fault in the strata,
placing the beds on the east of it on a higher level than those on the
west; and at other places merely to have produced a curvature in the
strata. Prestwich states that he was unable to give its exact course,
but its effect, at all events upon the water supply of London, is
important, as, in conjunction with the first or Thames valley
disturbance, it cuts off the supplies from the whole of Kent, and
interferes most materially with the supply from Essex; for in its course
up the valley of the Lea it either brings up the Lower Tertiary strata
to the surface, as at Stratford and Bow, or else, as farther up the
valley, it raises them to within 40 or 60 feet of the surface.
The Tertiary district thus appears, on a general view, to be divided
naturally into four portions by lines running nearly north and south,
the former line passing immediately south, and the latter east of
London, which stands at the south-east corner of the north-western
division, and consequently it must not be viewed as the centre of one
large and unbroken area, so far as the Tertiary strata are concerned.
CHAPTER II.
THE NEW RED SANDSTONE.
This formation has been already alluded to at pp. 5 and 8; it is, next
to the chalk and lower greensand, the most extensive source of water
supply from wells we have in England, and although the two formations
mentioned occupy a larger area, yet, owing to geographical position, the
new red sandstone receives a more considerable quantity of rainfall,
and, owing to the comparative scarceness of carbonate of lime, yields
softer water.
The new red sandstone is called on the Continent "the Trias," as in
Germany and parts of France it presents a distinct threefold division.
Although the names of each of the divisions are commonly used, they are
in themselves local and unessential, as the same exact relations between
them do not occur in other remote parts of Europe or in England, and are
not to be looked for in distant continents. The names of the divisions
and their English equivalents are:
1. Keuper, or red marls.
2. Muschelkalk, or shell limestones (not found in this country).
3. Bunter sandstone, or variegated sandstone.
The strata consist in general of red, mottled, purple, or yellowish
sandstones and marls, with beds of rock-salt, gypsum pebbles, and
conglomerate.
The region over which triassic rocks outcrop in England stretches across
the island from a point in the south-western part of the English Channel
about Exmouth, Devon, north-north-eastward, and also from the centre of
this band along a north-westward course to Liverpool, thence dividing
and running north-east to the Tees, and north-west to Solway Firth.
In central Europe the trias is found largely developed, and in North
America it covers an area whose aggregate length is some 700 or 800
miles.
The beds, in England, may be divided as follows;
Average Thickness.
KEUPER--Red marls, with rock-salt and gypsum 1000 ft.
Lower Keuper sandstones, with trias
sandstones and marls (waterstones) 250 ft.
Dolomitic conglomerate
BUNTER--Upper red and mottled sandstone 300 ft.
Pebble beds, or uncompacted conglomerate 300 ft.
Lower red and mottled sandstone 250 ft.
The Keuper series is introduced by a conglomerate often calcareous,
passing up into brown, yellow, or white freestone, and then into thinly
laminated sandstones and marls. The other subdivisions are remarkably
uniform in character, except in the case of the pebble beds, which in
the north-west form a light red pebbly building stone, but in the
central counties becomes generally an unconsolidated conglomerate of
quartzose pebbles.
The following tabulated form, due to Edward Hull, Esq., M.A., shows the
comparative thickness and range of the Triassic series along a
south-easterly direction from the estuary of the Mersey, and also shows
the thinning away of all the Triassic strata from the north-west towards
the south-east of England, which Hull was amongst the first to
demonstrate.
THICKNESS AND RANGE OF THE TRIAS IN A S.E. DIRECTION FROM THE MERSEY.
------------------+----------------+---------------+---------------
| Lancashire | | Leicestershire
Names of Strata. | and | Staffordshire.| and
| West Cheshire. | | Warwickshire.
------------------+----------------+---------------+---------------
KEUPER SERIES-- | | |
Red marl | 3,000 | 800 | 700
Lower Keuper | | |
sandstone | 450 | 200 | 150
| | |
BUNTER SERIES-- | | |
Upper mottled | | |
sandstone | 500 | 50 to 200 | absent
Pebble beds | 500 to 750 | 100 to 300 | 0 to 100
Lower mottled | | |
sandstone | 200 to 500 | 0 to 100 | absent
------------------+----------------+---------------+---------------
The formation may be looked upon as almost equally permeable in all
directions, and the whole mass may be regarded as a reservoir up to a
certain level, from which, whenever wells are sunk, water will always be
obtained more or less abundantly, This view is very fairly borne out by
experience, and the occurrence of the water is certainly not solely due
to the presence of the fissures or joints traversing the rock, but to
its permeability, which, however, varies in different districts. In the
neighbourhood of Liverpool the rock, or at least the pebble bed, is less
porous than in the neighbourhood of Whitmore, Nottingham, and other
parts of the midland counties, where it becomes either an unconsolidated
conglomerate or a soft crumbly sandstone. Yet wells sunk even in the
hard building stone of the pebble beds, either in Cheshire or
Lancashire, always yield water at a certain variable depth. Beyond a
certain depth the water tends to decrease, as was the case in the St.
Helen's public well, situated on Eccleston Hill. At this well an attempt
was made, in 1868, to increase the supply by boring deeper into the
sandstone, but without any good result. When water percolates downwards
in the rock we may suppose there are two forces of an antagonistic
character brought into play; there is the force of friction, increasing
with the depth, and tending to hinder the downward progress of the
water, while there is the hydrostatic pressure tending to force the
water downwards; and we may suppose that when equilibrium has been
established between these two forces, the further percolation will
cease.
The proportion of rain which finds it way into the rock in some parts of
the country must be very large. When the rock, as is generally the case
in Lancashire, Cheshire, and Shropshire, is partly overspread by a
coating of dense boulder clay, almost impervious to water, the quantity
probably does not exceed one-third of the rainfall over a considerable
area; but in some parts of the midland counties, where the rock is very
open, and the covering of drift scanty or altogether absent, the
percolation amounts to a much larger proportion, probably one-half or
two-thirds, as all the rain which is not evaporated passes downwards.
The new red sandstone, as remarked, may be regarded, in respect to
water supply, as a nearly homogeneous mass, equally available
throughout; and it is owing to this structure, and the almost entire
absence of beds of impervious clay or marl, that the formation is
capable of affording such large supplies of water; for the rain which
falls on its surface and penetrates into the rock is free to pass in any
direction towards a well when sunk in a central position. If we consider
the rock as a mass completely saturated with water through a certain
vertical depth, the water being in a state of equilibrium, when a well
is sunk, and the water pumped up, the state of equilibrium is destroyed,
and the water in the rock is forced in from all sides. The percolation
is, doubtless, much facilitated by joints, fissures, and faults, and in
cases where one side of a fault is composed of impervious strata, such
as the Keuper marls, or coal measures, the quantity of water pent up
against the face of the fault may be very large, and the position often
favourable for a well. An instance of the effect of faults in the rock
itself, in increasing the supply, is afforded in the case of the well at
Flaybrick Hill, near Birkenhead. From the bottom of this well a heading
was driven at a depth of about 160 feet from the surface, to cut a fault
about 150 feet distant, and upon this having been effected the water
flowed in with such impetuosity that the supply, which had been 400,000
gallons a day, was at once doubled.
The water from the new red sandstone is clear, wholesome, and pleasant
to drink; it is also well adapted for the purposes of bleaching, dyeing,
and brewing; at the same time it must be admitted that its qualities as
regards hardness, in other words, the proportions of carbonates of limes
and magnesia it contains, are subject to considerable variation,
depending on the locality and composition of the rock. As a general
rule, the water from the new red sandstone may be considered as
occupying a position intermediate between the _hard_ water of the chalk,
and the _soft_ water supplied to some of our large towns from the
drainage of mountainous tracts of the primary formations, of which the
water supplied from Loch Katrine to Glasgow is perhaps the purest
example, containing only 2·35 grains of solid matter to the gallon.
Having besides but a small proportion of saline ingredients, which,
while they tend to harden the water, are probably not without benefit in
the animal economy, the water supply from the new red sandstone
possesses incalculable advantages over that from rivers and surface
drainage. Many of our large towns are now partially or entirely supplied
with water pumped from deep wells in this sandstone; and several from
copious springs gushing forth from the rock at its junction with some
underlying impervious stratum belonging to the primary series.
CHAPTER III.
WELL SINKING.
Previous to sinking it will be necessary to have in readiness a stock of
buckets, shovels, picks, rope, a pulley-block or a windlass, and barrows
or other means of conveying the material extracted away from the mouth
of the sinking. After all the preliminary arrangements have been made,
the sinking is commenced by marking off a circle upon the ground 12 or
18 inches greater in circumference than the intended internal diameter
of the well. The centre of the well as commenced from must be the centre
of every part of the sinking; its position must be carefully preserved,
and everything that is done must be true to this centre, the plumb-line
being frequently used to test the vertical position of the sides.
[Illustration: Drum Curb Plan.
Fig. 17.]
[Illustration: Drum Curb Section.
Fig. 18.]
To sink a well by underpinning, an excavation is first made to such a
depth as the strata will allow without falling in. At the bottom of the
excavation is laid a curb, that is, a flat ring, whose internal diameter
is equal to the intended clear diameter of the well, and its breadth
equal to the thickness of the brickwork. It is made of oak or elm planks
3 or 4 inches thick, either in one layer fished at the joints with iron,
or in two layers breaking joint, and spiked or screwed together. On
this, to line the first division of the well, a cylinder of brickwork,
technically called steining, is built in mortar or cement. In the centre
of the floor is dug a small pit, at the bottom of which is laid a small
platform of boards; then, by cutting notches in the side of the pit,
raking props are inserted, their lower ends abutting against a foot
block, and their upper ends against the lowest setting, so as to give
temporary support to the curb with its load of brickwork. The pit is
enlarged to the diameter of the shaft above; on the bottom of the
excavation is laid a new curb, on which is built a new division of the
brickwork, giving permanent support to the upper curb; the raking props
and their foot-blocks are removed; a new pit is dug, and so on as
before. Care should be taken that the earth is firmly packed behind the
steining.
A common modification of this method consists in excavating to such a
depth as the strata will admit without falling in. A wooden curb is laid
at the bottom of the excavation, the brick steining laid upon it and
carried to the surface. The earth is then excavated flush with the
interior sides of the well, so that the earth underneath the curb
supports the brickwork above. When the excavation has been carried on as
far as convenient, recesses are made in the earth under the previous
steining, and in these recesses the steining is carried up to the
previous work. When thus supported the intermediate portions of earth
between the sections of brickwork carried up are cut away and the
steining completed.
In sinking with a drum curb, the curb, which may be either of wood or
iron, consists of a flat ring for supporting the steining, and of a
vertical hollow cylinder or drum of the same outside diameter as the
steining, supporting the ring within it and bevelled to a sharp edge
below. The rings, or ribs, of a wooden curb are formed of two
thicknesses of elm plank, 1-1/2 inch thick by 9 inches wide, giving a
total thickness of 3 inches.
[Illustration: Iron Drum Curb Plan.
Fig. 19.]
[Illustration: Iron Curb Enlarged Segment.
Fig. 20.]
Fig. 17 is a plan of a wooden drum curb, and Fig. 18 a section showing
the mode of construction. The outside cylinder or drum is termed the
lagging, and is commonly made from 1-1/2-inch yellow pine planks. The
drum may be strengthened if necessary by additional rings, and its
connections with the rings made more secure by brackets. In large curbs
the rings are placed about 3 feet 6 inches apart. Fig. 19 is a plan, and
Fig. 20 an enlarged segment of an iron curb. When the well has been sunk
as far as the earth will stand vertical, the drum curb is lowered into
it and the building of the brick cylinder commenced, care being taken to
complete each course of bricks before laying another, in order that the
curb may be loaded equally all round. The earth is dug away from the
interior of the drum, and this, together with the gradually increasing
load, causes the sharp lower edge of the drum to sink into the earth;
and thus the digging of the well at the bottom, the sinking of the drum
curb and the brick lining which it carries, and the building of the
steining at the top, go on together. Care must be taken in this, as in
every other method, to regulate the digging so that the well shall sink
vertically. Should the friction of the earth against the outside of the
well at length become so great as to stop its descent before the
requisite depth is attained, a smaller well may be sunk in the interior
of the first well. A well so stopped is said to be earth-fast. This plan
cannot be applied to deep wells, but is very successful in sandy soils
where the well is of moderate depth.
The curbs are often supported by iron rods, fitted with screws and nuts,
from cross timbers over the mouth of the well, and as the excavation is
carried on below, brickwork is piled on above, and the weight of the
steining will carry it down as the excavation proceeds, until the
friction of the sides overpowers the gravitating force or weight of the
steining, when it becomes earth-bound; then a set-off must be made in
the well, and the same operation repeated as often as the steining
becomes earth-bound, or the work must be completed by the first method
of underpinning.
When the rock to be sunk through is unstratified, or if stratified, when
of great thickness, recourse must be had to the action of explosive
agents. The explosives most frequently used for this purpose are
guncotton, dynamite, lithofracteur, and gunpowder. Lithofracteur is now
often employed, and always with considerable success, as its power is
similar to that of dynamite, but, what is particularly important in
vertical bore-holes, its action is intensely local; it is, moreover,
safe, does not generate fumes more harmful than ordinary gunpowder,
requires smaller holes, and but little tamping. The dangerous character
of guncotton has hitherto prevented its adoption for ordinary
operations, while the comparatively safe character and convenient form
of gunpowder have commended it to the confidence of workmen, and hence
for sinking operations this explosive is generally employed. We shall
therefore, in treating of blasting for well sinking, consider these
operations as carried out by the aid of gunpowder alone.
The system of blasting employed in well sinking is that known as the
small-shot system, which consists in boring holes from 1 to 3 inches
diameter in the rock to be disrupted to receive the charge. The position
of these holes is a matter of the highest importance from the point of
view of producing the greatest effects with the available means, and to
determine them properly requires a complete knowledge of the nature of
the forces developed by an explosive agent. This knowledge is rarely
possessed by sinkers. Indeed, such is the ignorance of this subject
displayed by quarrymen generally, that when the proportioning and
placing the charges are left to their judgment, a large expenditure of
labour and material will produce very inadequate results. In all cases
it is far more economical to entrust these duties to one who thoroughly
understands the subject. The following principles should govern all
operations of this nature.
The explosion of gunpowder, by the expansion of the gases suddenly
evolved, develops an enormous force, and this force, due to the pressure
of a fluid, is exerted equally in all directions. Consequently, the
surrounding mass subjected to this force will yield, if it yield at all,
in its weakest part, that is, in the part which offers least resistance.
The line along which the mass yields, or line of rupture, is called the
line of least resistance, and is the distance traversed by the gases
before reaching the surface. When the surrounding mass is uniformly
resisting, the line of least resistance will be a straight line, and
will be the shortest distance from the centre of the charge to the
surface. Such, however, is rarely the case, and the line of rupture will
therefore in most instances be an irregular line, and often much longer
than that from the centre direct to the surface. Hence in all blasting
operations there will be two things to determine, the line of least
resistance and the quantity of powder requisite to overcome the
resistance along that line. For it is obvious that all excess of powder
is waste; and, moreover, as the force developed by this excess must be
expended upon something, it will probably be employed in doing mischief.
Charges of powder of uniform strength produce effects varying with their
weight, that is, a double charge will move a double mass. And as
homogeneous masses vary as the cube of any similar line within them, the
general rule is established that charges of powder to produce similar
results are to each other as the cubes of the lines of least resistance.
Hence when the charge requisite to produce a given effect in a
particular substance has been determined by experiment, that necessary
to produce a like effect in a given mass of the same substance may be
readily determined. As the substances to be acted upon are various and
differ in tenacity in different localities, and as, moreover, the
quality of powder varies greatly, it will be necessary, in undertaking
sinking operations, to make experiments in order to determine the
constant which should be employed in calculating the charges of powder.
In practice, the line of least resistance is taken as the shortest
distance from the centre of the charge to the surface of the rock,
unless the existence of natural divisions shows it to lie in some other
direction; and, generally, the charge requisite to overcome the
resistance will vary from 1/15 to 1/35 of the cube of the line, the
latter being taken in feet and the former in pounds. Thus, suppose the
material to be blasted is chalk, and the line of least resistance 4
feet, the cube of 4 is 64, and taking the proportion for chalk as 1/30,
we have 64/30 = 2-2/15 lb. as the charge necessary to produce
disruption.
[Illustration: Jumper.
Fig. 21.]
When the blasting is in stratified rock, the position of the charge will
frequently be determined by the natural divisions and fissures; for if
these are not duly taken into consideration, the sinker will have the
mortification of finding, after his shot has been fired, that the
elastic gases have found an easier vent through one of these flaws, and
that consequently no useful effect has been produced. The line of least
resistance, in this case, will generally be perpendicular to the beds of
the strata, so that the hole for the charge may be driven parallel to
the strata and in such a position as not to touch the planes which
separate them. This hole should never be driven in the direction of the
line of least resistance, and when practicable should be at right-angles
to it.
The instruments employed in boring the holes for the shot are iron rods
having a wedge-shaped piece of steel welded to their lower ends and
brought to an edge so as to cut into the rock. These are worked either
by striking them on the head with a hammer, or by jumping them up and
down and allowing them to penetrate by their own weight. When used in
the former manner they are called borers or drills; in the latter case
they are of the form Fig. 21, and are termed jumpers. Recently power
jumpers worked by compressed air, and drills actuated in the same manner
have been very successfully employed. Holes may be made by these
instruments in almost any direction; but when hand labour only is
available, the vertical can be most advantageously worked. Hand-jumpers
are usually about 4 feet 8 inches in length, and are used by holding in
the direction of the required hole, and producing a series of sharp
blows through lifting the tool about a foot high and dropping it with an
impulsive movement. The bead divides a jumper into two unequal lengths,
of which the shorter is used for commencing a bore-hole, and the longer
for finishing it. Often the bit on the long length is made a trifle
smaller than the other to remove any chance of its not following into
the hole which has been commenced.
Drills and jumpers should be made of the best iron, preferably Swedish,
for if the material be of an inferior quality it will split and turn
over under the repeated blows of the mall, and thus endanger the hands
of the workman who turns it, or give off splinters that may cause
serious injury to those engaged in the shaft. Frequently they are made
entirely of steel, and this material has much to recommend it for this
purpose; the length of drills varies from 18 inches to 4 feet, the
different lengths being put in successively as the sinking of the hole
progresses. The cutting edge of the drills should be well steeled, and
for the first, or 18-inch drill, have generally a breadth of 2 inches;
the second, or 28-inch drill, may be 1-3/4 inch on the edge; the third,
or 3-foot drill, 1-1/2 inch, and the fourth, or 4-foot drill, 1-1/4
inch.
[Illustration: Funnel and Pipe for Shot-hole Acid Pouring.
Fig. 22.]
The mode of using the drill in the latter case is as follows; The place
for the hole having been marked off with the pick, one man sits down
holding the drill in both hands between his legs. Another man then
strikes the drill with a mall, the former turning the drill partially
round between each blow to prevent the cutting edge from falling twice
in the same place.
The speed with which holes may be sunk varies of course with the
hardness of the rock and the diameter of the hole. At Holyhead the
average work done by three men in hard quartz rock with 1-1/2-inch
drills was 14 inches an hour; one man holding the drill, and two
striking. In granite of good quality, it has been ascertained by
experience that three men are able to sink with a 3-inch jumper 4 feet
in a day; with a 2-1/2-inch jumper, 5 feet; with a 2-1/4-inch, 6 feet;
with a 2-inch, 8 feet; and with a 1-3/4-inch, 12 feet. A strong man with
a 1-inch jumper will bore 8 feet in a day. The weight of the hammers
used with drills is a matter deserving attention; for if too heavy they
fatigue the men, and consequently fewer blows are given and the effect
produced lessened; while, on the other hand, if too light, the strength
of the workman is not fully employed. The usual weight is from 5 to 7
lb.
As the labour of boring a shot-hole in a given kind of rock is dependent
on the diameter, it is obviously desirable to make the hole as small as
possible, due regard being had to the size of the charge; for it must be
borne in mind in determining the diameter of the boring that the charge
should not occupy a great length in it. Various expedients have been
resorted to for the purpose of enlarging the hole at the bottom so as to
form a chamber for the powder. If this could be easily effected, such a
mode of placing the charge would be highly advantageous, as a very small
bore-hole would be sufficient, and the difficulties of tamping much
lessened. One of these expedients is to place a small charge at the
bottom of the bore and to fire it after being properly tamped. The
charge being insufficient to cause fracture, the parts in immediate
contact with it are compressed and crushed to dust, and the cavity is
thereby enlarged. The proper charge may then be inserted in the chamber
thus formed by boring through the tamping. Another method, applicable
chiefly to calcareous rock, has been tried with satisfactory results at
Marseilles. When the bore-hole has been sunk to the required depth, a
copper pipe, Fig. 22, of a diameter to fit the bore loosely, is
introduced, the end A reaching to the bottom of the hole, which is
closed up tight at B with clay so that no air may escape. The pipe is
provided with a bent neck C. A small leaden pipe _e_, about half an inch
in diameter, with a funnel _f_ at the top, is introduced into the copper
pipe at D and passed down to within about an inch of the bottom. The
annular space between the leaden and copper pipes at _g_ is filled with
a packing of hemp. Dilute nitric acid is then poured through the funnel
and leaden pipe. The acid dissolves the calcareous rock at the bottom,
causing an effervescence, and a substance containing the dissolved lime
is forced out of the orifice C. This process is continued until from the
quantity of acid consumed it is judged that the chamber is sufficiently
enlarged. Other acids, such as muriatic or sulphuric, will produce the
same effects, but the result of the chemical solution will of course
depend upon the nature of the stone.
After the shot-hole has been bored, it is cleaned out and dried with a
wisp of hay, and the powder poured down; or, when the hole is not
vertical, pushed in with a wooden rammer. The quantity of powder should
always be determined by weight. One pound, when loosely poured out, will
occupy about 30 cubic inches, and 1 cubic foot weighs 57 pounds. A hole
1 inch in diameter will therefore contain ·414 ounce for every inch of
depth. Hence to find the weight of powder to an inch of depth in any
given hole, we have only to multiply ·414 ounce by the square of the
diameter of the hole in inches, and we are enabled to determine either
the length of hole for a given charge, or the charge in a given space.
It is important to use strong powder in blasting operations, because, as
a smaller quantity will be sufficient, it will occupy less space, and
thereby save labour in boring.
[Illustration: Clay Iron.
Figs. 24, 25.]
[Illustration: Pricker.
Fig. 23.]
When the hole is in wet stone, means must be provided for keeping the
powder dry. For this purpose, tin cartridges are sometimes used. These
are tin cylinders of suitable dimensions, fitted with a small tin stem
through which the powder is ignited. The effect of the powder is,
however, much lessened by the use of these tin cases. Generally a paper
cartridge, well greased to prevent the water from penetrating, will give
far more satisfactory results. When the paper shot is used, the hole
should, previous to the insertion of the charge, be partially filled
with stiff clay, and a round iron bar, called a clay-iron or bull, Figs.
24, 25, driven down to force the clay into the interstices of the rock
through which the water enters. By this means the hole will be kept
comparatively dry. The bull is withdrawn by placing a bar through the
eye near the top of the former, provided for that purpose, and lifting
it straight out. The cartridge is placed upon the point of a pricker and
pushed down the hole. The pricker, shown in Fig. 23, is a taper piece of
metal, usually of copper to prevent accidents, pointed at one end and
having a ring at the other. When the cartridge has been placed in its
position by this means, a little oakum is laid over it, and a Bickford
fuse inserted. This fuse is inexpensive, very certain in its effects,
not easily injured by tamping, and is unaffected by moisture. The No. 8
fuse is preferred for wet ground; and when it is required to fire the
charge from the bottom in deep holes, No. 18 is the most suitable.
When the line of least resistance has been decided upon, care must be
taken that it remains the line of least resistance; for if the space in
bore-hole is not properly filled, the elastic gases may find an easier
vent in that direction than in any other. The materials employed to fill
this space are, when so applied, called tamping, and they consist of the
chips and dust from the sinking, sand, well-dried clay, or broken brick
or stones. Various opinions are held concerning the relative value of
these materials as tamping. Sand offers very great resistance from the
friction of the particles amongst themselves and against the sides of
the bore-hole; it may be easily applied by pouring it in, and is always
readily obtainable. Clay, if thoroughly baked, offers a somewhat greater
resistance than sand, and, where readily procurable, may be
advantageously employed. Broken stone is much inferior to either of
these substances in resisting power. The favour in which it is held by
sinkers and quarrymen, and the frequent use they make of it as tamping,
must be attributed to the fact of its being always ready to hand, rather
than to any excellent results obtained from its use. The tamping is
forced down with a stemmer or tamping bar similar to Figs. 26, 27, too
frequently made of iron, but which should be either of copper or bronze.
The tamping end of the bar is grooved on one side, to admit of its
clearing the pricker, or the fuse, lying along the side of the hole. The
other end is left plain for the hand or for being struck with a hammer.
All tamping should be selected for its freedom from particles likely to
strike fire, but it must not be overlooked that the cause of such a
casualty may lie in the sides of the hole itself. Under these
circumstances is seen the advisability of using bronze or copper tamping
tools, and of not hammering violently on the tamping until a little of
it has been first gently pressed down to cover over the charge, because
the earlier blows on the tamping are the most dangerous in the event of
a spark occurring. A little wadding, tow, paper, or a wooden plug is
sometimes put to lie against the charge before any tamping is placed in
the hole.
[Illustration: Tamping Bar.
Figs. 26, 27.]
[Illustration: Metal Cone Plug.
Fig. 28.]
To lessen the danger of the tamping being blown out, plugs or cones of
metal of different shapes are sometimes inserted in the hole. The best
forms of plug are shown in Figs. 28 and 29; Fig. 28 is a metal cone
wedged in on the tamping with arrows, and Fig. 29 is a barrel-shaped
plug.
When all is ready, the sinkers, with the exception of one man whose duty
it is to fire the charge, are either drawn out of the shaft, or are
removed to some place of safety. This man then, having ascertained by
calling and receiving a reply that all are under shelter, applies a
light to the fuse, shouts "Bend away," or some equivalent expression,
and is rapidly drawn up the shaft.
To avoid shattering the walls of a shaft, no shot should be placed
nearer the side than 12 inches. The portion of stone next the wall sides
of the shaft left after blasting is removed by steel-tipped iron wedges
7 or 8 inches in length. These wedges are applied by making a small hole
with the point of the pick and driving them in with a mall. The sides
may be then dressed as required with the pick.
[Illustration: Barrel Shaped Plug.
Fig. 29.]
After some 30 or 40 feet have been sunk the air at the bottom of the
well may be very foul, especially in a well where blasting operations
are being carried on, or where there is any great escape of noxious
gases through fissures. Means must then be provided for applying at the
surface a small exhaust fan to which is attached lengths of tubing
extending down the well. Another good plan is to pass a 4 or 6 inch pipe
down the well, bring it up with a long bend at surface, and insert a
steam jet; a brick chimney is frequently built over the upper end of the
pipe to increase the draught, and the lower end continued down with
flexible tubing. With either fan or steam jet, the foul air being
continuously withdrawn, fresh air will rush down in its place. This is
far better than dashing lime-water down the well, using a long wooden
pipe with a revolving caphead, or pouring down a vertical pipe water
which escaped at right-angles, the old expedients for freshening the air
in a well.
A means of increasing the yield of wells, which is frequently very
successful, is to drive small tunnels or headings from the bottom of the
well into the surrounding water-bearing stratum.
[Illustration: Fig. 30.
Sectional Plan.
Water-bearing Stratum.]
As an example, let Fig. 30 represent a sectional plan of a portion of
the water-bearing stratum at the bottom of the shaft. This stratum is
underlaid by an impervious stratum, and, consequently, the water will
flow continuously through the former in the direction of the dip, as
shown by the arrow and the dotted lines. That portion of the stratum to
the rise of the shaft, S, which is included within vertical lines
tangent to the circle at the points _m_ and _n_, will be drained by the
shaft. The breadth of this portion will, however, be extended beyond
these lines by the relief to the lateral pressure afforded by the shaft,
which relief will cause the fillets of water to diverge from their
original course towards the shaft, as shown in the figure. Hence the
breadth of drainage ground will be _a b_, and it is evident that the
shaft, S, can receive only that water which descends towards it through
this space. But if tunnels be driven from the shaft along the strike of
the stratum, as at _m c_, _n d_, these tunnels will obviously intercept
the water which flows past the shaft. By this means the drainage ground
is extended from _a b_ to _a´ b´_, and the yield of the well
proportionately increased.
It should be remarked that when the strata is horizontal or depressed in
the form of a basin, that is, when it partakes more of the character of
a _reservoir_ than a _stream_, the only use of tunnels is to facilitate
the ingress of water into the shaft, and in such case they should
radiate from the shaft in all directions. They are also of service in
case of accident to the pumps, as the time they take to fill up allows
of examination and repairs being made in that time to the pumps, which
could not be got at if the engines stopped pumping and the water rose
rapidly up the shaft.
The size of the headings is usually limited by the least dimensions of
the space in which miners can work efficiently, that is about 4-1/2 feet
high and 3 feet wide. The horse-shoe form is generally adopted for the
sides and top, the floor being level, for the drawing off of the water
by the pumps is quite sufficient to cause a flow, unless of course the
dip of the stratum in which the tunnels are driven is such as to warrant
an inclination. Where there is any water it is not possible to drive
them with a fall, for the men would be drowned out.
The cost of some headings in the new red sandstone which the writer
recently inspected, varied from 30_s._ a yard in ordinary stone, to
4_l._ 10_s._ a yard in very hard stone.
The foregoing remarks do not apply to headings driven in the chalk,
where it is the usual practice to select the largest feeder issuing from
a fissure and follow that fissure up, unless the heading is merely to
serve as a reservoir, when the direction is immaterial.
The sides of wells usually require lining or steining, as it is termed,
with some material that will prevent the loose strata of the sides of
the excavation falling into the well and choking it. The materials that
have been successfully used in this work are brick, stone, timber, and
iron. Each description of material is suitable under certain conditions,
while in other positions it is objectionable. Brickwork, which is
universally used in steining wells in England, not unfrequently fails in
certain positions; through admitting impure water when such water is
under great pressure, or from the work becoming disjointed from
settlement due to the draining of a running sand-bed, or the collapse of
the well. Stone of fair quality, capable of withstanding compressive
strains, is good in its way; but, inasmuch as it requires a great deal
of labour to fit it for its place, it cannot successfully compete with
brickwork in the formation of wells, more especially as it has no merits
superior to those of brick when used in such work; however, if in any
locality, by reason of its cheapness, it can be used, care should be
taken to select only such as contains a large amount of silica; indeed,
in all cases it is a point of great importance in studying the nature of
the materials used in the construction of wells, to select those which
are likely to be the most durable, and at the same time preserve the
purity of the water contained in the well; and this is best secured by
silicious materials.
Timber is objectionable as a material to be used in the lining of wells,
on account of its liability to decay, when it not only endangers the
construction of the well, but also to some extent fouls the water. It is
very largely used under some circumstances, especially in the
preliminary operations in sinking most wells. It is also successfully
used in lining the shafts of the salt wells of Cheshire, and will
continue entire in such a position for a great number of years, as the
brine seems to have a tendency to preserve the timber and prevent its
decay. Iron is of modern application, and is a material extensively
employed in steining wells; and, as it possesses many advantages over
materials ordinarily used, its use is likely to be much extended. It is
capable of bearing great compressive strains, and of effectually
excluding the influx of all such waters as it may be desirable to keep
out, and is not liable to decay under ordinary circumstances. Baldwin
Latham mentions instances in his practice where recourse has been had to
the use of iron cylinders, when it was found that four or five rings of
brickwork, set in the best cement, failed to keep out brackish waters;
and, if the original design had provided for the introduction of these
cylinders, it would have reduced the cost of the well very materially.
[Illustration: _Plan at Top_
Fig. 31.]
[Illustration: Elevation.
Fig. 32.]
[Illustration: _Plan at Bottom_
Fig. 33.]
The well-sinker has often, in executing his work, to contend with the
presence of large volumes of water, which, under ordinary circumstances,
must be got rid of by pumping; but by the introduction of iron
cylinders, which can be sunk under water, the consequent expense of
pumping is saved.
When sinking these cylinders through water-bearing strata, various tools
are used to remove the soil from beneath them. The principal is the
mizer, which consists of an iron cylinder with an opening on the side
and a cutting lip, and which is attached to a set of boring rods and
turned from above.
The valve in the old form of mizer is subject to various accidents which
interfere with the action of the tool; for instance, pieces of hard soil
or rock often lodge between the valve and its seat, allowing the
contents to run out whilst it is being raised through water. To remedy
this defect the eminent well-sinker, Thomas Docwra, designed and
introduced the improved mizer, shown of the usual dimensions in Figs. 31
to 36; Fig. 31 being a plan at top, Fig. 32 an elevation, Fig. 33 a plan
at bottom, Fig. 35 a section, Fig. 34 a plan of the stop _a_, and Fig.
36 a plan of the valve. It consists of an iron cylinder, conical shaped
at bottom, furnished with holes for the escape of water, and attached to
a central shank by means of stays. The shank extends some 7 inches
beyond the bottom, and ends in a point, while the upper part of the
shank has an open slot, to form a box-joint, Figs. 37 to 39, with the
rods. The conical bottom of the mizer has a triangular-shaped opening;
on the outside of this is fitted a strong iron cutter, and on the inside
a properly-shaped valve, seen in section and plan in Figs. 35 and 36.
When the mizer is attached to and turned by means of the boring rods,
the _débris_, sand, or other soil to be removed, being turned up by the
lip of the cutter, enters the cylinder, the valve, whilst the mizer is
filling, resting against a stop. After the mizer is charged, which can
be ascertained by placing a mark upon the last rod at surface and noting
its progress downwards, the rods are reversed and turned once or twice
in a backward direction; this forces the valve over the opening and
retains the soil safely in the tool.
[Illustration: Plan of the Stop.
Fig. 34.]
[Illustration: A Section.
Fig. 35.]
[Illustration: Plan of the Valve.
Fig. 36.]
Fig. 40 is a pot mizer occasionally used in such soils as clay mixed
with pebbles; there is no valve, as the soil is forced upwards by the
worm on the outside, and falls over the edge into the cone.
[Illustration: Box Joint.
Figs. 37-39.]
[Illustration: Pot Mizer.
Fig. 40.]
Mizers are fastened to the rods by means of the box-joint, shown in
Figs. 37 to 39, as a screw-joint would come apart on reversing.
As many as five or six different sized mizers, ranging from 1 foot 6
inches to 9 feet in diameter, can be used successively, the smallest
commencing the excavation, and the larger ones enlarging it until it is
of the requisite size.
[Illustration: Picker.
Figs. 41-43.]
As an accessory, a picker, shown by the three views, Figs. 41 to 43,
Fig. 42 indicating its correct position when in operation, is employed
where the strata is too irregular or compact to be effectually cleared
away by the cutter of the mizer. The picker is fixed upon the same rods
above the mizer, and is used simultaneously, being raised and lowered
with that tool.
[Illustration: Scratcher.
Figs. 44, 45.]
The cutting end of the picker is frequently replaced by a scratcher,
Figs. 44, 45. This useful tool rakes or scratches up the _débris_ thrown
by the mizer beyond its own working range, and causes it to accumulate
in the centre of the sinking, where it is again subjected to the action
of the mizer.
Brick steining is executed either in bricks laid dry or in cement, in
ordinary clay 9-inch work being used for large wells, and half-brick, or
4-1/2-inch work, for small wells.
[Illustration: Brick Steining.
Sectional Plans.
Figs. 46-48.]
Figs. 46 and 47 show the method of laying for 9-inch work, and Fig. 48
for 4-1/2 inches. The bricks are laid flat, breaking joint; and to keep
out moderate land-springs clay, puddle, or concrete is often introduced
at the back of the steining; for most purposes concrete is the best, as,
in addition to its impervious character, it adds greatly to the strength
of the steining. A ring or two of brickwork in cement is often
introduced at intervals, varying from 5 feet to 12 feet apart, to
strengthen the shaft, and facilitate the construction of the well.
Too much care cannot be bestowed upon the steining; if properly executed
it will effectually exclude all objectionable infiltration, but badly
made, it may prove a permanent source of trouble and annoyance. Half the
wells condemned on account of sewage contamination really fail because
of bad steining.
CHAPTER IV.
WELL BORING.
The first method of well boring known in Europe is that called the
Chinese, in which a chisel suspended by a rope and surrounded by a tube
of a few feet in length is worked up and down by means of a spring-pole
or lever at the surface. The twisting and untwisting of the rope
prevents the chisel from always striking in the same place; and by its
continued blows the rock is pounded and broken. The chisel is withdrawn
occasionally, and a bucket or shell-pump is lowered, having a hinged
valve at the bottom opening upwards, so that a quantity of the _débris_
becomes enclosed in the bucket, and is then drawn up by it to the
surface; the lowering of the bucket is repeated until the hole is
cleared, and the chisel is then put to work again.
Fig. 49 is of an apparatus, on the Chinese system, which may be used
either for hemp-rope or wire-rope, and which was originally made for
hoop-iron. At A, Fig. 49, is represented a log of oak wood, which is set
perpendicularly so deep in the ground as to penetrate the loose gravel
and pass a little into the rock, and stand firm in its place; it is well
rammed with gravel and the ground levelled, so that the butt of the log
is flush with the surface of the ground, or a few feet below. Through
this log, which may be, according to the depth of loose ground, from 5
feet to 30 feet long, a vertical hole is bored by an auger of a diameter
equal to that of the intended boring in the rock. On the top of the
ground, on one side of the hole, is a windlass whose drum is 5 feet in
diameter, and the cogwheel which drives it 6 feet; the pinion on the
crank axle is 6 inches. This windlass serves for hoisting the spindle or
drill, and is of a large diameter, in order to prevent short bends in
the iron, which would soon make it brittle.
[Illustration: Chinese System.
Fig. 49.]
In all cases where iron, either hoop-iron or wire-rope, is used, the
diameter of the drum of the windlass used must be sufficiently large to
prevent a permanent bend in the iron. On the opposite side of the
windlass is a lever of unequal leverage, about one-third at the side of
the hole, and two-thirds at the opposite side, where it ends in a cross
or broad end where men do the work. The workmen, with one foot on a
bench or platform, rest their hands on a railing, and work with the
other foot the long end of the lever. In this way the whole weight of
the men is made use of. The lift of the bore-bit is from 10 to 12
inches, which causes the men to work the treadle from 20 to 24 inches
high. Below the treadle, T, is a spring-pole, S, fastened under the
platform on which the men stand, the end of this spring-pole is
connected by a link to the working end of the lever, or to the rope
directly, and pulls the treadle down. When the bore-spindle is raised by
means of the treadle, the spring-pole imparts to it a sudden return, and
increases by these means the velocity of the bit, and consequently that
of the stroke downwards.
This method has been generally disused, iron or wood rods substituted in
the place of the rope, and a variety of augers and chisels instead of
the simple chisel, with appliances for clearing the bore-hole of
_débris_. Figs. 50 to 56 show examples of an ordinary set of well boring
tools. Fig. 52 is a flat chisel; Fig. 53 a [V]-chisel; and Fig. 54 a
[T]-chisel. These chisels are made from wrought-iron, and when small are
usually 18 inches long, 2-1/2 inches extreme breadth, and weigh some
4-1/2 lb.; the cutting edge being faced with the best steel. They are
used for hard rocks, and whilst in operation need carefully watching
that they may be removed and fresh tools substituted when their sides
are sufficiently worn to diminish their breadth. If this circumstance is
not attended to the size of the hole decreases, so that when a new
chisel of the proper size is introduced it will not pass down to the
bottom of the hole, and much unnecessary delay is occasioned in
enlarging it. In working with the chisel, the borer keeps the tiller, or
handles, in both hands, one hand being placed upon each handle, and
moves slowly round the bore, in order to prevent the chisel from falling
twice, successively, in the same place, and thus preserve the bore
circular. Every time a fresh chisel is lowered to the bottom it should
be worked round in the hole, to test whether it is its proper size and
shape; if this is not the case the chisel must be raised at once and
worked gradually and carefully until the hole is as it should be. The
description of strata being cut by the chisel can be ascertained with
considerable accuracy by a skilful workman from the character of the
shock transmitted to the rods.
When working in sandstone there is no adherence of the rock to the
chisel when drawn to the surface, but with clays the contrary is the
case. Should the stratum be very hard, the chisel may be worn and blunt
before cutting three quarters of an inch, it must therefore be raised to
the surface and frequently examined; however, 7 or 8 inches may be bored
without examination, should the nature of the stratum allow of such
progress being made.
[Illustration: Well-boring Tools.
Figs. 50-56.]
Ground augers, Figs. 50, 51, and 56, are similar in action to those used
for boring wood, but differ in shape and construction. The common earth
auger, Fig. 50, is 3 feet in length, having the lower two-thirds
cylindrical. The bottom is partially closed by the lips, and there is an
opening a little up one side for the admission of soft or bruised
material. Augers are only used for penetrating soft rock, clay, and
sand; and their shape is varied to suit the nature of the strata
traversed, being open and cylindrical for clays having a certain degree
of cohesion, conical, and sometimes closed, in quicksands. Augers are
sometimes made as long as 10 feet, and are then very effective if the
strata is soft enough to permit of their use. The shell is made from 3
feet to 3-1/2 feet in length, of nearly the same shape as the common
auger, sometimes closed to the bottom, Fig. 56, or with an auger nose,
Fig. 51; in either case there is a clack or valve placed inside for the
purpose of retaining borings of a soft nature or preventing them from
being washed out in a wet hole. Fig. 59 shows a wad-hook for withdrawing
stones, and Fig. 58 a worm-auger.
The Crow's Foot, Fig. 55, is used when the boring rods have broken in
the bore-hole, for the purpose of extracting that portion remaining in
the hole; it is the same length, and at the foot the same breadth as the
chisels. When the rods have broken, the part above the fracture is drawn
out of the bore-hole and the crow's foot screwed on in place of the
broken piece; when this is lowered down upon the broken rod, by careful
twisting the toe is caused to grip the broken piece with sufficient
force to allow the portion below the fracture to be drawn out of the
bore-hole. A rough expedient is to fasten a metal ring to a rope and
lower it over the broken rod, when the rod cants the ring, and thus
gives it a considerable grip; this is often very successful. Fig. 57 is
a worm used for the same purpose. A bell-box, Fig. 60, is frequently
employed for drawing broken rods; it has two palls fixed at the top of
the box, which rise and permit the end of the rod to pass when the box
is lowered, but upon raising it the palls fall and grip the rod firmly.
A spiral angular worm, similar to Fig. 57, is also applied for
withdrawing tubes.
[Illustration: Withdrawing Tools.
Figs. 57-59.]
[Illustration: Bell-box.
Fig. 60.]
Of these withdrawing tools the crow is the safest and best, as it may be
used without that intelligent supervision and care absolutely necessary
with the worms and wad-hooks, or the bell-box.
The boring rods, Figs. 61, 62, are in 3, 6, 10, 15, or 20 feet lengths,
of wrought-iron, preferably Swedish, and are made of different degrees
of strength according to the depth of the hole for which they are
required; they are generally 1 inch square in section: at one end is a
male and at the other end a female screw for the purpose of connecting
them together. The screw should not have fewer than six threads. One of
the sides of the female screw frequently splits and allows the male
screw to be drawn out, thus leaving the rods in the hole. By constant
wear, also, the screw may have its thread so worn as to become liable to
slip. Common rods being most liable to accident should be carefully
examined every time they are drawn out of the bore-hole, as an
unobserved failure may occasion much inconvenience, and even the loss of
the bore-hole. In addition to the ordinary rods there are short pieces,
varying from 6 inches to 2 feet in length, which are fixed at the top,
as required, for adjusting the rods at a convenient height.
[Illustration: Boring Rods.
Fig. 61, 62.]
[Illustration: Hand-dog.
Fig. 63.]
[Illustration: Lifting Dog.
Fig. 64, 65.]
[Illustration: Tillers.
Fig. 66.]
Fig. 63 is a hand-dog; Figs. 64 and 65, a lifting dog; Fig. 66, the
tillers or handles by which the workmen impart a rotary motion to the
tools. The tillers are clamped to the topmost boring rod at a convenient
height for working. Fig. 61, a top rod with shackle. Fig. 67, a
spring-hook. When in use this should be frequently examined and kept in
repair.
[Illustration: Spring-hook.
Fig. 67.]
Lining tubes are employed to prevent the bore-hole falling in through
the lateral swelling of clay strata, or when passing through running
sand. The tubes are usually of iron, of good quality, soft, easily bent,
and capable of sustaining an indent without fracture. Inferior tubes
occasion grave and costly accidents which are frequently irreparable, as
a single bad tube may endanger the success of an entire boring.
Wrought-iron tubes with screwed flush joints, Fig. 68, are to be
recommended, but they are supplied brazed, Fig. 69, or riveted, Fig. 70,
and can be fitted with steel driving collars and shoes. Cast-iron tubes
are constantly applied; they should have turned ends with wrought-iron
collars and countersunk screws.
[Illustration: Tubes with Screwed Joints.
Fig. 68.]
[Illustration: Tubes with Brazed or Riveted Joints.
Figs. 69, 70.]
Cold-drawn wrought-iron tubes have been used, and are very effective as
well as easily applied, but their relatively high cost occasions their
application to be limited.
[Illustration: Stud Block.
Fig. 71.]
Fig. 71 shows a stud-block, which is used for suspending tubing either
for putting it down or for drawing it up. It consists of a block made to
fit inside the end of the tube, and attached to the rods in the usual
way. In the side of the block is fixed an iron stud for slipping into a
slot, similar to a bayonet-joint, cut in the end of the tube, so that it
may be thus suspended. Figs. 72 to 74 show various forms of
spring-darts, and Fig. 75 a pipe-dog, for the same purpose. Sometimes a
conical plug, with a screw cut around the outside for tightening itself
in the upper end of the tube, is used for raising and lowering tubing.
Figs. 76 and 77 are of tube clamps, and Fig. 78 tongs for screwing up
the tubes. Fig. 79 is of an ordinary form of sinker's bucket.
[Illustration: Spring-dart.
Fig. 72.]
[Illustration: Spring-Dart.
Fig. 73.]
[Illustration: Spring-dart.
Fig. 74.]
[Illustration: Pipe-dog.
Fig. 75.]
[Illustration: Tube Clamp.
Fig. 76.]
[Illustration: Tube Clamp.
Fig. 77.]
[Illustration: Tongs.
Fig. 78.]
[Illustration: Sinker's Bucket.
Fig. 79.]
Fig. 80 is a pipe-dolly, used for driving the lining tubes; the figure
shows it in position ready for driving.
When a projection in the bore-hole obstructs the downward course of the
lining tubes, the hole can be enlarged below the pipes by means of a
rimer, Fig. 81. It consists of an iron shank, to which is bolted two
thin strips, bowed out in to the form of a drawing pen. The rimer is
screwed on to the boring rods, and forced down through the pipes; when
below the last length of pipe the rimer expands, and can then be turned
round, which has the effect of scraping the sides and enlarging that
portion of the hole subject to its operation. Fig. 82 is of an improved
form of rimer, termed a riming spring. It will be seen that this
instrument is much stronger than the ordinary rimer, in consequence of
the shank being extended through its entire length, thus rendering the
scraping action of the bows very effective, whilst the slot at the foot
of the bows permits of its introduction into, and withdrawal from, the
tubing.
[Illustration: Pipe-dolly.
Fig. 80.]
[Illustration: Rimer.
Fig. 81.]
[Illustration: Riming Spring.
Fig. 82.]
In England, for small works, the entire boring apparatus is frequently
arranged as in Fig. 83, the tool being fixed at the end of the
wrought-iron rods instead of at the end of a rope, as in the Chinese
method. Referring to Fig. 83, A is the boring tool; B the rod to which
the tool is attached; D D the levers by which the men E E give a
circular or rotating motion to the tool; F, chain for attaching the
boring apparatus to the pole G, which is fixed at H, and by its means
the man at I transmits a vertical motion to the boring tool.
[Illustration: Boring Apparatus.
English, for Small Works.
Fig. 83.]
The sheer-legs, made of sound Norway spars not less than 8 inches
diameter at the bottom, are placed over the bore-hole for the purpose of
supporting the tackle K K for drawing the rods out of or lowering them
into the hole, when it is advisable to clean out the hole or renew the
chisel. It is obvious that the more frequently it is necessary to break
the joints in drawing and lowering the rods, the more time will be
occupied in changing the chisels, or in each cleaning of the hole, and
as the depth of the hole increases the more tedious will the operation
be. It therefore becomes of much importance that the rods should be
drawn and lowered as quickly as possible, and to attain this end as long
lengths as practicable should be drawn at each lift. The length of the
lift or off-take, as it is termed, depending altogether upon the height
of the lifting tackle above the top of the bore-hole, the length of the
sheer-legs for a hole of any considerable depth should not be less than
30 to 40 feet; and they usually stand over a small pit or surface-well,
which may be sunk, where the clay or gravel is dry, to a depth of 20 or
30 feet. From the bottom of this pit the bore-hole may be commenced, and
here will be stationed the man who has charge of the bore-hole while
working the rods.
[Illustration: Boring Platform.
For Deep or Difficult Wells.
Fig. 84.]
The arrangement, Fig. 84, is intended for either deep or difficult
boring. A regular scaffolding is erected upon which a platform is built.
The boring chisel A is, as in the last instance, coupled by means of
screw-couplings to the boring rods B. At each stroke two men stationed
at E E turn the rod slightly by means of the tiller D D. A rope F, which
is attached to the boring tool, is passed a few times round the drum of
a windlass G, the end of the rope being held by a man at I. When the
handles are turned by the men at L L the man at I pulls at the rope end,
the friction between the rope and the drum of the windlass is then
sufficient to raise the rods and boring tool, but as soon as the tool
has been raised to its intended height the man at I slackens his hold
upon the rope, and as there is insufficient friction on the drum to
sustain the weight of the boring tools, they fall. By a repetition of
this operation the well is bored, and after it has been continued a
sufficient length of time the tiller is unscrewed, and a lifting dog,
attached to the rope from the windlass, is passed over the top of the
rods, and then a short top rod with a shackle is screwed on. The two men
at the windlass draw up the rods as far as the height of the
scaffolding or sheer-legs will allow, when a man at E, Fig. 84, by
passing a hand-dog or a key upon the top of the rod under the lowest
joint drawn above the top of the hole, takes the weight of the rods at
this joint, the men at L having lowered the rods for this purpose; with
another key the rods are unscrewed at this joint, the rope is lowered
again, the lifting dog put over the rod, another top rod screwed on, the
rods lifted, and the process continued until the chisel is drawn from
the hole and replaced by another, or, if necessary, replaced by some
other tool.
When a deep boring is undertaken, direct from the surface, the operation
had best be conducted with the aid of a boring sheer-frame such as is
shown in the frontispiece. This consists of a framework of timber balks,
upon which are erected four standards, 27 feet in height, and 9 inches ×
1 foot thick, 3 feet 8 inches apart at bottom, and 1 foot 2 inches at
top, as seen in the front and rear elevations. The standards are tied by
means of cross pieces, upon which shoulders are cut which fit into
mortise holes, and are fastened by means of wooden keys, the standards
being surmounted by two head pieces 5 feet long, mortised and fitted.
Upon the head pieces two independent cast-iron guide pulleys are
arranged in bearings; over these pulleys are led the ends of two ropes
coiling in opposite directions upon the barrel of a windlass moved by
spur gearing, and having a ratchet stop attached to a pair of diagonal
timbers, connected with the left-hand legs or standards of the sheers,
near the ground. These ropes are used for raising or lowering the
lengths of the boring rod.
Eight feet below the bearings of the top pulleys, a pair of horizontal
traverses is fixed across the frame, supporting smaller pulleys mounted
on a cast-iron frame, which is capable of motion between horizontal
wooden slides. Over these pulleys is led a rope from a plain windlass
fixed to the right-hand legs of the frame, to be used for raising or
lowering the shell to extract the _débris_ or rubbish from the hole.
The lever, 15 feet long, and 9 inches × 6 inches in section, is
supported by an independent timber frame. It has a cast-iron cap,
fastened by means of two iron straps, cast with lugs through which bolts
are passed, these being tightened with nuts in the ordinary manner. The
bearing-pins at a are 1-1/2 inch in diameter, and also form part of the
lower strap. Upon the cap is an iron hook, to this a chain is attached
carrying the spring-hook which bears the top shackle of the rods. The
top of the bore-hole is surrounded by a wooden tube 1 foot in diameter,
and surrounded by a hinged valve, whose action is similar to that of a
clack-valve; this has a hole in the centre for the rods to pass up and
down freely. The valve permits of the introduction and withdrawal of the
tools, and at the same time prevents anything from above falling into
the bore-hole.
[Illustration: Diminishing Tube Diameter Arrangement.
Fig. 85.]
The lever is applied by pressure upon its outer end, and as the relation
of the long to the short arm is as 4 to 1, a depression of 2 feet in the
one case produces an elevation of 6 inches in the other, the minimum
range of action, the maximum being 26 inches.
With the sheer-frame the boring tools are worked in the same manner as
in the preceding arrangements, Figs. 83, 84; but its portability,
compactness, and adaptation of means to the required end, render its use
desirable wherever it is possible to obtain it.
[Illustration: Driving Ring for wrought-iron Tubes.
Fig. 86.]
When in the progress of the work it is found that the auger does not go
down to the depth from which it was withdrawn, after trial, tubing will
generally be necessary. The hole should be enlarged from the surface,
or, if not very deep, commenced afresh from the surface with a larger
auger, and run down to nearly the same depth; the first length of tube
is then driven into the hole, and when this is effected another tube,
having similar dimensions to the first, is screwed into its upper end,
and the driving repeated, and so on until a sufficient number of pipes
have been used to reach to the bottom of the hole. If the ordinary auger
is now introduced through these tubes it will have free access to the
clay or sand, and after a few feet deeper have been bored another pipe
may be screwed on, and the whole driven farther down. In this way from
10 to 20 feet of soft stratum may be bored through. If the thickness of
the surface clay or sand is considerable the method here mentioned will
not be effective, as the friction of the pipes caused by the pressure
of the strata will be so great that perhaps not more than 80 or 100 feet
can be driven without the pipes being injured. It will then be necessary
to put down the first part of the bore-hole with a large auger, and
drive in pipes of larger diameter; the hole is continued of smaller
diameter, and lined with smaller tubes projecting beyond the large
tubes, as in Fig. 85, until the necessity for their use ceases. It will
be evident that to ensure success the tubing, whatever it is made of,
should be as truly cylindrical as possible, straight, and flush surface,
both outside and in. It will also be evident that in thus joining pieces
of tubing together, the thickness ought to have a due proportion to the
work required, and the force likely to be used in screwing or driving
them down. Wrought-iron tubes, when driven, must be worked carefully, by
means of a ring made of wrought-iron, from 1-1/2 to 2 inches in height
and 3/4 inch thick, and of the form shown in Fig. 86; or driven with a
pipe-dolly such as that in Fig. 80. The ring, or the dolly, is screwed
into the lowermost boring rod and worked at the same rate and in a
similar manner to the chisel, due regard being had to the depth at which
the driving is being done, as the weight of the boring rods will
materially affect the strength of the blow delivered. Cast-iron tubing
may be driven hard with a monkey. To withdraw broken or defective
tubing quickly, two hooks attached to ropes are lowered down from
opposite sides of the bore-hole, caught on the rim of the lowermost
tube, and power applied to haul the tubing up bodily.
Figs. 87 to 91 show good methods of forming tube or pipe joints both in
cast and wrought-iron, when not screwed.
[Illustration: Fig. 87.]
[Illustration: Fig. 88.]
[Illustration: Fig. 89.]
[Illustration: Fig. 90.]
[Illustration: Fig. 91.]
P. S. Reed, an English mining engineer, gives the following instance of
replacing defective tubing in a boring which had been pursued to the
depth of 582-1/2 feet, but which, owing to circumstances which were
difficult to determine, had become very expensive, and made slow
progress.
The 582-1/2 feet had been bored entirely by manual labour; but Reid
recommended the erection of a horse-gin, in which the power was applied
to a 40-inch drum placed upon a vertical axle, the arms of which
admitted of applying two horses, and men at pleasure, the power gained
being in the proportion of one to ten at the starting-point for the
horses.
Upon the upright drum a double-ended chain was attached, which worked
over sheer-legs erected immediately over the hole, so as to attain an
off-take for the rods of 60 feet, and so as that, in the act of raising
or lowering, there might always be one end of the chain in the bottom,
ready to be attached, and expedite the work as much as possible.
These arrangements being made, it was soon found that there was a defect
in the tubing which was inserted to the depth of 109 feet, and the
defect was so serious, in permitting the sand to descend and be again
brought up with the boring tools, as to render it very difficult to tell
in what strata they really were; this increased to such a degree as to
cause the silting up of the hole in a single night to the extent of 180
feet, and it occupied nearly a fortnight in clearing the hole out again.
On carefully examining into this defect, it appeared that the water rose
in the hole to the depth of 74 feet from the surface; and that at this
point it was about level with the high-water mark on the Tees, about two
miles distant, with which it was no doubt connected by means of
permeable beds, extending from the arenaceous strata at a depth of 100
feet.
On commencing to bore, the motion of the rods in the hole caused the
vibration of the water between a range of 40 feet at the bottom of the
tubing, and so disturbed the quiescent sand as to cause it to run down
through the faults in the lower end of the tubing.
This tubing was made of galvanized iron plates, riveted together and
soldered; at the top of the hole it was in three concentric circles,
which had been screwed and forced down successively until an obstacle
was met with at three different places. So soon as the outer circle
reached the first depth, all hope appears to have vanished, from those
who bored the earlier part of the work, of getting the tube farther; a
second tube was, therefore, inserted, which seems to have advanced as
far as the second obstacle, where it, in its turn, was abandoned; and a
third one advanced until it rested in the strata at the lower part of
the lias freestone of a blue nature, as found on the rocks at Seaton
Carew, and in the bed of the Leven, near Hutton Rudby. The diameter of
the first tubing was 3-7/8 inches external and 3-1/2 inches internal;
the second tube was 3-1/4 inches external, and 3 inches internal
diameter; and the third tube was 2-3/4 inches external and 2-1/2 inches
internal diameter.
Such being the account gathered from the workmen who superintended the
earlier part of the boring, it became necessary to decide upon the best
cause to remedy the evil. At first sight it would have appeared easy
enough to have caught the lower end of the tubes by means of a fish-head
properly contrived, and thus to have lifted them out of the hole, and
replaced them with a perfect tube, such as a gas-tube, with faucet
screw-joints; but, on attempting this, it soon became evident that
however good the tubing which might have been adopted, it would be a
work of the greatest difficulty to extract when once it was regularly
fixed and jammed into its place by the tenacious clayey strata
surrounding it; and the difficulty of extracting it, in the present
case, was even enhanced by the inferior quality and make of the tubing;
in short, that, unless by crumpling it up in such a manner as to destroy
the hole, it was impossible to extract this tubing by main force.
There was, therefore, no other choice left but to attempt cutting it
out, inch by inch; though before doing so, force was applied to the
bottom of the tubing, to the extent of upwards of 30 tons, the only
result being the loss of several pieces of steel down the hole, which
had to be brought up with a powerful magnet.
After much mature consideration and contrivance, it was determined to
order such tubing as would at the same time present as little obstacle
as possible to the clay to be passed through on the outside, as well as
surround the largest of the three tubes then in the hole, and present no
obstacle to their being withdrawn through its interior.
These tubes were made 12 feet in length, flush outside and in, the lower
portion being steeled for 6 inches from the bottom end, so as to cut its
way and follow down the space, and cover that exposed by the old tubes
when cut and drawn, as shown in Fig. 92.
In order to commence operations, and avoid too much clay going down to
the bottom of the hole, a straw-plug was firmly fixed in the lias
portion of the hole. The lower portion of the new tubes was then screwed
around the old ones by means of powerful clamps, attached to the
exterior in such a manner as to avoid injuring the surface; and when
they could be screwed no farther, the knife or cutter, Figs. 92 to 94,
was introduced inside the old tubing. Some force was needed to get this
knife down into the tubing, but the spring a giving so as to
accommodate itself to the hole, permitted its descent to the distance
required; this being effected, it was turned round so that the steel
cutter, shown at _b_, being forced against the sides of the tube, cut it
through in the course of ten minutes or a quarter of an hour's turning.
See section at _b_, _c_, Fig. 93.
[Illustration: The Action of the Knife or Spring-cutter.
Prepared for Cutting.
Fig. 92.]
[Illustration: The Action of the Knife or Cutter.
Ready for Removal after Cutting.
Fig. 93.]
[Illustration: Back View of Knife or Cutter.
Fig. 94.]
The old tubes being three-ply, three of these knives or cutters were
required to cut out the three tubes, the inner one being detached first,
and then the two exterior ones; and so soon as these latter were cut out
as far as they had been forced into the clay, the work became simplified
into following down the interior tubing by the new tubes, as shown by
the dotted lines. From _d_ at the lower end, it was found that the old
inner tube had been so damaged or torn, either by the putting in or
hammering it down, as to leave a vent or fissure for the sand to
descend, and thus spoil the whole of the work for all future success in
the boring, to say nothing of the very great cost of lifting the sand
out, and subsequent most arduous labour to put the hole right.
Boring was recommenced after about a month's labour in taking out the
old tubings, leaving the new ones firmly bedded into the lias
formation, 112 feet from the surface, and the hole was subsequently
bored to a depth of 710 feet in the new red sandstone formation,
proceeding at the rate of about 3 feet in the twelve hours, and leaving
the hole so as, if requisite, it might be widened out to 4 inches
diameter. Fig. 92 shows the action of the knife and spring-cutter when
forced down into the tubing, ready to commence cutting. It also shows
the lower end of the new tubing, enclosing the others at the
commencement of the work. The joints of the new tubes were made by means
of a half-lap screw. Fig. 94 is a back view of the knife or cutter _b_.
Fig. 93 shows the action of the spring and cutter when the requisite
length is cut through and ready for lifting; the position of the tube
being maintained perpendicular, or nearly so, by the ball or thickening
on the rods at K, and the lower end of the tube being supported by the
projecting steel cutter at _b_, the dotted lines from _d_ showing the
position of the new steel-ended tube when screwed down ready for another
operation. In boring deeper after the tubes were removed, three wooden
blocks were used round the rods in the new tube to keep them plumb.
In some cases it is necessary to widen out holes below the sharp edge of
tubing, so as to permit its descent. This is effected with a rimer,
Figs. 81 and 82, and is an operation requiring great care and attention.
To reduce the stoppages for the withdrawal of _débris_ the system of
Fauvelle was introduced, but it is now very little practised on the
Continent, and not at all in Great Britain. The principles upon which it
was founded were: first, that the motion given to the tool in rotation
was simply derived from the resistance that a rope would oppose to an
effort of torsion; and therefore that the limits of application of the
system were only such as would provide that the tool should be safely
acted upon; and, secondly, that the injection of a current of water,
descending through a central tube, should wash out the _débris_ created
by the cutting tool at the bottom. The difficulties attending the
removal of the _débris_ were great; and though the system of Fauvelle
answered tolerably well when applied to shallow borings, it was found
to be attended with such disadvantages when applied on a large scale,
that it has been generally abandoned. The quantity of water required to
keep the boring tool clear is a great objection to the introduction of
this system, especially as in the majority of cases Artesian wells are
sunk in such places as are deprived of the advantage of a large supply.
In the ordinary system of well boring, innumerable breakages and delays
occur when a boring is required to be carried to any depth exceeding 200
or 300 feet, owing to the buckling of the rods, the crystallization of
the iron by the constant jarring at each blow, and particularly the
increased weight of the rods as the hole gets deeper. It follows from
this, that where the excavation is very deep, there is considerable
difficulty in transmitting the blow of the tool, in consequence of the
vibration produced in the long rod, or in consequence of the torsion;
and, for the same reason, there is a danger of the blows not being
equally delivered at the bottom. It has been attempted to obviate this
difficulty, but without much success, by the use of hollow rods,
presenting greater sectional area than was absolutely necessary for the
particular case, in order to increase their lateral resistance to the
blows tending to produce vibration.
Boring is usually executed by contract. The approximate average cost in
England may be taken at 1_s._ 3_d._ a foot for the first 30 feet; 2_s._
6_d._ a foot for the second 30 feet; and continue in arithmetical
progression, advancing 1_s._ 3_d._ a foot for every additional 30 feet
in depth. This does not include the cost of tubing, conveyance of plant
and tools, professional superintendence, or working in rock of unusual
hardness, such as hard limestone and whinstone. A clause is usually
inserted in the contract, to the effect that, if any unforeseen
difficulty is met with in the course of the work, it is then paid for by
the day, at a rate previously determined upon, until the difficulty has
been overcome.
CHAPTER V.
AMERICAN TUBE WELL.
This well consists of a hollow wrought-iron tube about 1-3/4 inch
diameter, composed of any number of lengths from 3 to 11 feet, according
to the depth required. The water is admitted into the tube through a
series of holes, which extend up the lowest length to a height of 2-1/2
feet from the bottom.
The position for a well having been selected, a vertical hole is made in
the ground with a crowbar to a convenient depth; the well tube _a_,
having the clamp _d_, monkey _c_, and pulleys _b_, Fig. 95, previously
fixed on it, is inserted into this hole.
The clamp is then screwed firmly on to the tube from 18 inches to 2 feet
from the ground, as the soil is either difficult or easy; each bolt
being tightened equally, so as not to indent the tube.
The pulleys are next clamped on to the tube at a height of about 6 or 7
feet from the ground, the ropes from the monkey having been previously
rove through them.
[Illustration: Tube Well Assembly.
Fig. 95.]
The monkey is raised by two men pulling the ropes at the same angle.
They should stand exactly opposite each other, and work together
steadily, so as to keep the tube perfectly vertical, and prevent it from
swaying about while being driven. If the tube shows an inclination to
slope towards one side, a rope should be fastened to its top and kept
taut on the opposite side, so as gradually to bring the tube back to the
vertical. When the men have raised the monkey to within a few inches of
the pulleys, they lift their hands suddenly, thus slackening the ropes
and allowing the monkey to descend with its full weight on to the clamp.
The monkey is steadied by a third man, who also assists to force it down
at each descent. This man, likewise, from time to time, with a pair of
gas-tongs, turns the tube round in the ground, which assists the process
of driving, particularly when the point comes in contact with stones.
Particular attention must be paid to the clamp, to see that it does not
move on the tube; the bolts must be tightened up at the first appearance
of any slipping.
When the clamp has been driven down to the ground, the monkey is raised
off it, the screws of the clamp are slackened, and the clamp is again
screwed to the tube, about 18 inches or 2 feet from the ground. After
this, the monkey is lowered on to it, and the pulleys are then raised
until they are again 6 or 7 feet from the ground.
The driving is continued until but 5 or 6 inches of the well tube remain
above the ground, when the clamp, monkey, and pulleys are removed, and
an additional length of tube screwed on to that in the ground. This is
done by first screwing a collar on to the tube in the ground, and then
screwing the next length of tube into the collar, till it buts against
the lower tube; a little white-lead must be placed on the threads of the
collar before the ends of the tubes are screwed into it.
The driving can thus be continued until the well has obtained the
desired depth. Soon after another length has been added, the upper
length should be turned round a little with the gas-tongs, to tighten
the joints, which have a tendency to become loose from the jarring of
the monkey. Care must be taken, after getting into a water-bearing
stratum, not to drive through it, owing to anxiety to get a large
supply. From time to time, and always before screwing on an additional
length of tube, the well should be sounded, by means of a small lead
attached to a line, to ascertain the depth of water, if any, and
character of the earth which has penetrated through the holes perforated
in the lower part of the well tube. As soon as it appears that the well
has been driven deep enough, the pump is screwed on to the top and the
water drawn up. It usually happens that the water is at first thick, and
comes in but small quantities; but after pumping for some little time,
as the chamber round the bottom of the well becomes enlarged, the
quantity increases and the water becomes clearer.
When sinking in gravel or clay, the bottom of the well tube is liable to
become filled up by the material penetrating through the holes; and
before a supply of water can be obtained, this accumulation must be
removed by means of the cleaning pipes.
The cleaning pipes are of small diameter, 1/2-inch externally, and the
several lengths are connected together in the same way as the well
tubes, by collars screwing on over the adjoining end of two pipes.
To clear the well, one cleaning pipe after another is lowered into the
well, until the lower end touches the accumulation; the pipes must be
held carefully, for if one were to drop into the well it would be
impossible to get it out without drawing the well. A pump is then
attached to the upper cleaning pipe by means of a reducing socket; the
lower end of the cleaning pipe is then raised and held about an inch
above the accumulation by means of the gas-tongs: water is next poured
down the well outside the cleaning pipe, and, being pumped up through
the cleaning pipe, brings up with it the upper portion of the
accumulation; the cleaning pipe is gradually lowered, and the pumping
continued until the whole of the stuff inside the well tube is removed.
The pump is then removed from the cleaning pipe, and the cleaning pipes
are withdrawn piece by piece; and finally the pump is screwed on to the
upper end of the tube well, Fig. 96, which is then in working order.
[Illustration: Tube Well in Working Order.
Fig. 96.]
The tube being very small, is in itself capable of containing only a
limited supply of water, which would be exhausted by a few strokes of
the pump; the condition, therefore, upon which alone these tube wells
can be effective, is that there shall be a free flow of water from the
outside through the apertures into the lower end of the tube. When the
stratum in which the water is found is very porous, as in the case of
gravel and some sorts of chalk, the water flows freely; and a yield has
been obtained in such situations as great and rapid as the pump has been
able to lift, that is 600 gallons an hour. In some other soils, such as
sandy loam, the yield in itself may not be sufficiently rapid to supply
the pump; in such cases, the effect of constant pumping is to draw up
with the water from the bottom a good deal of clay and sand, and so
gradually to form a reservoir, as it were, around the foot of the tube,
in which water accumulates when the pump is not in action, as is the
case in a common well. In dense clays, however, of a close and very
tenacious character, the American tube well is not applicable, as the
small perforations become sealed, and water will not enter the tube.
When the stratum reached by driving is a quicksand, the quantity of sand
drawn up from the water will be so great, that a considerable amount
will have to be pumped before the water will come up clear; and even in
some positions, when the quicksand is of great extent, the effect of the
pumping may be to injure the foundations of adjoining buildings on the
surface of the ground.
The tube well cannot itself be driven through rock, although it might be
used for drawing water from a subjacent stratum through a hole bored in
the rock to receive it.
Subject to these conditions, these tube wells afford a ready and
economical means for drawing water to the surface from a depth not
exceeding 27 or 28 feet.
CHAPTER VI.
WELL BORING AT GREAT DEPTHS.
The first well that was executed of great depth, and which gave rise to
the adoption of tools which directed public attention to the art of well
boring, was that for the city of Paris by Mulot, at the Abattoir of
Grenelle. This was commenced in the year 1832; and after more than eight
years' incessant labour, water rose, on the 26th of February, 1842, from
the total depth of 1798 feet. Subsequent to this, many wells have been
sunk on the Continent, with the hope of attaining the brine springs so
often met with in the Rhine provinces, or the springs destined for the
supply of towns, and which are even deeper than the well of Grenelle,
reaching in some cases to the extraordinary depth of 2800 feet; but all
of them, like the Grenelle well, of small diameter. In their
construction, however, the German engineers introduced some important
modifications of the tools employed; and, amongst other inventions,
Euyenhausen imparted a sliding movement to the striking part of the tool
used for comminuting the rock, so as to fall always through a certain
distance; and thus, while he produced a uniform action upon the rock at
the bottom, he avoided the jar of the tools. Kind also began to apply
his system to the working of the large excavations for the purpose of
winning coal. Whilst the art was in this state, and when he had already
executed some very important works in Germany, Belgium, the North of
France, Creuzot, and Seraing, the Municipal Council of Paris determined
to entrust him with the execution of a new well they were about to sink
at Passy.
In sinking the well of Passy, the weight of the trepan for comminuting
the rock was about 1 ton 16 cwt., 1800 kilog.: the height through which
it fell was about 60 centimètres; and its diameter was 3 feet 3-7/16
inches, 1 mètre. The rods were of oak, about 8 inches on the side, and
the dimensions of the cutting tool were limited to 3 feet 3-7/16 inches
because it worked the whole time in water; but generally the class of
borings Kind undertook were of such a description as justified resorting
to tools of great dimensions. When sinking the shafts for winning coal,
his operations required to be carried on with the full diameters of 10
feet or 14 feet; and he then drove a boring of 3 feet 4 inches diameter
in the first instance, and subsequently enlarged this excavation. There
can be no objection to executing Artesian borings of this diameter,
other than the probable exhaustion of the supply; particularly as it is
now known that the yield of water by these methods is proportionate to
the diameter of the column; though, strange as it may appear, the first
opposition to Kind's plan of sinking the well of Passy was founded upon
the assumption that he would not meet with a larger supply of water from
the subcretaceous formations than had been met with at Grenelle, where
the diameter of the boring was at the bottom not more than 8 inches. It
is now, however, proved that there is a direct gain in adopting the
larger borings, not only as regards the quantity of water to be derived
from them, but also in their execution, arising from the fact that the
tools can be made more secure against the effects of torsion or of
concussion against the sides of the excavation, which is the cause of
the most serious accidents met with in well sinking.
The trepan of M. Kind contains some peculiar details, which are shown in
Figs. 97, 98. The trepan is composed of two principal pieces, the frame
and the arms, both of wrought-iron, with the exception of the teeth of
the cutting part, which are of cast steel. The frame has at the bottom a
series of holes, slightly conical, into which the teeth are inserted,
and tightly wedged up, Fig. 99. These teeth are placed with their
cutting edges on the longitudinal axis of the frame that receives them;
and at the extremity of the frame there are formed two heads, forged out
of the same piece with the body of the tool, which also carries two
teeth, placed in the same direction as the others, but double their
width, in order to render this part of the tool more powerful. By
increasing the dimensions of these end teeth, the diameter of the boring
can be augmented, so as to compensate for the diminution of the clear
space caused by the tubing, necessarily introduced for security in
traversing strata disposed to fall in, or for the purpose of allowing
the water from below to escape at an intermediate level.
[Illustration: The Trepan of M. Kind.
Figs. 97-99.]
Above the lower part of the frame of the trepan is a second piece
composed of two parts bolted together, and made to support the lower
portion of the frame. This part of the machinery also carries two teeth
at its extremities, which serve to guide the tool in its descent, and to
work off the asperities left by the lower portion of the trepan. Above
this, again, are the guides of the machinery, properly speaking,
consisting of two pieces of wrought-iron, arranged in the form of a
cross, with the ends turned up, so as to preserve the machinery
perfectly vertical in its movements, by pressing against the sides of
the boring already executed. These pieces are independent of the blades
of the trepan, and may be moved closer to it or farther away from it, as
may be desired. The stem and the arms are terminated by a single piece
of wrought-iron, which is joined to the frame with a kind of
saddle-joint, and is kept in its place by means of keys and wedges. The
whole of the trepan is finally jointed to the great rods that
communicate the motion from the surface, by means of a screw-coupling,
formed below the part of the tool which bears the joint; this
arrangement permits the free fall of the cutting part, and unites the
top of the arms and frame, and the rod, Fig. 100. It has been proposed
to substitute for this screw-coupling a keyed joint, in order to avoid
the inconvenience frequently found to attend the rusting of the screw,
which often interposes great difficulties in cases where it becomes
necessary to withdraw the trepan.
[Illustration: The Rod.
Fig. 100.]
The sliding joint is the part of Euyenhausen's invention most
unhesitatingly adopted by Kind, and it is one of the peculiarities of
his system as contrasted with the processes formerly in use. So long as
his operations were confined to the small dimensions usually adopted for
Artesian borings, he contented himself with making a description of
joint with a free fall; a simple movement of disengagement regulating
the height fixed by the machinery itself, like the fall of the monkey in
a pile-driving machine; but it was found that this system did not answer
when applied to large borings, and it also presented certain dangers.
Kind then, for the larger class of borings, availed himself of sliding
guides, so contrived as to be equally thrown out of gear when the
machinery had come to the end of the stroke, and maintained in their
respective positions by being made in two pieces, of which the inner one
worked upon slides, moving freely in the piece that communicated the
motion to the striking part of the machinery. The two parts of the tool
were connected with pins, and with a sliding joint, which, in the Passy
well, was thrown out of gear by the reaction of the column of water
above the tool unloosing the click that upheld the lower part of the
trepan, Figs. 101 to 103. The changes thus made in the usual way of
releasing the tool, and in guiding it in its fall were, however, matters
of detail; they involved no new principle in the manner of well boring:
and the modern authorities upon the subject consider that there was
something deficient in Kind's system of making the column of water act
upon a disc by which the click was set in motion. This system, in fact,
required the presence of a column of water not always to be commanded,
especially when the borings had to be executed in the carboniferous
strata.
[Illustration: Sliding Joints.
Figs. 101-103.]
[Illustration: wrought-iron Joints.
Fig. 104.]
[Illustration: Shell Elevation.]
The rods used for the suspension of the trepan, and for the transmission
of the blows to it, were of oak; and this alone would constitute one of
the most characteristic differences between the system of tools
introduced by Kind and those made by the majority of well-borers, but
which, like the disengagement of the tool intended to comminute the
rock, depended for its success upon the boring being filled with water.
The resistance that the wood offers, by its elasticity, to the effects
of any sudden jar, is also to be taken into account in the comparison of
the latter with iron, for the iron is liable to change its form under
the influence of this cause. The resistance to an effort of torsion need
not, however, be much dwelt on, for the turn given to the trepan is
always made when the tool is lifted up from its bed. For the purpose of
making the rods, Kind recommended that straight-grown trees, of the
requisite diameter, should be selected, rather than they should be made
of cut-timber, as there is less danger of the wood warping, and the
character of the wood is more homogeneous. He generally used these trees
in lengths of about 50 feet, and he connected them at the ends with
wrought-iron joints, fitting one into the other, Fig. 104. The ironwork
of the joints is made with a shoulder underneath the screw-coupling, to
allow the rods to be suspended by the ordinary crow's foot during the
operation of raising or lowering them. In the works executed at Passy
there was a kind of frame erected over the centre of the boring, of
sufficient height to allow of the rods being withdrawn in two lengths at
a time, thus producing a considerable economy of time and labour.
[Illustration: Shell Plan.
Figs. 105, 106.]
Nearly all the processes yet introduced for removing the products of the
excavation must be considered to be, more or less, defective, because
all are established on the supposition that the comminuting tool must be
withdrawn, in order that the shell, or other tool intended to remove the
products of the working of the comminutor, may be inserted. This remark
applies to Kind's operations at Passy and elsewhere, as he removed the
rock detached from the bottom of the excavation by a shell, Figs. 105,
106, which was a modification of the tool he invariably employs for this
purpose. It consisted of a cylinder of wrought-iron, suspended from the
rods by a frame, and fastened to it, a little below the centre of
gravity, so that the operation of upsetting it, when loaded, could be
easily performed. This cylinder was lowered to the level of the last
workings of the trepan, and the materials already detached by that
instrument were forced into the tool, by the gradual movement of the
latter in a vertical direction. Some other implements, employed by Kind
for the purpose of removing the products of the excavation in the shafts
for the coal-mines of the North of France, were ingenious, and well
adapted to the large dimensions of the shafts; but they were all, in
some degree, exposed to the danger of becoming fixed, if used in the
small borings of Artesian wells, by the minute particles of rocks
falling down between their sides and the excavation from above. Their
use was therefore abandoned, and the well of Passy was cleared out with
the shell, the bottom of which was made to open upwards, with a hinged
flap, which admitted the finer materials detached by the trepan. There
were also several tools for the purpose of withdrawing the broken parts
of the machinery from the excavation, or whatever substances might fall
in from above; and all were marked by a great degree of simplicity, but
they did not differ enough from those generally used for the same
purpose to merit further remarks. In fact, the accidents intended to be
guarded against or remedied are so precisely alike in all cases, that
there can be little variety in the manufacture of these instruments. But
there is no doubt that Kind deprived himself of a valuable appliance in
not using the ball-clack, _la soupape à boulet_, that other well-borers
employ, Fig. 107.
[Illustration: Ball-clack.
Fig. 107.]
At Passy great strength was given to the head of the striking tool, and
to the part of the machinery applied to turn the trepan, because the
great weight of the latter superinduced the danger of its breaking off
under the influence of the shock, and because the solidity of this part
of the machinery necessarily regulated the whole working of the tool.
The head of the boring arrangement was connected with the balance-beam
of the steam-engine by a straight link-chain, with a screw-coupling,
admitting of being lengthened as the trepan descended, Figs. 108, 109.
The balance-beam, in order to increase its elastic force in the upward
stroke, is in Kind's works made of wood, in two pieces; the upper one
being of fir and the lower one of beech. The whole of the machinery is
put in motion by steam, which is admitted to the upper part of the
cylinder, and presses it down, and thus raises the tool at the other end
of the beam to that part in connection with the cylinder. The
counterpoise to the weight of the tools is also placed upon the
cylinder-end of the beam. The cylinder receives the steam through ports
that are opened and closed by hand, like those of a steam-hammer; so
that the number of the strokes of the piston may be increased or
diminished, and the length of the strokes may be increased, as occasion
may require.
[Illustration: Side Elevations on Balance-beam Connection.
Figs. 108, 109.]
The balance-beam is continued beyond the point where the piston is
connected with it, and it goes to meet the blocks placed to check the
force of the blow given by the descent of the tool. The guides of the
piston-head are attached to the part of the machinery that acts in this
manner; but at Passy, Kind made the balance-beam work upon two free
plummer-blocks, or blocks having no permanent cover, that they might be
more easily moved whenever it was necessary to displace the beam, for
the purpose of taking up or letting down the rods, or for changing the
tools; for the balance-beam was always immediately over the centre of
the tools, and it therefore had to be displaced every time that the
latter were required to be changed. This was effected by allowing the
beam to slide horizontally, so as to leave the mouth of the pit open.
The counter-check, above mentioned, likewise prevented the piston from
striking the cylinder cover with too great a force, when it was brought
back by the weight of the tools to its original position. The operation
of raising and lowering the rods, or of changing the tools, was
performed at Passy by a separate steam-engine, and the shell was
discharged into a special truck, moving upon a railway expressly laid
for this purpose in the great tower erected over the excavation. All
these arrangements were in fact made with the extreme attention to the
details of the various parts of the work which characterizes the
proceedings of foreign engineers, and conduces so much to their success.
The beating, or comminution of the rock, was usually effected at Passy
at the rate of from fifteen strokes to twenty strokes a minute. The rate
of descent, of course, differed in a marked manner, according to the
nature of the rock operated upon; but, generally speaking, the trepan
was worked for the space of about eight hours at a time, after which it
was withdrawn, and the shell let down in order to remove the _débris_.
The average number of men employed in the gang, besides the foreman, or
the superintendent of the well, was about fourteen: they consisted of a
smith and hammerman, whose duty it was to keep the tools in order; and
two shifts of men entrusted with the excavation, namely, an
engine-driver and stoker, a chief workman, or sub-foreman, and three
assistants. The total time employed in sinking the shafts executed upon
this system in the North of France, where it has been applied without
meeting with the accidents encountered in the Passy well, was found to
be susceptible of being divided in the following manner: from 25 per
cent. to 56 per cent. was employed in manoeuvring the trepan; from 11
per cent. to 14-1/2 per cent. in raising and lowering the tools; from 19
per cent. to 21 per cent. in removing the materials detached from the
rocks, and cleaning out the bottom of the excavation; and from 8 per
cent. to 10-1/2 per cent. was lost, owing to the stoppage of the
engines, or to the accidents from broken tools, or to other causes
always attending these operations. In the well of Passy there was, of
course, a considerable difference in the proportions of the time
employed in the various details of the work; and the long period
occupied in obviating the effects of the slips which took place in the
clays, both in the basement beds of the Paris basin and in the
subcretaceous strata, would render any comparison derived from that well
of little value; but it would appear that, until the great accident
occurred, the various operations went on precisely as Kind had
calculated upon.
KIND-CHAUDRON SYSTEM.
[Illustration: Section through Kind-Chaudron Well at Durham.
Fig. 110.]
In the year 1872 Emerson Bainbridge, C.E., drew attention to the
Kind-Chaudron system of sinking mine shafts through water-bearing
strata, without the use of pumping machinery, in a paper read before the
Institute of Civil Engineers. As the operation is almost identical with
that which would have to be carried through in the case of a well sunk
through an upper series of water-bearing strata, of minor importance or
of impure quality, past rock and into the lower water strata, as for
instance through tertiaries and chalk into the lower greensand, the
following extract from Bainbridge's paper may be read with interest.
In the first place, it may be desirable to describe briefly the system
of sinking hitherto pursued in passing through strata yielding large
quantities of water. The most important sinkings of this character have
been carried out in the county of Durham, to the east of the point at
which the Permian overlie the carboniferous rocks. In this district
there is a thin bed of sand between the Permian rock and the coal
measures. Towards this bed the feeders of water are generally found to
increase, and in the sand there is usually a large reservoir of water.
The mode of sinking will be understood by reference to Fig. 110. Whilst
sinking in hard rock, it has ordinarily been the custom to place iron
curbs, or cribs, wherever a bed of stone appeared to form a natural
barrier between two distinct feeders of water. Thus it has frequently
happened that important feeders have been tubbed back, rendering much
less pumping power necessary than would have been required had all the
feeders been allowed to accumulate in the shaft. As will be seen by Fig.
110, the number of wedging cribs employed is no less than thirteen in
250 feet. The cribs forming the foundation of each set of tubbing are
generally much more massive and costly than the segments of tubbing.
[Illustration: Cast-iron Tubbing.
Fig. 111.]
[Illustration: Wooden Tubbing.
Fig. 112.]
[Illustration: End Elevation showing Arrangement at Extensive Sinking.
Fig. 113.]
[Illustration: Side Elevation showing Arrangement at Extensive Sinking.
Fig. 114.]
[Illustration: Pass Pipe.
Fig. 115.]
[Illustration: Pass Pipe.
Fig. 116.]
[Illustration: Ball Type of Pressure Equalizer.
Fig. 118.]
The process of fixing the crib is as follows;--The diameter of the shaft
is made about 30 inches larger than that of the inside of the tubbing.
When a bed of rock, which may be considered sufficiently hard and close
to separate the feeders above and below it, is reached, the shaft is
contracted to the diameter of the tubbing, and a smooth horizontal face
is made on which to place the wedging crib. The wedging crib, which
usually consists of segments about 4 feet long by 6 inches high by 14
inches wide, is then placed on the bed. To give the crib a firm and
secure position, it is tightly wedged with wood, both behind and between
the joints; the tubbing is then built upon it to the next wedging crib,
which rests upon a bell-shaped section of rock. When the tubbing nearly
reaches this crib, the rock is removed piece by piece, and the top ring
of tubbing is placed close up against the crib. It will thus be seen
that the fixing of each crib is a costly process, often causing
considerable delay.
In some cases, where it has been difficult to find suitable foundations
for intermediate wedging cribs, the whole of the water-bearing rocks
have been sunk through without attempting to stop the feeders
separately, and no tubbing has been placed in the shaft till the wedging
crib could be fixed below the lowest feeder. This process is more
expeditious where there are small quantities of water; but where the
water is excessive greater delay is caused by contending with it than
from putting in numerous sets of tubbing to stop the feeders separately.
The tubbing used in England has almost invariably been of cast-iron; on
the Continent, till recently, tubbing of wood has chiefly been used.
Illustrations of both descriptions are shown by Figs. 111 and 112.
Figs. 113, 114, show, in elevation, the plant and the arrangements
generally in use at extensive sinkings. Where the water is in large
quantities it is usually pumped by an engine erected for the purpose,
assisted by the engine or engines intended to be employed to raise the
coal. A small capstan engine is used for passing the men and material up
and down the pit during the sinking, such engine being provided also
with a drum on slow motion, which is used for heavy weights. The
continual pumping, the placing of cribs, and the fixing of the tubbing
are proceeded with till the lowest feeder is reached, when a hard bed is
sought for on which to fix the lowest wedging crib. In all cases the
water has to be pumped out before the wedging crib, which forms the
foundation of each set of tubbing, can be placed.
From this description it will be understood that the sinkers, who number
from ten to twelve at one time, working four hours at a shift in a pit,
say, 14 feet in diameter, are compelled to work in water until all the
tubbing is fixed. This causes a serious obstacle to blasting, and in
other ways delays the progress of the work.
The tubbing used for damming back the water is generally in segments
from 1 foot to 3 feet high, and about 4 feet in length, the thickness
varying from half an inch to 3-3/4 inches. It is kept in position by
packing with wood behind the joints; and is made water-tight by placing
between the segments pieces of wood sheeting about half an inch thick,
which are wedged when all the tubbing is fixed, usually twice with wood,
and sometimes once with iron wedges.
[Illustration: Pressure Equalizing Valve in the Wedging Crib.
Fig. 117.]
To equalize the pressure of water and gas behind the different sets of
tubbing, pass pipes, Figs. 115 and 116, are sometimes used. Another
expedient to effect this is to have a valve, working upwards, placed in
the wedging crib, Fig. 117. A ball is also sometimes used, Fig. 118.
The various modes of piercing beds of quicksand are;--By hanging tubbing
to that already fixed, and adding fresh rings as the sand is removed.
This is only practicable when the quantity of sand is inconsiderable. By
heavily weighting a cylinder of iron of the same size as the shaft, and
thus forcing it down through the sand. By keeping back the sand by the
use of piles--a resource that can only be recommended when the bed of
sand is not of great thickness. When the water is excessive, by using
pneumatic agency. As these operations are apart from our immediate
subject we need not further discuss them.
M. Chaudron's system, which is a modification of Kind's, is divisible
into the following distinct processes, which consist of;--
The erection of the necessary machinery on the surface, and the opening
of the mine.
The boring of the pits to the lowest part of the water-bearing strata.
The placing of the tubbing.
The introduction of cement behind the tubbing to complete its solidity.
The extraction of the water from the pits, and the placing of the
wedging cribs, or "faux cuvelage," below the moss box.
[Illustration: Surface Plant Elevation.
Fig. 119.]
[Illustration: Surface Plant Elevation.
Fig. 120.]
[Illustration: Surface Plant Plan.
Fig. 121.]
Figs. 119 to 121 show in elevations and in plan the plant usually
employed on the surface. O is a small capstan engine, having a cylinder
20 inches in diameter and a stroke of 32 inches, working on the third
motion. Attached to this engine, and working in the small pit C, is a
counterbalance weight. This engine is used for raising and lowering
boring tools, and for lifting the _débris_ resulting from the boring. As
far as the platform, which is about 10 feet from the surface, the pit
has a diameter of 19 feet, or 4 feet more than the diameter of the pit
below. A at level of about 38 feet above this platform there is a
tramway on which small trucks run, carrying the _débris_ cylinder on one
side, and the boring tools on the other. At a level of 48 feet above the
platform are placed supports for the wooden spears to which the boring
tools are attached. The machinery for boring is worked by a cylinder,
which has a diameter of 39-1/3 inches, and a full stroke of 39-1/3
inches, the usual stroke varying from 2 feet to 3 feet. A massive beam
of wood transmits motion from this cylinder to the boring apparatus, the
connection between the beam and the piston-rod and the beam and the
boring tools being made by a chain. The engine-man sits close to the
engine, and applies the steam above the piston only. The down stroke of
the boring tools is caused by the sudden opening of the exhaust, and a
frame then prevents the shock of the boring rods from being too severe.
The engines work at speeds varying from 12 to 18 strokes a minute,
according to the character of the strata passed through.
[Illustration: The Small Trepan.
Fig. 122.]
[Illustration: The Small Trepan.
Fig. 123.]
[Illustration: The Small Trepan.
Fig. 124.]
[Illustration: Plan of Guide B.
Fig. 125.]
[Illustration: Swivelled Ring Front Elevation.
Fig. 126.]
[Illustration: Swivelled Ring Side Elevation.
Fig. 127.]
[Illustration: Large Trepan Front Elevation.
Fig. 128.]
[Illustration: Large Trepan Side Elevation.
Fig. 129.]
[Illustration: Large Trepan Component,
Fig. 130.]
[Illustration: Trepan Teeth.
Figs. 131-134.]
After the working platform is fixed, the first boring tool applied is
the small trepan, Figs. 122 to 125. This tool is attached to the wooden
beam by the same arrangement shown by Fig. 109. The boring tools can be
lowered at pleasure by means of an adjusting screw. Next in order comes
the handle for boring. This is worked by four men on the platform, and
is turned by the aid of a swivel. Attached to the handle-piece are
wooden rods, made from Riga pitch pine. These rods are 59 feet in length
and 7-3/4 inches square. A swivelled ring, Figs. 126, 127, is attached
to the rope when raising and lowering the boring rods. The small trepan
cuts a hole 4 feet 8-3/4 inches in diameter, and has fourteen teeth,
fitted in cylindrical holes and secured by pins entering through
circular slots. The teeth are steeled. At a distance of 4 feet 4 inches
above the main teeth of the trepan there is an arm, with a tooth at each
end. This piece answers the purpose of a guide, and at the same time
removes irregularities from the sides of the hole. At a distance of 13
feet 6 inches above the main teeth are the actual guides, consisting of
two strong arms of iron fixed on the tool, and placed at right-angles to
each other. The hole made by the small trepan is not kept at any fixed
distance in advance of the full-sized pit, but the distance generally
varies from 10 to 30 yards. With the small trepan, which weighs 8 tons,
the progress varies from 6 to 10 feet a day.
The large trepan, Figs. 128 to 130, weighs 16-1/2 tons, is forged in
one solid piece, and has twenty-eight teeth. A projection of iron forms
the centre of this trepan, and fits loosely into the hole made by the
small trepan, acting as a guide for the tool. At a distance of 7 feet 6
inches above the teeth, a guide is sometimes fixed on the frame, but is
not furnished with teeth. At a distance of 13 feet 3 inches from the
teeth are two other guides at right-angles to each other. These guides
are let down the pit with the boring tool, the hinged part of the guides
being raised whilst passing through the beams at the top of the pit,
which are only 6 feet 7 inches apart. When the tool is ready to work,
the two arms are let down against the side of the pit, and are hung in
the shaft by ropes, thus acting as a guide for the trepan, which moves
through them. To provide against a shock to the spears when the trepan
strikes the rock on the down stroke, at the upper part of the frame a
slot motion is arranged, the play of which amounts to about half an
inch. The teeth of the large trepan are not horizontal, but are deeper
towards the inside of the pit, the face of the inside tooth being 3-3/4
inches lower than the outside. The object of this is to cause the
_débris_ to drop at once into the small hole, by the face of the rock at
the bottom of the pit being somewhat inclined. The teeth used, Figs. 131
to 134, are the same both for the large and the small trepan, and weigh
about 72 lb. each. As a rule, only one set of teeth is kept in use, this
set working for twelve hours, the alternate twelve hours being employed
in raising the _débris_. This time is divided in about the following
proportions;--Boring, twelve hours; drawing the rods, one hour to five
hours, according to depth; raising the _débris_, two hours; and lowering
the rods one hour to five hours. The maximum speed of the larger trepan
may be taken at about 3 feet a day. The ordinary distance sunk is not
more than 2 feet a day, and in flint and other hard rocks the boring has
proceeded as slowly as 3 inches a day.
[Illustration: Tools used in Boring.
Figs. 135, 136.]
[Illustration: Epicycloidal Hook.
Figs. 137, 138.]
[Illustration: Rod Disconnection Key.
Figs. 139, 140.]
[Illustration: Connections to the Trepan and Spears or Rods.
Fig. 141, 142.]
The _débris_ in the small bore-hole contains pieces of a maximum size of
about 8 cubic inches. In the large boring, pieces of rock measuring 32
cubic inches have been found. As a rule, however, the material is beaten
very fine, having much the appearance of mud or sand. In both the large
and the small borings the _débris_ is raised by a shell, similar to
Figs. 105, 106, and in this system consisting of a wrought-iron
cylinder, 3 feet 3 inches in diameter by 6 feet 9 inches long, and
containing two flap-valves at the bottom, through which the excavated
material enters. This apparatus is passed down the shaft by the
bore-rods, and it is moved up and down through a distance varying from 6
to 8 inches, for about a quarter of an hour, and is then drawn up and
emptied. In some cases where the rock is hard, three sizes of trepan are
used consecutively, the sizes being 5 feet, 8 feet, and 13 feet.
The several other tools and appliances used during the boring operations
are shown, Figs. 135 to 140, including the key, Figs. 139, 140, used at
the surface to disconnect the rods, the hook on which each rod is hung
after being raised to the high platform and there detached, the bar upon
which the hooks are moved, and the fork for suspending the rods or
tools from the rollers when it is desired to move the rods or tools from
above the shaft.
[Illustration: Connections to the Trepan and Spears or Rods.
Figs. 143-146.]
Figs. 141 to 146 are of the connections to the trepan and spears or
rods.
Should broken tools fall into the shaft, several varieties of apparatus
are used for their recovery. In case of broken rods of any kind having a
protuberance that can be clutched, a hook or crow, Figs. 137, 138, of an
epicycloidal form, enables the object to be taken hold of very readily.
Where the broken part has no shoulder which can be held, but is simply a
bar, the apparatus shown by Figs. 147, 148, is employed. This is
composed of two parts. The rods, the bottom of which have teeth inside,
are prevented from diverging by the cone and slide on the main rods.
When passed over a rod or pipe, they clutch it by means of the teeth,
and draw it up. Chaudron has, by this tool, raised a column of pipes 295
feet in length and 8 inches in diameter. An instrument, called a
"grapin," Figs. 149, 150, is used for raising broken teeth or other
small objects which may have fallen into the bottom of the shaft. This
tool also has one part sliding in the other, and is lowered with the
claws closed. The parts are moved by two ropes worked from the surface.
By weighting the cross-bar, which is attached to the moving parts, the
pressure desired can be exerted on the claws. The weight is then lifted,
the claws are opened, and are made to close upon the substance to be
raised. This instrument is now seldom required.
[Illustration: Pipe or Rod Recovery Tool.
Figs. 147, 148.]
[Illustration: Grapin.
Figs. 149, 150.]
In boring shafts in the manner described, without being able to prove in
the usual way the perpendicularity of the shaft, it might be feared that
the system would be open to objection on this account. It appears,
however, that in all cases where Chaudron has sunk shafts by this system
he has succeeded in making them perfectly vertical. This is ensured by
the natural effect of the treble guide, which the chisels and the two
sets of arms attached to the boring tools afford, and by the fact that
if the least divergence from a plumb-line is made by the boring tool,
the friction of the tool upon one side of the shaft is so great as to
cause the borers to be unable to turn the instrument.
Boring alternately with the large and the small instrument, the shaft is
at length sunk to the point at which the lowest feeder of water is
encountered. In a new district this has to be taken, to some extent, at
hazard; but where pits have been sunk previously, it is not difficult to
tell, by observing the strata, almost the exact point at which the
bottom of the tubbing may be safely fixed. This point being ascertained,
the third process is arrived at.
[Illustration: Hydraulic Test Apparatus.
Fig. 151.]
As the object of placing tubbing in a shaft is effectually to shut off
the feeders, which for water supply may have some bad qualities, and to
secure a water-tight joint at the base, it is important that the bed on
which the moss box has to rest should be quite level and smooth. This is
attained by the use of a tool, termed a "scraper," attached to the
bore-rods, the blades being made to move round the face of the bed
intended for the moss box. The tubbing employed is cast in complete
cylinders. At Maurage each ring has an internal diameter of 12 feet and
is 4 feet 9 inches high. Each ring has an inside flange at the top and
bottom, and also a rib in the middle, the top and bottom of the ring
being turned and faced. The rings of tubbing are attached to each other
by twenty-eight bolts 1·1 inch in diameter, passed through holes bored
in the flanges. The tubbing is suspended in the pit by means of six
r
Showing first 200,000 characters. Full text: gutenberg.org/ebooks/45703