is large, it will be better to arrange that the work be done at night. Where an invert to the arch is required, the work must be carried on during the night unless the road is free from traffic. It is better in all these operations to use hydraulic mortar, because it sets so quickly some hydraulic mortars become solid in a quarter of an hour either in the air or under water. Where the depth from the surface, and consequently the crush, is great, arches however strongly built are often destroyed by its force. It has been found that by packing the top and sides with sand, to a thickness of not less than one foot, the weight is distributed over the whole surface of the arch, and the walling has remained intact. The thickness of masonry required to resist a given pressure is less if packed firmly behind with sand than would be necessary if no packing be used. In the drawings Figs. 183 to 186, the different forms of arch are shown with the packing of sand filled in behind. : CHAPTER VIII. NARROW WORK AND METHODS OF WORKING. Shaft Pillars-Water-Levels-Cross-measure Drifts from Shafts sunk through inclined strata— Stone Drifts through faults-Longwall Method of Working-Post and Stall SystemDifferent Arrangements of Single Road Stall Working-Double Road Stall Method and its Modifications- Method of Working and Timbering adopted at the following Collieries:Celynen, Risca, and the Ocean-Wicket System of North Wales-The Bank System of South Yorkshire-Method of Working and Timbering adopted at the following Collieries : -Lundhill, Kiveton Park, High Park, Wearmouth, Silksworth, Florence, Great Fenton, Cannock and Rugely, Pemberton, Clifton Hall, Pendlebury, Sovereign, Radstock, Kingswood, Allanshaw, Cowdenbeath-Working thin seams in Northern France and BelgiumSquare-work Working of the Staffordshire thick coal seam-Working the thick coal seams of Poland, Upper Silesia, and Bohemia-Dealing with excessively thick coal seams by Longwall and Post and Stall-Questions and Answers bearing on the subjects of the Chapter. AFTER the shafts have been sunk, drivings will be necessary to win the coal, and one of the first things to consider is the size of pillar or pillars to be left for the support of the shaft. If no pillar were left, but a longwall face opened at once from the shaft on either side of it, the subsidence of the roof, except in very thin seams, consequent on such proceeding would disturb the strata near the pit, and might cause injury to the shaft-walling, displace the shaft-fittings, and entail a considerable after-expense in restoring the shaft to a working condition. The size of the shaft pillar or pillars should be such that, when the coal is worked away beyond a sufficient area round, the shaft will be unaffected by the "draw"-the lateral disturbance of the strata beyond the point actually worked. The depth from the surface, the nature of the strata above and below the coal seam, as well as that of the coal itself, and the amount of dip all influence this. For any depth to 100 yards, it may be sufficient to leave a pillar 40 yards square. Adopting this size as a minimum we may fix any size of pillar for greater depths by increasing the pillar 5 yards for every 20 yards in depth, so that for a shaft 150 yards deep, we should require a pillar 52 yards square, for a shaft 200 yards deep, 65 yards square, for a shaft 300 yards deep, 90 yards square, for a shaft 400 yards deep, 115 yards square, and so on. The shaft should always be in the centre of the pillar or pillars left for its support, to ensure the same amount of protection on each side. If water is likely to be met with, water-levels will be required, and these should be started some feet below the seam at the pit bottom. Roadways in the seam to be "water-level" should rise slightly, about ths of an inch per yard, to allow the water to flow out to the shaft. For the purpose of ventilation, the two shafts, which are necessary to every colliery, are also connected as soon as possible by driving in the seam. Levels are usually driven on both sides of the shaft, and there may be either two or three on each side, driven parallel to one another, and about 20 or 25 yards apart. They are generally driven from 7 to 10 feet wide; if the roof be very bad, it may be desirable to make them as narrow as 5 feet. With regard to height, if the seam is thicker than 7 or 8 feet, the level is usually carried that height, and the upper portion of coal left as the roof. If the seam is less than 5 feet high, the roof is ripped down or the bottom cut to make height on the upper level, and when finished the road should be 6 or 7 feet high. The levels may be timbered or walled if the top is bad. For the purpose of ventilation they are connected every 30 or 40 yards by cross-holings driven at right angles. As soon as a fresh one is cut through, the previous one is closed by a brick or stone stopping built in it so as to keep the air well up to the face. If the seams of coal to be won lie at a high angle of inclination, it is usual to sink the shafts below the seams and drive a cross-measure or stone drift, water level, from the shaft across the barren ground until it intercepts the seam or seams Fig. 187.-SECTION SHOWING CROSS-MEASURE DRIFT DRIVEN FROM THE SHAFT TO TWO COAL-SEAMS LYING AT AN ANGLE OF 30°. of coal. Fig. 187 is an example of this, where a cross-measure drift is shown extending from the shaft to two seams of coal which lie at an angle of 30 degrees. At the point where the drift intersects the seam of coal, water-levels are turned opposite each other from either side of the drift. For the purpose of ventilation, if a pair of drifts are not proceeding, air boxes or brattice are used as a temporary expedient, until a return air-way is driven in the seam of coal, back to the upcast shaft. At times it is desirable to make stone drifts from one seam to another, or through a fault to intercept the same seam of coal on its other side. Fig. 188 is intended to show an example of this kind. The workings after proceeding to the rise on No. I and No. 2 seams have stopped at a downthrow fault. If stone drifts be now carried at water level as shown in the two positions on the section from No. 2 seam, the upper cross-measure drift will cut the No. 1 seam on the other side of the fault, while the lower and longer one will prove the No. 2 seam on the far side of the fault, and these seams being connected by an air pit, working may be resumed on them inside the fault. Instead of driving the two drifts as shown, the upper one might be continued water level past the point of cutting the No. 1, till it reaches the No. 2 seam, after which, to form a return air-way, the air-shaft between the seams must be sunk inside the fault, and another stone drift must be driven parallel to the first, and extending from No. 1 to No. 2 seams on opposite sides of the fault. In Fig. 189, the workings of two seams lying level are shown as having proceeded against an upthrow fault, and here the cross-measure drift is made to rise Scale. 3 Chains to 1 Irech. rapidly in order to cut the seams thrown up. After the cross-measure drift has proved the two seams as shown on the section, it will be necessary that another be driven parallel to the first, either all the way, or parallel to the first through the part of its course from No. 1 to No. 2 seams on opposite sides of the fault, when the working of No. 2 seam where just proved through the fault may be continued, and an air-pit sunk to connect the two seams at the higher side of the fault. The rock between seams may be removed upwards or downwards from the lower to the upper seam or vice versa, for the purpose of making an air-shaft. |