Machine Mining in Anthracite Mines.-Owing to the exhaustion of the thicker seams and the rapidly increasing cost of mining the thinner seams, the anthracite operators in northeastern Pennsylvania have begun to use undercutting machines of the ordinary bituminous-coal mine type. The method of using the machines differs in no way from that employed in the soft-coal fields, although numerous ingenious expedients have been adopted to overcome local difficulties. The machines employed are mostly of the shortwall style that propel themselves along the face. In one place they are being used on a 15° pitch and can be used on pitches of as much as 20° to 25°. these pitching places, after the sumping cut is made, an iron rail is placed back of the machine and parallel to the face of the room. This rail, which is held in place by jacks, holds the machine up to the face and along it the machine slides to the end of the cut. The rail is, of course, moved up after each cut is made and the coal shot down. The machine has the necessary power to pull itself I up to the face and to make the sumping cut at the same time.

DRAWING PILLARS

In

The work of drawing back, or robbing, that is, removing the pillars left in the first working, should be commenced as soon as the rooms are worked up their full length, whenever this is possible. If this is delayed, and the openings left to stand for any length of time, the roof will settle heavily on the pillars and there will be danger of crushing them and thus losing the coal. Fig. 1 shows the way of drawing the pillars in rooms turned off one side of a single pair of entries where the drawing begins next to the main entries and progresses inbye. The drawing of the pillars has been completed in rooms 1, 2, 3, and 4, down to the entry pillars a, which are left and these rooms are closed; the work on pillars 5, 6, and 7 is in progress; rooms 8 and 9 have reached the limit, and the

work on the pillar separating rooms 8 and 9 will now begin. The rooms inside of number 9 have not been completed, and the last room on this pair of entries has just been turned. After the cross-entries b and c have been completed and all of the rooms off them completed and the pillars between the rooms drawn, the pillars between the entries b and c and the stumps a will be drawn back to the main entries d. It is advantageous to carry on the pillar drawing systematically and to keep the ends of the pillars in a line to avoid excessive pressure being brought on a single pillar by the drawing of the pillars on either side of it. Unless the ends of the pillars are kept in line, there is also increased danger from falls, and the work of drawing timber is made more difficult. If rooms are driven toward each other from adjoining cross-entries, a pillar should be left between the ends of the rooms and removed when the pillars between the rooms are taken out.

1515

Fig. 2, page 635, shows the reverse method of drawing the ribs, that is, from the inby end of the entries or headings toward the haulage roads. The cross-headings are driven in pairs off the main headings, and off of one of these cross-headings other headings called butt headings, are driven. These butt headings divide the mine into panels and they are driven their full length, but so as to leave a chain pillar c between the end of the butt headings and the next pair of cross-headings. The rooms are then started from the inbye end of the butt headings, and as soon as the first or inbye room has been driven up to its full length, leaving a chain pillar between it and the next pair of butt headings, the pillars are drawn, as shown. As soon as the pillars have been drawn back to the butt headings, the pillars between the butt headings, and the

FIG. 2

chain pillar between the lower butt heading and the next lower tier of rooms are drawn back as shown. The advantage of this method over that illustrated in Fig. 1 is that both the room pillars and the entry pillars are drawn back in one operation, instead of the room pillars being first drawn and the entry pillars and stumps drawn subsequently. A greater portion of the total amount of coal is probably obtained in this manner when it is possible to carry it out. The method of drawing back the pillars illustrated in Fig. 2 is similar to that just described, but it is extended over a much larger area. The crossheadings a are driven to the boundary of the property before the robbing is begun; from these headings the butt headings b are driven, and off these

narrow rooms, as shown. The pillars over the entire length of the property are then brought back at one time.

Work of Drawing:-When the work of drawing the pillar is to begin, a cut-through is driven from the face of each room to the face of the room adjoining so as to give a free face across the end of the pillar. There are a number of ways of attacking the pillar, the choice of a method being determined by local conditions and

FIG. 3

custom.

Fig. 3 illustrates one way of drawing back pillars separating wide rooms in which there is a track along each rib of the pillar. The work of drawing back the pillars is shown as having just begun, the pillars having been cut through at the face,

and the first shots having removed the coal at each corner of the pillar. The second holes a will remove the remainder of the first slice across the pillar and the holes b the first cut of the second slice. The coal is thus removed in steps. A row of props c keeps up the roof along the face of the pillar. As the pillar is drawn back a sufficient distance, a second row of props similar to that shown at c is stood across the face of the pillar, parallel to the first row c, which is then withdrawn. In pillar drawing, the back timber should be drawn only so fast as to throw a proper weight on the pillar. If this weight is excessive, the end of the pillar is crushed. An excessive weight is also thrown on the pillars by leaving too much timber standing. Just how much timber to use can be determined only by experience. If the pillar is very wide, a slice or skip may be taken off it from the entry to the face before the pillar is cut across at the face.

When the pillars have been drawn back to the entry stump at the mouth of the room, where the room begins to narrow toward the neck, care should be taken to break the roof, if necessary, back

to the entry pillar or stump. With a hard roof, it may be necessary to place one or two shots in the roof at this point. By this means the entry pillars are relieved of excessive pressure due to the settlement in the abandoned rooms, which have been closed by the drawing of the pillars.

Fig. 4 shows other methods very commonly used for drawing pillars in both flat and inclined seams, known as splitting a pillar. In the method shown in (a), the opening b is driven up the center of the pillar as wide as the strength of the roof will permit without crushing the pillars left between this opening and the rooms. Each of these small side pillars is then drawn back in slices by a method similar to that shown in Fig. 3. In Fig. 4 (b), the pillar is shown divided into a number of small pillars by cross-cuts c. Each of

(b) FIG. 4

these small pillars is then divided lengthwise as shown at d. In (c), the pillar is divided up its full length by a narrow place e. This narrow place and the break-throughs f divide the original pillars into a number of small pillars g. These small pillars are next broken up by other cross-cuts h, leaving still smaller pillars, and these are then taken out by shots, as shown.

Fig. 5 shows some of the methods used in robbing the pillars in steep pitching, thick beds of anthracite. To get the coal out of the pillar at the left of A, a skip is taken off the side, as shown. Successive skips are thus taken off until the whole is removed, the miner keeping the manway open to the heading below as a means of retreat. The pillar between A and B is very similarly

worked. To remove that between B and C, a narrow chute or heading is driven up the middle, and cross-cuts put to the right and left a few yards from the upper end. Shots are placed in the four blocks of coal thus formed, as shown, and they are fired simultaneously by battery. This operation is repeated in each descending portion unless the pillar begins to run. pillar from which the coal has started to run is shown to the right of C.

A

Delayed Pillar Drawing.-The work of drawing pillars between the rooms is sometimes preferably delayed until the entries have been driven to the boundary and the rooms also worked up to that point, when the work of drawing pillars will be commenced at the boundary and proceed uniformly toward the mine opening. This may be necessary in the working of two beds separated by only a few feet of solid strata where a number of overlying beds are worked; or in certain cases where the bed is overlaid by water-bearing strata, and where the breaking of the roof rock would result in the flooding of the mine.

FIG. 5

When this method is used, a constantly increasing extent of airways and roadways must be kept open and in repair, until the robbing begins, while the difficulties of ventilation are also increased. Again, the pillars first formed are last removed and there may be a loss from depreciation of the pillar coal due to weathering and also from the crushing of the pillars, unless much larger pillars are left than are required when the pillars are drawn as soon as the rooms are finished. With fairly thick and very soft coals, the rapid working up of the rooms and equally quick drawing of the ribs as soon as the rooms are driven their full distance, is essential to economical working; for delay in extracting ribs and pillars in such circumstances results in their getting crushed and the coal lost or largely ground to slack.

Precautions in Pillar Drawing.-When two or more overlying seams are worked simultaneously, the pillars in the lower seam should not be removed until the upper seams have been worked out and the pillars drawn.

It is not generally advisable to attempt to draw the pillars in a limited area surrounded by a district in which the pillars are not drawn, particularly under a hard roof, as an excessive weight will then be thrown on the pillars left standing and a disturbance set up that may extend a long distance from the immediate district from which the pillars are removed.

In very gaseous mines, the pillars are sometimes not taken out until the workings have reached a considerable distance from the shaft, in order that there may not be accumulations of gas in the gob and waste near the shaft, since it is often more difficult to prevent gas accumulations in robbed workings than when the pillars are left standing. If the coal is tender, the removal of the pillars should be delayed if the roof will not fall readily, because if they are taken out, excessive pressure may be brought on the entry pillars. In the case of bad roof, the pillars should be taken out as soon as possible, not only for economy, but also because, when the roof is bad and falls freely as the pillars are drawn, the débris soon sustains the superincumbent pressure and relieves the weight on the pillars next to the entries. Early drawing of pillars also concentrates the working district, and, excepting in a gaseous mine, reduces the area to be ventilated.

With a strong roof that does not break readily when the pillars are removed, great care must be taken that the removal of the room pillars does not bring sufficient weight on the entry pillars to crush them. A weak roof falls freely and soon fills up the gobs, thus partly sustaining the pressure from the roof and relieving somewhat the weight on the pillars along the side and main roads, due to the layers of rock immediately above the coal.

If the roof has fallen in the rooms before the work of drawing begins, the strata above the pillar can usually be kept up comparatively easily by props about the working face, without great danger to the miner, but where the roof remains over the rooms, excessive pressure is often thereby thrown on the pillars and the work of drawing is very dangerous and treacherous; under such circumstances, the whole pillar can rarely be removed, as it will usually crush before it can be taken out.

In drawing pillars, their ends should be kept in a straight line, for if they

are not, some pillars are subjected to greater pressure than others, valuable coal is lost, and the work is materially interfered with.

Especial attention should be paid to the effect of the removal of the pillars on the surface and the overlying strata, particularly if the latter are waterbearing or contain running materials, such as quicksand.

The work of drawing pillars is particularly dangerous where faults or slips are frequent in the roof, or top coal is to be taken down, or where pot bottoms, sink holes, boulders, etc. are of frequent occurrence in the roof, or where the workings underlie or approach buried valleys or extensive beds of quicksand. Where the pillars are crushed and creviced, blown-out shots are liable to occur in their working. Undermining, in pillar work, should be done with caution. Pillar coal can sometimes be undermined with machines, but the practice is not general and hand work is usually depended on. Small stumps,

or portions of pillars should not be left scattered through the gob, as they interfere with the uniform breaking of the top.

The conditions at different mines are so varied that no general rule can be laid down to suit all. There is probably no more dangerous work in mining than pillar drawing and the method adopted depends largely on local conditions and on the experience of the miner.

LONGWALL SYSTEM OF MINING

SYSTEMS OF LONGWALL

In the longwall system of mining, the coal is taken out in a single operation, the face of the workings advancing in an unbroken line or wall; no pillars of coal are left to support the roof which is allowed to fall and settle behind the miners as the workings advance. The accompanying illustration shows the general system of longwall followed in the United States. From the shaft, or point from which the workings are opened out, a number of roads are kept

open for haulage purposes to the working face, as shown. Roads a, d, e, h and i are called main roads, or main entries, as they are kept open permanently. The roads d and e are sometimes called diagonal main roads, or main diagonal roads, as they cut diagonally across the workings between the straight roads a and h and a and i. Each of the broken straight lines represents a temporary road from the face to the nearest haulage road. The distance between any two of these broken lines constitutes a working place, or a room. The crossroads b, c, f, g, j, k are driven off the main roads usually at an angle of about 45°, and their purpose is to cut off the working rooms from time to time so that haulage distances may be reduced. The distance between crossentries, or the limiting lengths, of the rooms is determined by the time it is possible to keep the temporary roads from the face to the main or cross-roads

open.

The waste material from the seam, roof, or floor, is built in the form of pack walls along each side of the roadways and in the spaces between, from which the coal has been taken out. The pack walls, or packs, lining the roadways, are called road packs, and those between the roads gob packs.