BOUNDARY SYSTEM IN FLAT SEAMS.
In this plan we present the boundary system as applied to flat seams and those under 30° of dip, or up to the point at which coal will slide by gravity down the incline of the bottom slate.
As the former is designed for pitching seams where the coal is run in shutes into the cars in the gangway, this is designed for seams where the cars must follow the miner, or where the seams of coal are not steep enough to admit of the use of shutes.
The plan here presented displays the shaft instead of the slope mode, since most horizontal seams exist in comparatively shallow basins, and can be reached by shaft with economy, and drained and worked with more availability by the latter than the former mode. The main gangway, or inlet air-course, leading from the dgwncast shaft, is a, and the return air-course is b; c is the branch gangway, at right angles with the main one, and between the boundaries, or in the boundary or "barrier" pillar. From c the chambers d, d, are turned at right angles to the branch gangway, and parallel with the main gangway.
The chambers are worked onward to meet those advancing from the opposite side r f the boundary, and, when together, the "withdrawing" process is commenced, and all the available coal extracted, leaving the boundary "worked out," and the superincumbent strata on the floor of the seam.
We have only given a view of the first ends of two boundaries,—the first likely to be opened in a mine after sinking the shaft. It will be noticed that a mine can be opened and put in active and productive condition by this method sooner than by any other ; and that a large force of miners can be put at work in a shorter time than by any of the old methods.
When the seams are horizontal, the cars can be moved to all parts of the mine by horses ; but when the dip is over 10°, gravity inclines must be used in the branch gang ways, with a drum and endless chain, or some other mechanical arrangement to take the cars up the incline. There are several plans in use for this purpose, all of which are applicable, and work rapidly and effectively.
By this mode, a seam of almost any dip can be mined under 40°, and even above that, if desirable; but when the dip is steep enough the former mode is the most effective. When the seam is nearly flat, it may be best to carry the railroad in the middle of the chamber ; but when the dip is considerable, it is best on the dip side, against the pillar, and parallel with it, on account of the drainage of water and the handling of the coal. The inequalities of the dip do not affect this mode of mining, and confusion and dis order are almost impossible.
The seam may change from the horizontal to an inclination of 30° in a short distance, without materially affecting the order and plans of the respective boundary workings, since each is distinct and separate from the other. The boundaries may be small or large, as existing circumstances may dictate ; but we would recommend 500 yards as about the proper length, parallel with the main gangway. The breadth may be equal or greater than the length along the main avenue, but it should not be much, if any, over a square. If the extent of the property in the same basin is extremely wide, then the pillars between the boundaries should be large and strong, and several boundaries laid out along the branch gangways, and of course the barrier pillars left until the most distant side boundary is exhausted, and the withdrawal commenced at the "far end." This would not often happen in the anthracite region ; but in the bituminous, where the seams are nearly horizontal, it might be frequent and difficult to determine in which direction the main avenues should with most propriety be carried. But in any case of the kind the mode proposed is the only one available under all circumstances, and which may be carried to any extent without difficulty or derangement. The coal may be extracted entirely, or with but little loss, in large boundaries, and the super incumbent strata let gently down without danger to the remainder of the mine.