Coal and the coal mines

by Homer Greene

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.

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.