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.