Coal and the coal mines

by Homer Greene

To the early miners the drift was the favorite mode of entry. Finding an exposed seam of coal in the face of a ledge or cliff, they would dig in on it and bring the coal out from the opening in wheelbarrows. A place was selected, if possible, where a creek or river ran at the base of the ledge, and the coal was dumped from the wheelbarrow directly into a boat. In default of a water way a wagon road was built at the foot of the hill or cliff, a platform extended out over it, and the coal was thus loaded from the wheelbarrow into the wagon.

CROSS SECTION OF DRIFT OR GANGWAY WITH TIMBERS AND LAGGING.

The modern drift, though fashioned on an improved plan, is the simplest and least expensive way of making an entrance into a coal mine. The outline of the proposed opening is first marked out on the edge of the exposed coal seam. From fifteen to eighteen feet is an ordinary width to accommodate two tracks, and ten feet will readily accommodate one. Seven feet is an average height, though, if the seam be comparatively flat, the coal will be taken down until the rock is reached, even though a greater height should be attained. With this width and height the opening is cut into the hill through the coal seam. The floor of the drift must have a constant upward grade as it progresses inward, in order that the water may run out, and that loaded cars may be hauled more easily. The mouth of the drift must be above the level of the adjacent valley or stream, so that the water may be carried away, and the drift is therefore what is known as a water-level opening. It is usually necessary to support the roof and sides of the drift by timbers joined together in the form of a bent, and placed more or less closely to each other. These timbers are also sometimes lined by sticks placed behind and over them horizontally, and known as “lagging.” It will be seen that the conditions under which the opening by drift may be made place a serious limitation on the use of this method. It will also now be seen why the drift is the simplest and most economical mode of making an entrance to a mine. In this method there is no expense for removing earth or for cutting through rock, nor any cost at any time of pumping water or of hoisting coal. When the fact is remembered that it sometimes costs from $50,000 to $100,000 to sink a deep shaft through hard rock, and that to this amount must be added the cost of buildings, machinery, and repairs, and the perpetual cost of pumping water and of hoisting coal, the economy of the drift method will be appreciated. But the day of drift mining in the anthracite regions has gone by. Those portions of the coal beds lying above water level have been largely mined out, and the areas of coal that are now accessible by drift are very limited. In the bituminous districts, however, where the seams lie comparatively flat and the coal is mostly above water level, the method by drift is still almost universally used.

Next to a drift, the tunnel is the simplest and most economical method, under certain circumstances, of making an entrance into a mine. This is a passage driven across the measures, and at right angles to the seam, in order to reach coal which at the point of opening is not exposed. The tunnel is usually driven into the side of a hill. The earth is first dug away until the rock is exposed, or, if the soil be too deep for that, only enough of it is taken to make a vertical face for the mouth of the tunnel. The opening is then driven into the hill at about the same width and height that a drift would be made, and in practically the same manner. If there is a section of earth tunneling at the mouth, the timbering must be close, and the lagging will be of heavy planks. When the solid rock is reached, however, it is not often that any timbering is necessary, the sides and roof being so hard and firm as not to need support. This passage is driven against the face of a coal seam, and when the coal is finally reached the tunnel proper ends, a passage is opened to the right and one to the left along the strike of the seam, and from these gangways the coal is mined. The tunnel, like the drift, must be above water level, and its floor must have a descending grade toward the mouth, to carry off water. The expense of the tunnel, and its superiority to the slope or shaft, will depend upon the distance through which the rock must be pierced before coal is reached. It is especially advisable, therefore, before opening a tunnel, to have an accurate map of the location and dip of the coal seams to be struck by it, otherwise no approximate calculation can be made of the extent or cost of the work.

In the anthracite districts, where the seams are sharply pitching, tunnels are driven in the interior of a mine from the workings of a seam already opened across the intervening measures to strike an adjacent seam. In this way two, three, or more coal seams can be worked, and the coal can all be brought out at one surface opening. This is virtually the only kind of tunneling that is now done in the anthracite regions; for, as has already been explained, the coal that lay above water level and was thus accessible by tunnel has now been mostly mined out.

If there is an outcrop of coal on the tract to be mined, and the dip of the seam is more than twenty degrees, it is usually advisable to enter the mine by means of a slope. This is a passage which, beginning at the outcrop, follows the coal seam down until the necessary depth is reached. It is driven in the coal. The distinction between the drift and the slope is that the drift is driven from the surface on the strike of the seam while the slope is driven on its dip. Where the coal seam comes within a moderate distance of the surface, as at an anticlinal ridge, a slope may be driven through the rock until the coal is reached at the axis, and from that point follow the seam down. Sometimes a shaft is sunk to the top of an anticlinal ridge, and from its foot two slopes are driven, one down each side of the roll, in opposite directions. If the seam is very irregular, or if it is much broken by faults, there may be a great deal of rock cutting to be done in order to preserve the uniformity of grade necessary for the slope. The cost may, indeed, in this case, amount to more than would have been sufficient to sink a shaft to the same depth, although, as a rule, the entrance by slope should cost only about one fourth of that by shaft.

CROSS SECTION OF SLOPE WITH DOUBLE TRACK.

The same methods are employed in sinking a slope as are used in driving a drift, except that generally the timbering need not be so heavy. The minimum height of the slope is about 6½ feet, the width at the top, or collar, about 8 feet, and the width at the bottom, or spread, about 12 feet. If a double track is desired the spread should be 18 feet and the collar 14 feet. In the Wyoming region, where the dip is usually less than twenty degrees, with infrequent outcrops, the slope is not in general use; but in the Southern coal field, where the dip varies from twenty degrees to the vertical, the slope is the most common method of entering a mine. There the opening is driven down for a distance of 300 feet, at which point gangways are started out to right and left, along the strike, and chambers driven from them back toward the surface. This is called the first lift. The slope is then continued downward for another distance of 300 feet, new gangways and chambers are laid off, and this is called the second lift. This process is continued until the synclinal basin is reached.