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in the wisdom of flushing. We know that we have been repaid and that it has all been worth while.

Culm fills are more effective in fighting mine fires than clay. Clay in drying contracts from the roof and ribs leaving air spaces; culm packs tight and remains so. Kingston average dip is 12°. Occasionally in rolls or heavy dip or isolated places we run a box or pipe up the chamber for drainage back through the battery. It is well to have packed mine rock inside the wood battery to reinforce and take the weight off the wood. The more cog or packed rock in a chamber, the more effective the silting; the rock acts like headers, stretchers, or binders in a wall or solid cog. Silting is a big asset also in its supporting of walls, headings, etc., making them air-tight and thus assisting ventilation. It also fills the spaces of old workings that otherwise might be dangerous from accumulation of gas. Silting districts should be patrolled and examined for gas, both explosive and noxious. In some cases we have found it to advantage to take out the lower part of the pillar first, then fill with silt, working backwards up the grade, pulling the coal to the upper level.

REVERSE MINING OF THICK OR THIN COAL BESS

One problem in silting old workings in an anthracite bed is the filling of just one working place at a time. To fill places one by one requires the construction of a battery at the lower end of each place. Batteries are expensive to build and often expensive to maintain while the silt is being flushed in behind them under pressure.

Silting an area where first mining has been done is not as difficult as silting an area that has been mined, silted, and, at some later time, reopened. In silting a first-mined area that has been mined by modern methods only rarely is it necessary to silt each working place separately unless the gangway must be kept open. The pillars left are large enough to prevent caving of the workings until all first mining has been done in that section or gangway; then batteries to hold back the silt and keep it out of other workings can be built at points on the gangway and airway, dividing them into sections of three or four rooms each. Silting can then start from the inside and retreat, the flow alternating from room to room. When one section is silted, the pipe can be withdrawn to the next. In other words, the silt can practically be allowed to broadcast itself over the section until it reaches the roof; thus it is quite possible that only two batteries will be needed to silt two to five rooms.

When such a silted area is reopened to recover the pillars, new gangways are driven at the lower end of the silted area and slices are taken off of the pillars as they are driven up. If the roof is so weak that the silt filling will not keep it up, resilting must be done in order to take the whole pillar by slabbing or slicing. Then the problem is to build batteries in each of the places so that the space left by the slice taken from the pillar can be filled with silt. If a battery is placed at the lower end of such a room, it not only has to silted pillar; even an old battery if present has to be strengthened, lest the silt when turned in rushes out through the adjoining silted pillar. If such a battery is built across both places, then as the silt rises in the room to the first cross heading this immediately becomes a point of weakness in the silt that lies in the adjoining rooms, and it may be necessary to build batteries at the ends of these rooms to retain the silt they hold in place. Therefore the work becomes extremely expensive.

SILTING REOPENED AREAS AT NO. 2 COLLIERY

To decrease the expense of batteries and allow silting to be conducted at any time without endangering with rushes of silt the lives of the men who are working beyond the point being silted, the Kingston Coal Co. at its No. 2 colliery has evolved a new method of mining that obviates the danger and difficulties involved in resilting areas that have been reopened. This method was first tested successfully in the Orchard bed, which is 4 to 5 feet thick. This method permits any one room to be silted without interfering in any way with work in other rooms or endangering men by rushes of silt.

Description of system.—The system is known as "reverse mining," and is being used in the Cooper, Bennett, Ross, and Red Ash beds at the No. 2 colliery in the No. 2 shaft area of the Kingston Coal Co., Edwardsville, Pa.

Where a gangway is driven through the whole first-mined area that has been silted, the slices in the pillar are turned to the dip instead of the rise, and as the pillars are about 36 feet thick an 18-foot slice is taken along one side advancing. (See map, fig. 43.) Then the rib on one side is coal and on the other side silt. These slabs are taken from- alternate pillars, leaving between each working place one pillar of coal and two pillars of silt to support the roof.

When the slice has been taken along this pillar to its lowest point— that is, to the gangway immediately below the one on which second mining is started—the work is stopped, and that working place is immediately silted. If first mining has been completed in the whole territory, the space left by slabbing the pillar can be filled with silt. It is not necessary to build a battery at the lower end because the lower gangway and workings have already been silted and there are no voids, but the silt therein is so porous that the water used in silting the space in the pillar percolates downward through the old workings to the lowest level in the mine. The mine pumps which usually are at this point lift the water to the surface. The men at work in adjoining chambers are protected by a pillar of coal and a pillar of the room to be silted is up the dip from these men, silting is postponed until the men have driven the place to the limit.

After the space has been filled with silt it is allowed to stand for about 4 months. The time required for silting a room 18 feet wide, 18 feet high, and about 400 to 500 feet long is about 10 workingdays.

Eventually work can start again on the remainder of the pillar, which can be entirely recovered. When all of the pillar is recovered there will be an opening with ribs wholly of silt; this space in turn is filled, and the roof then rests on a bed of silt instead of coal.

At this colliery it is customary to recover every other pillar, replacing it with silt, while the gangway is advancing through the old workings. When the limit of the property is reached the remaining pillars are mined on the retreat by the same method as on the advance, and the entire area is thus filled with silt. When silting is completed work can start on the gangway immediately above on the pitch of the coal, and all the coal can be recovered therefrom without any danger to the mine or to the men if the precautions already mentioned are taken. (See figs. 44 and 45.)

Advantages of system.—This system of mining has some advantages, as the places are working .to the dip and the weight of the overlying strata is continually bearing on the face of the coal tending to weaken it; indeed, in many places little or no explosive is required to loosen the coal. The most that is used is about one 25-pound keg of powder for every 35 cars of coal, less than 1 pound per car or about threequarters of a pound per ton.

Extensive timbering is required for roof support after the coal pillars have been removed and the weight is resting on the pillars of silt. If the roof is weak, it is apt to slab, and the slabs must be kept from falling on the men at work.

In this type of mining it is unnecessary to drive crosscuts between the workings for ventilation; as the workings are to the dip, and the main body of air is above, the air naturally tends to circulate through each working place. Fresh air passes downward along the bottom of the place to the face, then turns, rises to the roof, and passes back out of the place along the roof. This ventilation is very strong; if an oil lamp is held on the floor the air current down the pitch blows the flame almost horizontal.

In the Bennett bed, where the same system is used, it is not necessary to take two slices or slabs to a pillar. The whole pillar can be taken advancing, as old-time mining methods were used in this bed and comparatively small, narrow pillars which can all be recovered at one time were left.

In 1925 the largest section where second mining by this method

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Figure 44.—Face of room being driven in pillar in reverse mining. These chambers are being driven downhill from an upper gangway through the old pillars. The new pillars are of silt, which has been placed in the old room. See Figure 45

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