[Illustration: 819
_Fig._ 819.
1. The _brace-head_.
2. The common rod.
3. The double-box rod; intermediate piece.
4. The common chisel.
5. The indented chisel.
6. Another of the same.
7. The cross-mouthed chisel.
8. The wimble.
9. The sludger, for bringing up the mud.
10. The rounder.
11. The key for supporting the train of rods at the bore-mouth.
12. The key for screwing together and asunder the rods.
13. The topit, or top-piece.
14. The beché, for catching the rod when it breaks in the bore.
15. The runner, for taking hold of the topit.
16. The tongued chisel.
17. The right-handed worm screw.
18. The left-handed do.
19. The finger-grip or catch.]
We shall now explain the manner of conducting a series of bores in
searching ground for coal.
[Illustration: 820]
_Fig._ 820. represents a district of country in which a regular survey
has proved the existence and general distribution of coal strata, with a
dip to the south, as here shown. In this case, a convenient spot should
be pitched upon in the north part of the district, so that the
successive bores put down may advance in the line of the dip. The first
bore may therefore be made at No. 1., to the depth of sixty yards. In
the progress of this perforation, many diversities and alternations of
strata will be probably passed through, as we see in the sections of the
strata; each of which, as to quality and thickness, is noted in the
journal, and specimens are preserved. This bore is seen to penetrate the
strata _d_, _c_, _b_, _a_, without encountering any coal. Now, suppose
that the dip of the strata be one yard in ten, the question is, at what
distance from bore No. 1. in a south direction, will a second bore of 60
yards strike the first stratum _d_, of the preceding? The rule obviously
is, to multiply the depth of the bore by the dip, that is, 60 by 10, and
the product 600 gives the distance required; for, by the rule of three,
if 1 yard of depression corresponds to 10 in horizontal length, 60 yards
of depression will correspond to 600 in length. Hence the bores marked
1, 2, 3, 4, and 5, are successively distributed as in the figure, the
spot where the first is let down being regarded as the point of level to
which the summits of all the succeeding bores are referred. Should the
top of No. 2. bore be 10 yards higher or lower than the top of No. 1.,
allowance must be made for this difference in the operation; and hence a
surface level survey is requisite. Sometimes ravines cut down the
strata, and advantage should be taken of them, when they are
considerable.
In No. 2. a coal is seen to occur near the surface, and another at the
bottom of the bore; the latter seam resting on the first stratum _d_,
that occurred in bore No. 1.; and No. 2. perforation must be continued a
little farther, till it has certainly descended to the stratum _d_. Thus
these two bores have, together, proved the beds to the depth of 120
yards.
No. 3. bore being placed according to the preceding rule, will pass
through two coal-seams near the surface, and after reaching to nearly
its depth of 60 yards, it will touch the stratum _h_, which is the upper
stratum of bore No. 2.; but since a seam of coal was detected in No. 2.,
under the stratum _h_, the proof is confirmed by running the borer down
through that coal. The field has now been probed to the depth of 180
yards. The fourth bore is next proceeded with, till the two coal-seams
met in No. 3. have been penetrated; when a depth of 240 yards has been
explored. Hence No. 4. bore could not reach the lower stratum _a_,
unless it were sunk 240 yards.
The fifth bore (No. 5.) being sunk in like manner, a new coal-seam
occurs within a few yards of the surface; but after sinking to the depth
at which the coal at the top of the fourth bore was found, an entirely
different order of strata will occur. In this dilemma, the bore should
be pushed 10 or 20 yards deeper than the 60 yards, to ascertain the
alternations of the new range of superposition. It may happen that no
coals of any value shall be found, as the figure indicates, in
consequence of a slip or dislocation of the strata at B, which has
thrown up all the coals registered in the former borings, to such an
extent that the strata _b_, _a_, of the first bore present themselves
immediately on perforating the slip, instead of lying at the depth of
300 yards (5 × 60), as they would have done, had no dislocation
intervened. Some coal-fields, indeed, are so intersected with slips as
to bewilder the most experienced miner, which will particularly happen
when a lower coal is thrown upon one side of a slip, directly opposite
to an upper coal situated on the other side of it; so that if the two
seams be of the same thickness, erroneous conclusions are almost
inevitable.
When a line of bores is to be conducted from the dip of the strata
towards their outcrop, they should be placed a few yards nearer each
other than the rule prescribes, lest the strata last passed through be
overstepped, so that they may disappear from the register, and a
valuable coal-seam may thereby escape notice. In fact, each successive
bore should be so set down, that the first of the strata perforated
should be the last passed through in the preceding bore; as is
exemplified by viewing the bores in the retrograde direction, Nos. 4. 3.
and 2. But if the bore No. 2. had gone no deeper than _f_, and the bore
No. 1. been as represented, then the stratum _e_, with its immediately
subjacent coal, would have been overstepped, since none of the bores
would have touched it; and they would have remained unnoticed in the
journal, and unknown.
[Illustration: 821]
When the line of dip, and consequently the line of bearing which is at
right angles to it, are unknown, they are sought for by making three
bores in the following position.--Let _fig._ 821. be a horizontal
diagram, in which the place of a bore, No. 1., is shown, which reaches
a coal-seam at the depth of 50 yards; bore No. 2. may be made at B, 300
yards from the former; and bore No. 3. at C, equidistant from Nos. 1.
and 2., so that the bores are sunk at the three angles of an equilateral
triangle. If the coal occur in No. 2. at the depth of 30 yards, and in
No. 3. of 44 yards, it is manifest that none of the lines A B, B C, or C
A is in the line of level, which for short distances may be taken for
the line of bearing, with coal-seams of moderate dip. But since No. 1.
is the deepest of the three bores, and No. 3. next in depth, the line A
C joining them must be nearer the line of level, than either of the
lines A B or B C. The question is, therefore, at what distance on the
prolonged line B C is the point for sinking a bore which would reach the
coal at the same depth as No. 1., namely 50 yards. This problem is
solved by the following rule of proportion: as 14 yards (the difference
of depth between bores 2. and 3.) is to 300 yards (the distance between
them), so is 20 (the difference of depth betwixt 1. and 2.) to a fourth
proportion, or _x_ = 428 yards, 1 foot, and 8 inches. Now, this
distance, measured from No. 2., reaches to the point D on the prolonged
line B C, under which point D the coal will be found at a depth of 50
yards, the same as under A. Hence the line A D is the true level line of
the coal-field; and a line B F G drawn at right angles to it, is the
true dip-line of the plane which leads to the outcrop. In the present
example the dip is 1 yard in 14-1/2; or 1 in 14-1/2, to adopt the
judicious language of the miner; or the sine is 1 to a radius of 14-1/2,
measured along the line from B to F. By this theorem for finding the
lines of dip and level, the most eligible spot in a coal-field for
sinking a shaft may be ascertained.
Suppose the distance from B to G in the line of dip to be 455 yards;
then, since every 14-1/2 gives a yard of depression, 455 will give 30
yards, which added to 30 yards, the depth of the bore at B, will make 60
yards for the depth of the same coal-seam at G. Since any line drawn at
right angles to the line of level A D is the line of dip, so any line
drawn parallel to A D is a level line. Hence, if from C the line C E be
drawn parallel to D A, the coal-seam at the points E and C will be found
in the same horizontal plane, or 44 yards beneath the surface level,
over these two points. The point E level with C may also be found by
this proportion: as 20 yards (the difference in depth of the bores under
B and A) is to 300 yards (the distance between them), so is 14 yards
(the difference of depth under B and C) to 210 yards, or the distance
from B to E.
As boring for coal is necessarily carried on in a line perpendicular to
the horizon, and as coal seams lie at every angle of inclination to it,
the thickness of the seam as given obliquely by the borer, is always
greater than the direct thickness of the coal; and hence the length of
that line must be multiplied by the cosine of the angle of dip, in order
to find the true power of the seam.
_Of fitting or winning a coal-field._--In sinking a shaft for working
coal, the great obstacle to be encountered, is water, particularly in
the first opening of a field, which proceeds from the surface of the
adjacent country; for every coal-stratum, however deep it may lie in one
part of the basin, always rises till it meets the alluvial cover, or
crops out, unless it be met by a slip or dike. When the basset-edge of
the strata is covered with gravel or sand, any body or stream of water
will readily percolate downwards through it, and fill up the porous
interstices between the coal-measures, till arrested by the face of a
slip, which acts as a valve or flood-gate, and confines the water to one
compartment of the basin, which may, however, be of considerable area,
and require a great power of drainage.
In reference to water, coal-fields are divided into two kinds; 1., level
free coal; 2., coal not level free. In the practice of mining, if a
coal-field, or portion of it, is so situated above the surface of the
ocean that a level can be carried from that plane till it intersects the
coal, all the coal above the plane of intersection is said to be level
free; but if a coal-field, though placed above the surface of the ocean,
cannot, on account of the expense, be drained by a level or gallery, but
by mechanical power, such a coal-field is said to be not level free.
Besides these general levels of drainage, there are subsidiary levels,
called off-takes or drifts, which discharge the water of a mine, not at
the mouth of the pit, but at some depth beneath the surface, where, from
the form of the country, it may be run off level free. From 20 to 30
fathoms off-take is an object of considerable economy in pumping; but
even less is often had recourse to; and when judiciously contrived, may
serve to intercept much of the crop water, and prevent it from getting
down to the dip part of the coal, where it would become a heavy load on
a hydraulic engine.
Day levels were an object of primary importance with the early miners,
who had not the gigantic pumping power of the steam-engine at their
command. Levels ought to be no less than 4 feet wide, and from 5 feet
and a half to 6 feet high: which is large enough for carrying off
water, and admitting workmen to make repairs and clear out depositions.
When a day-level, however, is to serve the double purpose of drainage
and an outlet for coals, it should be nearly 5 feet wide, and have its
bottom gutter covered over. In other instances a level not only carries
off the water from the colliery, but is converted into a canal for
bearing boats loaded with coals for the market. Some subterranean canals
are nine feet wide, and twelve feet high, with 5 feet depth of water.
[Illustration: 822 823]
If in the progress of driving a level, workable coals are intersected
before reaching the seam which is the main object of the mining
adventure, an air-pit may be sunk, of such dimension as to serve for
raising the coals. These air-pits do not in general exceed 7 foot in
diameter; and they ought to be always cylindrical. _Fig._ 822.
represents a coal-field where the winning is made by a day-level; _a_ is
the mouth of the gallery on a level with the sea; _b_, _c_, _d_, _e_,
are intersected coal-seams, to be drained by the gallery. But the coals
beneath this level must obviously be drained by pumping. A represents a
coal-pit sunk on the coal _e_; and if the gallery be pushed forward, the
coal-seams _f_, _g_, and any others which lie in that direction, will
also be drained, and then worked by the pit A. The chief obstacle to the
execution of day-levels, is presented by quicksands in the alluvial
cover, near the entrance of the gallery. The best expedient to be
adopted amid this difficulty is the following:--_Fig._ 823. represents
the strata of a coal-field A, with the alluvial earth _a_, _b_,
containing the bed of quicksand _b_. The lower part, from which the
gallery is required to be carried, is shown by the line B _d_. But the
quicksand makes it impossible to push forward this day-level directly.
The pit B C must therefore be sunk through the quicksand by means of
_tubbing_ (to be presently described), and when the pit has descended a
few yards into the rock, the gallery or drift may then be pushed forward
to the point D, when the shaft E D is put down, after it has been
ascertained by boring that the rock-head or bottom of the quicksand at F
is a few yards higher than the mouth of the small pit B. During this
operation, all the water and mine-stuff, are drawn off by the pit B; but
whenever the shaft E D is brought into communication with the gallery,
the water is allowed to fill it from C to D, and rise up both shafts
till it overflows at the orifice B. From the surface of the water in the
deep shaft at G, a gallery is begun of the common dimensions, and pushed
onwards till the coal sought after is intersected. In this way no
drainage level is lost. This kind of drainage gallery, in the form of an
inverted syphon, is called a drowned or a blind level.
When a coal-basin is so situated that it cannot be rendered level free,
the winning must be made by the aid of machinery. The engines at present
employed in the drainage of coal-mines are:--
1. The water-wheel, and water-pressure engine.
2. The atmospheric steam-engine of Newcomen.
3. The steam-engine, both atmospheric and double stroke, of Watt.
4. The expansion steam-engine of Woolf.
5. The high-pressure steam-engine, without a condenser.
The depth at which the coal is to be won, or to be drained of moisture,
regulates the power of the engine to be applied, taking into account the
probable quantity of water which may be found, a circumstance which
governs the diameter of the working barrels of the pumps. Experience has
proved, that in opening collieries, even in new fields, the water may
generally be drawn off by pumps of from 10 to 15 inches diameter;
excepting where the strata are connected with rivers, sand-beds filled
with water, or marsh-lands. As feeders of water from rivers or sand-beds
may be hindered from descending coal-pits, the growth proceeding from
these sources need not be taken into account; and it is observed, in
sinking shafts, that though the influx which cannot be cut off from the
mine, may be at first very great, even beyond the power of the engine
for a little while, yet as this excessive flow of water is frequently
derived from the drainage of fissures, it eventually becomes manageable.
An engine working the pumps for 8 or 10 hours out of the 24, is reckoned
adequate to the winning of a new colliery, which reaps no advantage from
neighbouring hydraulic powers. In the course of years, however, many
water-logged fissures come to be cut by the workings, and the coal seams
get excavated towards the outcrop, so that a constant increase of water
ensues, and thus a colliery which has been long in operation, frequently
becomes heavily loaded with water, and requires the action of its
hydraulic machinery both night and day.
[Illustration: 824 825 826]
_Of Engine Pits._--In every winning of coal, the shape of the engine-pit
deserves much consideration. For shafts of moderate depth, many forms
are in use; as circular, oval, square, octagonal, oblong rectangular,
and oblong elliptical. In pits of inconsiderable depth, and where the
earthy cover is firm and dry, any shape deemed most convenient may be
preferred; but in all deep shafts, no shape but the circular should be
admitted. Indeed, when a water-run requires to be stopped by tubbing or
cribbing, the circular is the only shape which presents a uniform
resistance in every point to the equable circumambient pressure. The
elliptical form is the next best, when it deviates little from the
circle; but even it has almost always given way to a considerable
pressure of water. The circular shape has the advantage, moreover, of
strengthening the shaft walls, and is less likely to suffer injury than
other figures, should any failure of the pillars left in working out the
coal cause the shaft to be shaken by subsidence of the strata. The
smallest engine-pit should be ten feet in diameter, to admit of the
pumps being placed in the lesser segment, and the coals to be raised in
the larger one, as shown in _fig._ 824., which is called a double pit.
If much work is contemplated in drawing coals, particularly if their
masses be large, it would be advantageous to make the pit more than 10
feet wide. When the area of a shaft is to be divided into three
compartments, one for the engine pumps, and two for raising coals, as in
_fig._ 825., which is denominated a triple pit, it should be 12 feet in
diameter. If it is to be divided into four compartments, and made a
quadrant shaft, as in _fig._ 826., with one space for the pumps, and
three for ventilation and coal-drawing, the total circle should be 15
feet in diameter. These dimensions are, however, governed by local
circumstances, and by the proposed daily discharge of coals.
The shaft, as it passes through the earthy cover, should be securely
faced with masonry of jointed ashler, having its joints accurately
bevelled to the centre of the circle. Specific directions for building
the successive masses of masonry, on a series of rings or cribs of oak
or elm, are given by Mr. Bald, article MINE, _Brewster’s Encyclopædia_,
p. 336.
[Illustration: 827]
When the alluvial cover is a soft mud, recourse must be had to the
operation of tubbing. A circular tub, of the requisite diameter, is made
of planks from 2 to 3 inches thick, with the joints bevelled by the
radius of the shaft, inside of which are cribs of hard wood, placed from
2 to 4 feet asunder, as circumstances may require. These cribs are
constructed of the best heart of oak, sawn out of the natural curvature
of the wood, adapted to the radius, in segments from 4 to 6 feet long,
from 8 to 10 inches in the bed, and 5 or 6 inches thick. The length of
the tub is from 9 to 12 feet, if the layer of mud have that thickness;
but a succession of such tubs must be set on each other, provided the
body of mud be thicker. The first tub must have its lower edge thinned
all round, and shod with sharp iron. If the pit be previously secured to
a certain depth, the tub is made to pass within the cradling, and is
lowered down with tackles till it rests fair among the soft alluvium. It
is then loaded with iron weights at top, to cause it to sink down
progressively as the mud is removed from its interior. Should a single
tub not reach the solid rock (sandstone or basalt), then another of like
construction is set on, and the gravitating force is transferred to the
top. _Fig._ 827. represents a bed of quicksand resting on a bed of
impervious clay, that immediately covers the rock. A is the finished
shaft; _a a_, the quicksand; _b b_, the excavation necessarily sloping
much outwards; _c c_, the lining of masonry; _d d_, the moating or
puddle of clay, hard rammed in behind the stone-work, to render the
latter water-tight. In this case, the quicksand, being thin in body, has
been kept under for a short period, by the hands of many men scooping it
rapidly away as it filled in. But the most effectual method of passing
through beds of quicksand, is by means of cast-iron cylinders; called,
therefore, cast-iron tubbing. When the pit has a small diameter, these
tubs are made about 4 feet high, with strong flanges, and bolt holes
inside of the cylinder, and a counterfort ring at the neck of the
flange, with brackets: the first tub, however, has no flange at its
lower edge, but is rounded to facilitate its descent through the mud.
Should the pit be of large diameter, then the cylinders must be cast in
segments of 3, 4, or more pieces, joined together with inside vertical
flanges, well jointed with oakum and white lead. When the sand-bed is
thick, eighty feet, for instance, it is customary to divide that length
into three sets of cylinders, each thirty feet long, and so sized as to
slide within each other, like the eye tubes of a telescope. These
cylinders are pressed down by heavy weights, taking care to keep the
lower part always further down than the top of the quicksand, where the
men are at work with their shovels, and where the bottom of the pumps
hangs for withdrawing the surface water. This is an improvement adopted
of late years in the Newcastle district with remarkable success.
The engine pit being secured, the process of sinking through the rock is
ready to be commenced, as soon as the divisions of the pit formed of
carpentry, called brattices, are made. In common practice, and where
great tightness of jointing is not required, for ventilating inflammable
air, bars of wood, called buntons, about 6 inches thick, and 9 deep, are
fixed in a horizontal position across the pit, at distances from each
other of 10, 20, or 30 feet, according to circumstances. Being all
ranged in the same vertical plane, deals an inch and a half thick are
nailed to them, with their joints perfectly close; one half of the
breadth of a bunton being covered by the ends of the deals. In deep
pits, where the ventilation is to be conducted through the brattice, the
side of the buntons next the pumps is covered with deals in the same
way, and the joints are rendered secure by being caulked with oakum.
Fillets of wood are also fixed all the way down on each side of the
brattice, constituting what is called a double pit.
When a shaft is to have 3 compartments, it requires more care to form
the brattice, as none of the buntons stretch across the whole space, but
merely meet near the middle, and join at certain angles with each other.
As the buntons must therefore sustain each other, on the principle of
the arch, they are not laid in a horizontal plane, but have a rise from
the sides towards the place of junction of 8 or 9 inches, and are bound
together by a three-tongued iron strap. Fillets of wood are carried down
the whole depth, not merely at the joinings of the brattice with the
sides of the pit, but also at their central place of union; while wooden
pillars connect the centre of each set of buntons with those above and
below. Thus the carpentry work acquires sufficient strength and
stiffness.
In quadrant shafts the buntons cross each other towards the middle of
the pit, and are generally let into each other about an inch, instead of
being half-checked. _Fig._ 824. is a double shaft: A, the pump pit; B,
the pit for raising coal. _Fig._ 825. is a triple shaft; in which A is
the pump compartment; B and C are coal pits. _Fig._ 826. is a quadrant
shaft: A, the pump pit; B, pit of ventilation or upcast for the smoke; C
and D, pits for raising coals.
[Illustration: 828]
A depth of 75 fathoms is fully the average of engine pits in Great
Britain. In practice, it embraces three sets of pumps. Whenever the
shaft is sunk so low that the engine is needed to remove the water, the
first set of pumps may be let down by the method represented in _fig._
828.; where A is the pump; _a a_, strong ears through which pass the
iron rods connected with the spears _b b_; _c c_ are the lashings; _d_,
the hoggar pump; _e_, the hoggar; _f f_, the tackles; _g g_, the single
pulleys; _h h_, the tackle fold leading to the capstans; and _i_, the
pump-spears. By this mechanical arrangement the pumps are sunk in the
most gradual manner, and of their own accord, so to speak, as the pit
descends. To the arms of the capstans, sledges are fastened with ropes
or chains; these sledges are loaded with weights, as counterpoises to
the weight of the column of pumps, and when additional pumps are joined
in, more weight is laid on the sledges. As the sinking set of pumps is
constantly descending, and the point for the delivery of the water above
always varying, a pipe of equal diameter with the pumps, and about 11
feet long, but much lighter in the metal, is attached to _e_, and is
terminated by a hose of leather, of sufficient length to reach the
cistern where the water is delivered. This is called the hoggar-pipe. In
sinking, a vast quantity of air enters with the water, at every stroke
of the engine; and therefore the lifting stroke should be very slow, and
a momentary stop should take place before the returning stroke, to
suffer all the air to escape. As the working barrels are generally 9 or
10 feet long, and the full stroke of the engine from 7 to 8 feet, when
at regular work, it is customary to diminish the length of stroke, in
sinking, to about 6 feet; because, while the pumps are constantly
getting lower, the bucket in the working barrel has its working range
progressively higher.
The usual length for a set of pumps, is from 25 to 30 fathoms. Whenever
this depth is arrived at by the first set, preparations are made for
fixing firmly the _upper_ pit-cistern, into which the upper set of pumps
is to be placed, and the water of the second set is to be thrown. If a
strong bed of sandstone occurs, a scarcement of it is left projecting
about 3 feet into the shaft, which is formed in the course of sinking
into a strong chin or bracket, to sustain that part of the cistern in
which the superior set of pumps stands. A few feet beneath this
scarcement the shaft resumes its usual shape.
[Illustration: 829]
But although from 20 to 30 fathoms be the common length of a pump-lift,
it sometimes becomes necessary to make it much longer, when no place can
be found in the shaft for lodging a cistern, on account of the tubbing.
Hence a pump-lift has been occasionally extended to 70 fathoms; which
requires extraordinary strength of materials. The best plan for
collaring the pumps in the pit, and keeping them steady in a
perpendicular line, is to fix a strong bunton of timber under the joints
of each pipe; and to attach the pipes firmly to these buntons by an iron
collar, with screws and nuts, as represented in _fig._ 829.
The water obtained in sinking through the successive strata is, in
ordinary cases, conducted down the walls of the shaft; and if the strata
are compact, a spiral groove is cut down the sides of the shaft, and
when it can hold no more, the water is drawn off in a spout to the
nearest pump-cistern; or a perpendicular groove is cut in the side of
the shaft, and a square box-pipe either sunk in it, flush with the sides
of the pit, or it is covered with deal boards well fitted over the
cavity. Similar spiral rings are formed in succession downwards, which
collect the trickling streams, and conduct them into the nearest
cistern; or rings, made of wood or cast iron, are inserted flush with
the sides of the pipe; and the water is led from one ring to another,
through perpendicular pipes, until the undermost ring is full, when it
delivers its water into the nearest pump-cistern. Keeping the shaft dry
is very important to the comfort of the miners, and the durability of
the work.
When an engine shaft happens to pass through a great many beds of coal,
a gallery a few yards long is driven into each coal-seam, and a bore
then put down from one coal to another, so that the water of each may
pass down through these bores to the pump-cisterns.
[Illustration: 830]
While a deep pit is sinking, a register is kept of every part of the
excavations, and each feeder of water is measured daily, to ascertain
its rate of discharge, and whether it increases or abates. The mode of
measurement, is by noting the time, with a seconds watch, in which a
cistern of 40 or 50 gallons gets filled. There are three modes of
keeping back or stopping up these feeders; by plank tubbing; iron
tubbing; and by oak cribs. Let _fig._ 830. represent the sinking of a
shaft through a variety of strata, having a top cover of sand, with much
water resting on the rock summit. Each plane of the coal-measure rises
in a certain direction till it meets the alluvial cover. Hence, the
pressure of the water at the bottom of the tubbing that rests on the
summit of the rock, is as the depth of water in the superficial
alluvium; and if a stratum _a_ affords a great body of water, while the
superjacent stratum _b_, and the subjacent _c_, are impervious to water;
if the porous bed _a_ be 12 feet thick, while no water occurs in the
strata passed through from the rock head, until that depth (supposed to
be 50 fathoms from the surface of the water in the cover); in this case,
the tubbing or cribbing must sustain the sum of the two water pressures,
or 62 fathoms; since the stratum _a_ meets the alluvial cover at _d_,
the fountain head of all the water that occurs in sinking. Thus we
perceive, that though no water-feeder of any magnitude should present
itself till the shaft had been sunk 100 fathoms; if this water required
to be stopped up or tubbed off through the breadth of a stratum only 3
feet thick, the tubbing floodgate would need to have a strength to
resist 100 fathoms of water-pressure. For though the water at first
oozes merely in discontinuous particles through the open pores of the
sands and sandstones, yet it soon fills them up, like a myriad of tubes,
which transfer to the bottom the total weight of the hydrostatic column
of 100 fathoms; and experience shows, as we have already stated, that
whatever water occurs in coal-pits or in mines, generally speaking,
proceeds from the surface of the ground. Hence, if the cover be an
impervious bed of clay, very little water will be met with among the
strata, in comparison of what would be found under sand.
[Illustration: 831 832]
When several fathoms of the strata must be tubbed, in order to stop up
the water-flow, the shaft must be widened regularly to admit the kind of
tubbing that is to be inserted; the greatest width being needed for
plank-tubbing, and the least for iron-tubbing. _Fig._ 831. represents a
shaft excavated for plank-tubbing, where _a_, _a_, _a_ are the
impervious strata, _b_, _b_ the porous beds water-logged, and _c_, _c_
the bottom of the excavation, made level and perfectly smooth with
mason-chisels. The same precautions are taken in working off the upper
part of the excavation _d_, _d_. In this operation, three kinds of cribs
are employed; called wedging, spiking, and main cribs. Besides the stout
plank for making the tub, a quantity of well-seasoned and clean reeded
deal is required for forming the joints; called sheeting deal by the
workmen. This sheeting deal is always applied in pieces laid endwise,
with the end of the fibres towards the area of the pit. Since much of
the security from water depends on the tightness of the tub at its
jointing with the rock, several plans have been contrived to effect this
object; the most approved being represented in _fig._ 832. To make room
for the lower wedging crib, the recess is excavated a few inches wider,
as at _c_; and from _b_ to _c_, sheeting deals are laid all round the
circle, or a thin stratum of oakum is introduced. On this the wedging
crib _d_ is applied, and neatly jointed in the radius-line of the pit,
each segment being drawn exactly to the circle: and at each of its
segments sheeting deal is inserted. This wedging crib must be 10 inches
in the bed, and 6 inches deep. The vacuity _e_, at the back of the crib,
about 2 and a half inches wide, is filled with pieces of dry clean
reeded deal, inserted endwise; which is regularly wedged with one set of
wedges all round, and then with a second and a third set of wedges, in
the same regular style, to keep the crib in a truly circular posture. By
this process, well executed, no water can pass downwards by the back of
the crib. The next operation is to fix spiking cribs _f_, to the rock,
about 10 or 12 feet from the lower crib, according to the length of the
planks to be used for the tubs. They must be set fair to the sweep of
the shaft, as on them its true circular figure depends. The tubbing
deals _k_, must now be fixed. They are 3 inches thick, 6 broad, and
planed on all sides, with the joints accurately worked to the proper
bevel for the circle of the pit. The main cribs _g_, _g_, are then to be
placed as counterforts, for the support and strength of the tubbing. The
upper ends of the first set of tub-planks being cut square and level all
round, the second spiking crib _l_, is fixed, and another set of tubbing
deals put round like the former, having sheeting deal inserted betwixt
the ends of the two sets at _f_. When this is wedged, the cribs _h_,
_h_, are placed.
Oak cribbing is made with pieces of the best oak, from 3 to 4 feet long,
10 inches in the bed, and 7 or 8 inches deep.
The third mode of tubbing, by means of iron cylinders cast in segments,
is likely henceforth to supersede the wooden tubbing, from the great
reduction in the price of iron, and its superior strength and
durability. Each segment is adjusted piece to piece in the circular
recess of the pit cut out for their reception. The flange for the
wedging joint is best turned inwards. In late improvements of this plan,
executed by Mr. Buddle, where the pressure amounted to several hundred
feet, the segments were 6 feet long, 2 feet broad, and an inch thick,
counterforted with ribs or raised work on the back; the lip of the
flange was strong, and supported by brackets. These segments of the iron
cylinder are set true to the radius of the pit; and every horizontal and
perpendicular joint is made tight with a layer of sheeting deal. A
wedging crib is fixed at the bottom, and the segments are built up
regularly with joints like ashler-work. This kind of tubbing can be
carried to any height, till the water finds an outlet at the surface, or
till strata containing water can be tubbed off, as by the modes of
tubbing already described. A shaft finished in this manner presents a
smooth lining-wall of iron, the flanges being turned towards the outside
of the cylinders. In this iron tubbing, no screw bolts are needed for
joining the segments together; as they are packed hard within the pit,
like the staves of a cask. There is a shaft in the Newcastle district,
where 70 fathoms have been executed in this way, under the direction of
Mr. Buddle.
[Illustration: 833]
When a porous thin bed or parting betwixt two impervious strata, gives
out much water, or when the fissures of the strata, called cutters, are
very leaky, the water can be completely stopped up by the improved
process of wedging. The fissure is cut open with chisels, to a width of
two, and a depth of seven inches, as represented in _fig._ 833. The lips
being rounded off about an inch and a half, pieces of clean deal are
then driven in, whose face projects no further than the contour of the
lips; when the whole is firmly wedged, till the water is entirely
stopped. By sloping back the edges of the fissures, and wedging back
from the face of the stone, it is not liable to burst or crack off in
the operation, as took place in the old way, of driving in the wedge
directly.
[Illustration: 834]
_Ventilation of Engine Pits._--In ordinary cases, while the sinking of
the shaft is going on, the brattice walls produce a circulation, in
consequence of the air being slightly lighter in one compartment than in
another. If this does not occur, the circulation of air must be produced
by artificial means. The most approved contrivance is, to cover the
engine compartment of the shaft with deals, leaving apertures for the
pump-spears and tackling to pass through, with hatch-doors for the men,
and to carry a brick flue at least 3 feet square, in a horizontal
direction, from the mouth of that compartment to an adjoining high
chimney connected with a furnace, as represented in _fig._ 834. _a_,
_a_, are double doors, for the fireman to supply fuel by; _b_, the mouth
of the horizontal flue; _c_, the furnace; _d_, the ash-pit; _e_, the
furnace; _f_, the upright chimney for draught, from 50 to 100 feet high,
from 8 to 10 feet square at bottom, and tapering upwards to 3 or 4 feet
square inside. Such a furnace and chimney are also needed for
ventilating the coal-mine through all its underground workings. When a
great quantity of gas issues from one place in a pit, it is proper to
carry it up in a square wooden pipe, which terminating at some distance
above the surface in a helmet-shaped funnel, fitted to turn like a vane,
may cause considerable ventilation of itself; or the top of such a pipe
may be connected with a small fireplace, which will cause a rapid
current up through it, from the pit. The stones and rubbish produced in
sinking, are drawn up with horse-gins, when the pit is not deep; but in
all shafts of considerable depth, a steam engine is used, and the
workmen have now more confidence in them, as to personal safety, than in
machines impelled by horses.
The great collieries of Newcastle are frequently worked by means of one
shaft divided into compartments, which serves as an engine-pit, and
coal-pits, and by these the whole ventilation is carried on to an extent
and through ramifications altogether astonishing. This system has been
adopted on account of the vast expense of a large shaft, often amounting
to 60,000_l._ or 80,000_l._, including the machinery. The British
collieries, however, are in general worked by means of an engine-pit,
and a series of other pits, sunk at proper distances for the wants of
the colliery.