BOUNDARY SYSTEM OF MINING AND VENTILATION.
All will admit that the best mode of mining coal, and the most economical eventually, is to extend the workings, main avenues, and air-courses, to the extent of the "run," or boundary of the mine, and then work out all the coal in returning, by the "long wall," "breast and pillar," or "chamber and car" modes of mining.
The expense and time necessary to carry into effect this mode are its great and serious objections, and, in this country, may be considered effectual bars against its adoption. It is in use in some of the English collieries, in Lancashire ; but in the great Newcastle coal-field—the oldest and best-developed in Great Britain—it is but rarely, if ever, in use, partly owing to the same objections, but mainly because the system there generally in use presents all the benefits of the boundary system, as adopted near Manchester and elsewhere, and none of its objections; and a modification of this system to suit the peculiarities of our formations in the anthracite regions we now present. But we would here remark that the two plans presented embrace every character of dip, and may be adopted in horizontal as well as pitching seams; or the "workings" of any mine can be changed to this plan with benefit to ventilation and economy in mining.
The plan presented in figure 142 is designed for pitching seams,—say over 30°,—but may be used for all seams which are steep enough for the coal to run down the incline of the bottom slates without rehandling. All seams which do not admit of this should be worked by the plan represented in figure 144. This plan is equally applicable to shaft, slope, tunnel, or drift, above or below water-level: in fact, these considerations do not affect in any degree the mode proposed. We here present the slope method, as that is generally more applicable to steep-dipping seams than to those of low angles.
The plan represents the second "lift," showing the upper lift "worked out." In this design we present only enough of the mine—say 1000 yards on one side of the slope— to convey an impression of the plan proposed.
The slope and downcast air-course are represented by c, and the upcast air-course and pump-way by d. The gangway and inlet air-course is a, and the return air-course is b. The breasts are marked e, and the courses of the air denoted by arrows. The breasts communicate with the gangway by means of shutes, which may be one or two to each breast. To this point there is no change from the ordinary methods now in use, which
are as simple and effective as can be desired. But, in order to perfect the ventilation and secure the pillars from waste, we have laid out the mine in BOUNDARIES, which may be in small or large proportions, as the extent of the mine or character of the seam may suggest. We propose to make each boundary 500 yards in length, by the full breadth of the lift,—say from 200 to 300 feet, or more if found convenient. The boundary pillars must be larger than ordinary, and of sufficient strength to resist any pressure that might result from the excavation of the coal within the entire area, or 50,000 square yards of the boundary.
Nos. 1 and 2 represent two worked-out boundaries on the upper level; No. 3, one of the boundaries of the lower level. The principal object in laying out the mine into boundaries is twofold: first, to secure all the coal that is possible under any circum stance or by whatever method may be adopted, and to obtain it without injury to the permanent working of the mine, and without waiting for the usual "robbing" of the pillars. Second, to divide the air into separate columns and course it through each boundary and into the return air-course without carrying it through other portions of the mine. These two great objects are accomplished by this simple method without increasing the cost of original investment or the length of time required in opening out a colliery.
It will be observed that No. 4 may be under way at the same time with No. 3; or No. 3 may be exhausted without leaving a pillar, and yet no injury result to No. 4. In fact, the extraction of coal from a boundary of such considerable extent relieves the weight or pressure from the surrounding coal by bringing down the superincumbent strata over its immediate area, and thus letting down a certain amount of pressure, which would otherwise bear on the whole of the mine. In principle and effect it is the same as that followed and recommended by the best mining engineers; that is, the opening of the mine to its limits or main boundaries and extracting the coal in withdrawing or "working back." The only difference is that we lay out an extensive mine in several boundaries instead of one boundary, and effect the same ends with less time and expense.