A PLAN OF A COAL MINE.
The progress that has been made in the science of mining coal within
the last half century bears favorable comparison with the progress
that has been made in the other industrial sciences. To-day the ripest
experience and the best engineering skill in the land are brought
to bear upon the problems connected with coal mining. In comparison
with the marked ability employed and the marked success attained in
the mining enterprises of to-day, the efforts of the early miners are
almost amusing. The pick and the wedge were the chief instruments used
in getting out coal. Powder was not thought to be available until John
Flanigan, a miner for Abijah Smith, introduced it into the mines in
1818. It is said that when openings were first made for coal in the
vicinity of Pottsville shallow shafts were sunk, and the coal was
hoisted in a large vessel by means of a common windlass. As soon as the
water became troublesome, which was usually as soon as the shaft had
reached a depth of twenty or thirty feet, this opening was abandoned, a
new shaft sunk, and the process repeated.
The mine operator of to-day, having decided upon the shaft as the best
method of entry into his mine, sinks it to the bottom of the coal
bed, so that its longest dimension shall be with the dip of the seam.
Then from each side of the shaft, and at right angles to it, he cuts
a passage out through the coal with a width of from ten to fourteen
feet. These are the beginnings of the “gangways.” Then from each end
of the rectangular foot of the shaft he cuts another passage, at right
angles to the first one, about six or eight feet wide, and extending
to a distance of from fifteen to thirty feet. These are the first
“cross-headings.” At the extremities of the cross-headings passages are
now driven parallel to the gangways. These last passages are called
“airways.” When the gangways and airways have reached a distance of
from sixty to one hundred feet from the foot of the shaft they are
united by new cross-headings.
It is now apparent that two pillars of coal, each from fifteen to
thirty feet wide and from sixty to one hundred feet long are left on
each side of the shaft. Larger pillars than these may be left if the
roof about the shaft should need more support. It is also apparent,
the coal seam being inclined, that the level of one of the airways is
higher than the level of the gangway, and the level of the other airway
is lower.
It will be remembered that the design was to sink the shaft so that its
foot should be nearly to the bottom of the synclinal valley or basin.
If this has been done, then it is possible that the passage below the
foot of the shaft parallel to the gangway actually runs along the
synclinal axis. But if the bottom of the valley is still lower, the
cross-headings will be driven farther down and a new parallel passage
made, and, if necessary, still another. These openings now slope from
the foot of the shaft downward, and in them is collected not only the
water that may fall from the shaft, but, as the work advances, all the
water that comes from all parts of the mine. This basin which is thus
made to receive the mine water is called the “sump,” and from it the
water is pumped up through the shaft and discharged at the surface.
If the mine happens to be a very wet one it will require the constant
labor of the most powerful pumping engine to keep the level of the
water in the sump lower than the foot of the shaft. In some cases, in
older workings, a section of the mine which has been worked out and
abandoned is used for a sump, and then the water may cover an area many
acres in extent. When a shaft has been newly sunk, the openings for
the sump are the only ones that are made below the level of the foot
of the shaft or below the level of the gangway. Henceforth all the
workings will be made on the upper side of the gangway, extending up
the slope of the seam, until such time as it may be deemed advisable to
sink an inside slope to open a new set of workings on a lower level.
The main gangway on one side of the shaft and the airway above it are
now carried along simultaneously, and parallel with each other, and
are united at distances of from forty to sixty feet by cross-headings.
As soon as the last cross-heading is opened the one which immediately
preceded it is walled up as tightly as possible. This is to insure
ventilation. A current of air comes down the hoisting-way of the shaft,
passes into the gangway and along it to the last cross-heading, where
it crosses up into the airway and traverses the airway back to the
cross-heading that was driven up from the upper end of the foot of the
shaft. Passing down this cross-heading it comes to the air compartment
of the shaft, and is drawn out to the surface by a powerful fan. This
is the ventilating system of the mine in its simplest form. It is
apparent that if any of the cross-headings nearer to the shaft than the
last one should be left open, the air current would take a short course
through it up to the airway, and so back to the shaft, without going to
the extremity of the gangway at all. This gangway is the main artery of
the mine; it is the highway by which all the empty cars go in to the
working faces, and by which all the loaded cars come out to the foot
of the shaft; it is the general watercourse by which the entire mine
above it is drained, and by which the water is carried to the sump. In
comparatively flat seams its height is the height of the slate or rock
roof of the coal bed, but in steep pitching seams it is made seven or
eight feet high with a roof wholly or partly of coal. In some cases
the roof and sides are so firm that no timbering is required, and in
other cases the timbering must be close and heavy in order to give
the necessary support and security. The floor of the gangway must be
given a constantly ascending grade, usually from six inches to one foot
in every hundred feet, as it is driven inward. This is to facilitate
drainage and the movement of loaded cars.
Where the strata are horizontal, or nearly so, as in many of the
bituminous mines, the gangway may, and usually does, take a perfectly
straight course. This is also true where the line of strike has but a
single direction, no matter how steep the pitch of the seam may be. But
both of these conditions are so rare in the anthracite regions that
one seldom finds a gangway driven for any considerable distance in
one direction. The surface of an inclined coal seam is not dissimilar
to the surface of one side of a range of small hills. Any one who has
seen a railroad track winding in and out along such a range, keeping
to the surface of the ground and preserving a uniformity of grade, can
understand why, for the same reasons, the gangway must often change
direction in following the seam of coal. It must curve in around the
valleys and hollows that indent the seam in the same manner that a
surface railroad curves in around the depression where some hillside
brook runs down to meet the stream, the course of which the railroad
tries to follow; and it must strike out around the projections of the
seam in the same way in which a surface railroad bends out around the
projecting spurs of the hill range along which it runs. But the coal
seam is more irregular and more uncertain in its outline than the
hillside, and the curves in it are sharper and more varied. The surface
railroad too may shorten its route and relieve its curves by bridging
its small valleys and cutting through its narrow ridges. For the
gangway this cannot be done. As a rule the coal seam must be followed,
no matter where it leads. And it often leads in strange courses,――in
courses that at times curve back on themselves like a horseshoe and
point toward the foot of the shaft. The mining superintendent or
engineer never knows in advance just what tortuous course his main
artery may take. He cannot go over the ground and stake out his line
as a civil engineer does for a surface railway; he must build as he
advances, not knowing what the rock and coal may hide in the next
foot ahead of him. He must be prepared to encounter faults, fissures,
streams of water, diluvial deposits, and every other obstacle known to
mining engineers.
There are several systems of laying out a mine for actual working
after the gangway has been driven a sufficient distance. The one most
commonly in use in the anthracite region is known as the “pillar and
breast” system. In the bituminous mines it is called the “pillar and
room,” and in the mines of Great Britain the “bord and pillar.” It will
be borne in mind that the mine which is now being described is in the
Wyoming region, where the seams are comparatively flat, the entrance
usually by shaft, and the method of working is the pillar and breast
system. The gangway and airway are not driven far, not more than two
or three hundred feet, perhaps, before the openings are made for the
larger production of coal. Beginning on the upper side of the airway,
at such a distance from the shaft as will leave a reasonably large
sustaining pillar, perhaps from sixty to one hundred feet, an opening
is made and driven up the seam at right angles to the airway. This
opening is called a “chamber” or “breast.” In the bituminous districts
it is known as a “room.” The chamber is usually about twenty-four
feet wide, though where the roof is exceptionally good its width may
be increased to thirty-six feet. It is not often opened the full
width at the airway. Instead of this a narrow passage, large enough
to accommodate the mine car track, is driven up to a distance not
exceeding fifteen feet, and it is from this point that the chamber is
driven up at its full width. This narrow opening can be more easily
closed in case it is desired to prevent the passage of air through
it, and besides a greater proportion of coal is left in pillars along
the airway to prevent the passage from becoming blockaded by falls.
When the first chamber has been driven up a distance equal to its
width, a new chamber is begun parallel to it and on the side farthest
from the shaft. These two chambers are now separated by a wall of coal
from fourteen to twenty feet thick. If, however, the workings are
deep and there is danger from the weight of superincumbent strata,
the wall should be made as thick as the chamber is wide. When the new
chamber has been driven to a distance of twenty-five feet, or, if the
mine is free from gas and the ventilation is good, to a distance of
forty or sixty feet, the wall between the two chambers is pierced by
an opening from six to ten feet wide. This is called a cross-heading
or “entrance.” A partition is now built across the airway between the
openings to the two chambers, and the air current is thus forced up
into the last chamber, across through the entrance into the first, down
it to the airway again, and so in its regular course back to the foot
of the shaft. In the mean time progress has been made in the first
chamber, and by the time the second chamber has been driven another
distance of thirty or sixty feet, the entrance which will then be cut
through the wall will find the first chamber still in advance. The
inner extremity of the chamber is called the “face.” It is sometimes
spoken of also as the “breast,” though this last name is properly
that of the chamber as a whole. The wall of coal at the side of the
chamber is called the “rib.” A third chamber is now begun and driven up
parallel to the other two, then a fourth, a fifth, and so on; as many
chambers, indeed, as can be laid off in this way without deviating too
greatly from a right angle to the airway. But the face of the first
chamber is kept in advance of the face of the second, the face of the
second in advance of the face of the third, and so on, until the limit
of length is reached. This limit is determined, to some extent, by the
dip of the seam. In comparatively flat workings a set of chambers may
be driven in to a distance of five hundred, or even six hundred feet.
Where the pitch is steep, however, two hundred or three hundred feet is
the greatest length at which chambers can be economically worked. The
limit of length of chambers is sometimes determined also by an outcrop,
an anticlinal axis, a fault, or a boundary line. The wall of coal left
between any two chambers is divided by the entrances cut through it
into a line of pillars nearly uniform in size. As soon as the second
entrance from the airway is cut through the wall the first entrance is
blocked tightly up, and as soon as the third entrance is cut through
the second is closed, and so on to the extremity of the line of
pillars. This is to compel the air current to pass up to the very face
of the chamber before it can find a way across to the other chambers
and down again into the airway. If the air of the mine is bad, or if
the coal is giving off deleterious gases with rapidity, a “brattice” or
rude board partition is built from the lower side of the last entrance
diagonally up toward the face of the chamber to force the air to the
very point where men are working before it finds its way out through an
open entrance. These boards are sometimes replaced by a sheet of coarse
canvas, called brattice cloth, which is lighter, more easily handled,
and answers the same purpose.
[Illustration: A PLAN OF AN ANTHRACITE MINE WITH A SHAFT ENTRANCE.]
From the mine car track in the gangway a branch track is built,
crossing the airway and running up each chamber to its face. Up this
branch track a mule draws the empty car, and when it is loaded it is
let down to the gangway by the miner’s laborer. If the dip of the
chamber is too steep――more than ten degrees――for a mule to draw the
car up, a light car, used only in the chamber and called a “buggy,” is
pushed up by hand, and when the dip is too steep for this the coal is
pushed or allowed to slide down to the foot of the chamber. Chambers
are often driven up obliquely in order to reduce the grade, or are
curved in their course for the same reason.
When, on account of the steepness of pitch or a change in the direction
of the gangway, or for any other reason, one set of parallel chambers
is brought to a close, a new set is begun farther along with a
different course.
The direction in which a gangway, airway, or chamber is to be driven is
fixed by the mine boss. His bearings are obtained with a small miner’s
compass, and he marks on the roof, near the face of the opening, a
chalk line in the direction desired. The miner, sighting back on this
line, is thus able to take his course and to keep his opening straight.
Sets of chambers similar to those described are driven up from the
gangway along its entire length. This length may be limited by various
causes. A boundary line of property, a fault, a thinning out of the
coal seam, are some of them. They are usually driven, however, as far
as strict principles of economy will allow. A gangway that requires no
timbering and is easily kept in good working condition may be driven
to a distance of three or four miles. But where these conditions are
reversed, a mile may be as great a distance as coal can be hauled
through with economy. Beyond that limit it will be cheaper to sink
a new shaft or slope than to increase the distance for underground
haulage.
As the main gangway progresses inward it may separate into two
branches, each following a depression in the coal seam, and these
branches may separate into others; so that there may be a number of
gangways all keeping the same general level, from each of which sets
of chambers are driven. When the chambers tributary to a gangway
have reached their limit of length, and there is still an area of
coal above them to be mined, a new gangway is opened along the faces
of the chambers, or is driven just above them in the solid coal, and
from this, which is called a “counter-gangway,” new sets of chambers
are driven up the seam. It is often necessary to raise and lower cars
passing from one gangway to the other on an inclined plane, on which
the loaded cars, descending, and attached to one end of a rope, pull
up the light cars, ascending and attached to the other end, the rope
itself winding around a revolving drum at the head of the plane. This
system can be put into use on any incline where the gradient is one in
thirty, or steeper.
By this general system of gangways, counter-gangways, airways,
chambers, and planes, the area of coal lying on the upper side of the
main gangway and on both sides of the shaft is mined out, hauled by
mules to the foot of the shaft, and raised to the surface. On long
straight gangways the mule is sometimes replaced by a small mine
locomotive, and in these later days the electric engine has been
introduced into the mines as a hauling agent.
So far, however, in this mine which we are supposed to be working, not
a tap of a drill nor a blow of a pick has been made into the coal on
the lower side of the gangway save where the sump was excavated at the
foot of the shaft. If this shaft has been sunk nearly to the bottom
of the basin or synclinal axis, a short tunnel may be driven from the
main gangway through the rock or upper bench of coal across the valley
to the rise of the seam on the other side. A new gangway may here be
driven right and left, and this area of coal be made tributary to the
shaft already sunk. It often happens that a large body of coal lies
between the main gangway and the synclinal axis, for these two lines
may diverge greatly as they recede from the shaft. But chambers cannot
be driven down from the main gangway owing to the difficulties of
transportation and drainage. It therefore becomes necessary, in order
to work this area, to sink a slope from the main gangway down to or
toward the synclinal axis, and from the foot of this slope to drive a
new gangway. From this new gangway chambers will be opened extending up
the seam to the line of the main gangway, but not generally breaking
through into it. The coal is run down to the lower level gangway,
hauled to the foot of the slope, and hoisted up it to the main gangway.
It is apparent, however, that the inclined plane system cannot work
here; the conditions are reversed; the loaded cars are drawn up and
the light ones are let down. To do this work it is necessary to bring
into use a small steam stationary engine, or one working by compressed
air. A common method is to locate the steam engine on the surface
vertically above the head of the underground slope, and to carry power
to the sheaves below by wire ropes running down through bore holes
drilled for that purpose.
The system of slope mining by lifts, which is in common use in the
Middle and Southern anthracite districts, has been explained in a
preceding chapter. In this system the sump is always made by extending
the slope a short distance below the level of the gangway. This gangway
is driven from the foot of the slope to the right and left in the same
manner as in the Wyoming region, except that, the seam being so greatly
inclined, the gangway roof, or a part of it at least, will usually be
of coal instead of slate or rock, and in very steep pitching seams the
airway will be almost vertically above the gangway. The gangway is not
usually so crooked as where the workings are flat, and having been
started only three hundred feet down the slope from the surface, it
often follows the coal to some low point on the line of outcrop, and is
then known as a water level gangway, which is practically the same as a
drift.
The system of opening and working breasts differs somewhat from that
in use in the Northern field. Beginning at such a distance from the
foot of the slope as will leave a good thick slope pillar for its
protection, a narrow shute is driven up from the gangway into the coal
to a distance of perhaps thirty feet, at a height of six feet, and
with a width of from six to nine feet. It is then opened out to its
full width as a breast and continued up the seam toward the outcrop,
not often breaking through to daylight unless an airway or manway is
to be made. Parallel breasts are then laid off and worked out by the
usual pillar and breast system. If the dip is less than twelve or
fifteen degrees, the coal may be run down from the working face in a
buggy, dumped on to a platform or into the shute, and loaded thence
into a mine car standing on the gangway. If the dip is more than
fifteen degrees the pieces of coal will slide down the breast to the
shute, though if it is under twenty-five or thirty degrees the floor of
the breast should be laid with sheet iron to lessen the friction and
give greater facility in movement. In a steep-pitching breast a plank
partition is built across the shute just above the gangway, to hold
back the coal until it is desired to load a car with it. This partition
is called a “battery,” or, if there is a similar partition to hold the
coal in the breast, a “check battery.” In this partition there is an
opening through which the coal may be drawn when desired, and through
which the men may also go to their work, though a separate manway is
often provided. In these steep-pitching breasts the miner works by
standing on the coal which he has already mined, and which is held back
by the battery, in order to reach the uncut coal above him. There are
various systems of shutes, batteries, man ways, etc., in use, but
all are based on the same principles.
[Illustration: GROUND PLAN AND LONGITUDINAL SECTION OF CHAMBER.]
When the gangway of the first lift has reached its limit in both
directions, and the breasts from it have been worked up to their limit,
the slope is sunk to another distance of three hundred feet, and the
process is repeated. From the gangway of the second lift the breasts
are not extended up far enough to break through into the gangway
above; a wall of coal is left between that gangway and the faces of
the breasts, from fifteen to forty feet in thickness, known as the
“chain-pillar.” This is for the protection of the upper gangway against
falls and crushes, and is also necessary to hold back water from
escaping into the lower level. These lifts will continue, at distances
of about three hundred feet apart, until the synclinal valley is
reached.
When the method of opening the mine by a shaft is employed in these
steep-pitching seams, the shaft is sunk to the lowest level, and the
successive sets of gangways and breasts are laid off as the work
progresses upwards; that is, the slope method of extending the lifts
downwards is simply reversed.
The method of mining by tunnel and drift, and by slope in the flat
workings, is not different from the method already described for
shafts. So soon as the drift, tunnel, or slope has extended far enough
into the coal seam it becomes a gangway, chambers are laid off from
it, and mining goes on in the familiar mode.
Various modifications of the pillar and breast system are employed in
the anthracite coal mines, but no system is in use which is radically
different.
In the “long wall system,” common in Great Britain, and used to some
extent in the bituminous mines of Pennsylvania and the Western States,
the process of cutting coal is carried on simultaneously along an
extended face. The roof is allowed to fall, back of the workers, roads
being preserved to the gangway, and the roof at the face is temporarily
supported by an abundance of wooden props.
The descriptions of underground workings that have now been given have,
of necessity, been very general in their character. It is impossible,
in a limited space, to describe the various methods and modifications
of methods which are in use. No two mines, even in the same district,
are worked exactly alike. Sometimes they differ widely in plan and
operation. That system must be employed in each one which will best
meet its peculiar requirements. There is large scope here for the play
of inventive genius. There is scarcely a mine of any importance in
the entire coal region in which one cannot find some new contrivance,
some ingenious scheme, some masterpiece of invention devised to meet
some special emergency which may have arisen for the first time in
the history of mining. Yet the general features of all coal mining
methods must of necessity be the same in underground workings. No one
reasonably familiar with them could ever mistake a map of a coal mine
for a map of anything else under the sun.