England it is very fiequently employed. At a certain Yorkshire colliery, the coal, 4 feet thick, lay 60 yards below a heavily watered bed of rock, which was tubbed at the shafts. The coal was worked longwall, and there was plenty of dirt for making packs and stowing the goaf, and the water from the rock above did not come down in any large quantity. At another colliery the coal, about 5 feet thick, was about 60 yards below a water-bearing rock. The coal was worked pillar-and-stall; some packs were built, but not so many as in the first instance. Occasionally the water from the rock above, which was tubbed at the shafts, broke in, causing great expense. In some cases a thickness of 30 or 40 yards of shale are sufficient to keep the water from breaking through into the workings below, and in others it is probable that 100 yards would be insufficient. It is evident that the thickness required will depend not only on the nature of the shales, but also possibly upon the head or pressure of water in the rock, and upon the thickness of the seam of coal and the method of working. Thus if 2 feet of coal only were extracted, and the goaf tightly packed, there might be no fracture of the strata; whereas if 12 feet of coal were got, and no packs, there would be very serious fracture of the strata. Tub Cast-iron bing.-Fig. 178 shows some tubbing in plan and section. A wedging-curb is a cast-iron ring varying from 9 inches to 3 feet in width, but seldom exceeding 2 feet, and commonly about 15 to 18 inches in width, and 7 inches WEDGES WEDGES WEDGING CURB PLATES TUBBING BACK OF PLATE DESIGN. A WEDGING CURB FIG. 178.-Cast-iron tubbing. in depth; it is hollow inside, the metal being about 1 inch thick. There is a rebate, or step, a, on the inner side inch wider than the width of the base of the tubbing-plate. If another length of tubbing has to be brought up to this curb from below, there is then also a rebate on the under side. The curb is cast in segments of convenient length, say 5 feet. Where the curb has to be placed the shaft is belled out, so as to leave a space 3 or 4 inches in width at least at the back of the curb. The wide excavation is carried sufficiently high to allow the free use of a hammer; the curb-bed is cut out with a pick axe, chisel, and wedge so as to avoid the shattering action of explosives. It is carefully levelled; a sump is left in the middle, in which the water-barrel may dip. The segments of the curb are placed in a circle; between each joint is placed a board of soft pine about or inch thick. In order that the curb may be placed round the centre, an iron pin is fixed in the middle of the shaft (in a beam fixed across the sump) by means of a centre-line and plummet. A rod called a trammel is fixed by a collar over this pin; the end of this rod is moved round to see that each segment of the curb is an equal distance from the centre; blocks of yellow pine are placed with the grain vertical all round the curb between it and the strata, and tightly hammered WEDGES pitch pine (see a, a at the bottom. To make these wedges a resinous piece of pitch pine is selected, and cross-cut into blocks as thick as the length of the wedges; it is then split up, and the splinters carefully shaved and sharpened; these are then dried for some weeks or months, say in the boiler-room, and they become hard. These wedges are carefully driven in so as not to break them before they are driven up to the head. The wedging is done equally all round the shaft, the trammel being used to see that one segment is not driven nearer to the centre than the other; the blocks soon get too hard to admit wedges. Then a steel chisel (see b, Fig. 179) is driven into the blocks and extracted by the forked lever; the wedge is immediately put in the hole thus made, and driven in. As the wood gets harder it is necessary to use a two-handed hammer, d, to drive in the chisel, and the man has to jump on the extracting lever to get the chisel out. At this stage the long wedges cannot be driven home, and shorter wedges, 4 inches in length, are then used. The opening chisel is put in by holding it with the left hand and striking it with the small hammer till it just sticks in the top of the wedging; it is then struck with a two-handed hammer; if, instead of going in, it springs out, the wedging is sufficiently hard. It will take 72 hours, six men working at a time in 8-hour shifts, to wedge such a curb in a shaft 13 feet internal diameter. On the top of the curb the belled-out space is now filled up with brickwork set in cement or cement concrete, and grouted with cement all round, so that the wedging is securely covered up and kept down. The tubbing-plates, cast in segments of a convenient size (see Fig. 178), are now built up in a circle on the rebate of the curb; underneath each plate is a piece of board of yellow pine inch thick, with the grain pointing towards the centre of the shaft, and between each segment is a similar piece of board with the grain pointing towards the centre of the shaft. Long wedges are placed vertically behind the plates, the thick end of one wedge being put to the bottom, and then the other wedge driven down, liners being placed in the space if necessary; there will be a wedge at the centre of each plate and one at each joint. By driving in these large wedges the segments are squeezed tightly together. Some pitch pine wedges about 4 inches long are now driven into the vertical joints to make them tight. The tendency of these wedges is to widen the circle, while the tendency of the long wedges at the back is to close the circle; if the circle is too much widened, additional wedges must be put behind. When the vertical joints are thus made tight, the holes behind the tubbing may be filled up. This filling is sometimes done with any harmless material at hand, such as small shale, fire-clay, etc.; sometimes cement concrete is put in; other engineers prefer to fill up with finely riddled garden soil. The next ring of tubbing is now placed on the top of the first, wooden sheathing being placed in all the joints as before, the vertical joints of the second ring being in the middle of the plates of the lower ring; long wedges are driven in behind, and short wedges in the vertical joints as before, and the operation repeated until the tubbing has been carried up to the water-level or to some wedging-curb. When the tubbing stops at an upper wedging-curb, unless the length has been carefully calculated, it may be necessary to have a length of tubbingplates cast to make up the length. It is difficult to put wedges in behind the top ring of plates when it ends with a wedging-curb. The space behind this last ring may be filled up with cement concrete, so that the plates cannot be driven back by the wedging. After the length of tubbing has been all fixed, the horizontal joints may be wedged (it is evident that it would have been useless to wedge these previously, as the plates would have been merely lifted). It will probably be necessary to go over the vertical wedging again, and both horizontal and vertical joints will have to be wedged two or three times before the wedging is water-tight. When this is done, the hole in the centre of each plate may be plugged with a round block into which small wedges are driven. It is usual to fix a tap in one of the plates of the tubbing, and to carry a small pipe, say 2 inches in diameter, up to the water-level. The object of this pipe is to relieve the tubbing from any excess of pressure. Whether or not it has any such effect may be doubtful; it serves, however, the purpose of indicating the pressure of water at the back of the tubbing, and also may be convenient in case a supply of water is required. Design and Strength of Tubbing.-Design B (Fig. 180) is a very good one. The dimensions are given on the figure, which shows a back view of the plate and the section. When the plate is fixed in the shaft the ribs or flanges are not visible, the front of the plate being quite smooth. The strength of the plate. to resist bending_pressure lies in the ribs. The front plate connecting the ribs prevents the water entering the shaft; it will be seen <-------- 5.0 14 Shaft 4 10'Shaft 19'Shaft FIG. 180.-Tubbing-plate, design B, back elevation and vertical section. that in a plate 2 feet 6 inches high there are five horizontal ribs, and in a length of 5 feet 2 inches there are five vertical ribs. These ribs are stiffened by brackets. The lower flange or rib is 5 inches wide, the upper rib is 5 inches wide, and has a flange about inch thick standing up at the back about 4 inch high; this flange is continued down one side of the plate, and serves as a guide against which the next plate can be placed; it also prevents the wedging from driving the wooden sheathing back. In the centre of the plate is a hole through which the water can pass until the joints have been securely wedged; it is also convenient as a pin-hole for a shackle when lowering the plate down the pit. Width of Ribs.-The following rules for the design are in accordance with actual practice: the ribs, or flanges, to be not less than 4 inches on the bed for a shaft up to 10 feet in diameter; 4 inches for 10 feet to 13 feet; 5 inches for 13 feet to 16 feet (see Fig. 180); 5 inches for 16 feet to 19 feet; 6 inches for 19 feet to 22 feet. These dimensions, 4, 4, 5, 5, and 6 inches, are values of a (see Fig. 180, and also p. 128). Thickness of Plate and Ribs.-The following is a rule given by Greenwell for the thickness of the plate.' The ribs, or flanges, are all made the same thickness as the plate. Let D equal the diameter of the shaft in feet, and P equal the depth in feet below the water-level, x equals the thickness of the plate in feet; This rule is much quoted by writers; it is, however, obvious that to be useful it must be combined with other rules as to design and width of ribs, etc. P X D then +0'03 foot = x. If P = 300 and D = 14, then 50,000 300 X 14 = +0'030'114, which is the thickness of the plate in 50,000 feet. This multiplied by 12 = 1368 inch, or say 1 inch. If the plate is made on design B, with the width of ribs above given, and with the thickness of the plate and flanges according to this rule, and properly put in, it will never burst. Another design, A (Fig. 178), is similar to B, except that there are only three ribs instead of five, and the plate is therefore a little lighter; for very shallow depths the thickness given by the rule is too light, as it might be accidentally broken. For these shallow depths, therefore, the plates may be made thicker, and design A employed; in this case the thickness of the plate should be 50 per cent. stronger than that given by Greenwell's rule. This rule is useful for rough-and-ready calculations, but it is evidently not very accurate, because the thickness of the plate varies directly as the depth, while the strength to resist bending given by increased thickness is more than in direct proportion to this thickness. It is evident that tubbing made according to the above rule will not give the best possible disposition of the iron, in the case of deep shafts, where the tubbing is 200 yards and upwards in depth, and that the right way would be to increase the width of the flanges. Thus if a rib, or flange, is 5 inches wide (the thickness of plate is in all cases included in the width of the rib)-and that is a suitable width for a depth of 200 feet-then for a depth of 600 feet the width of the rib ought to be increased. If the ribs are regarded as girders, their strength will follow in proportion to the square of their width; thus a rib 8 inches wide will be four times as strong as a rib 4 inches wide; but if the ribs and plate are regarded merely as the stones of an arch, then their power of resistance will be in direct proportion to the weight of metal in the plate and horizontal ribs. It is best to take a middle course between these two points of view. If, therefore, the thickness of the plate and ribs is increased in proportion to the square root of the pressure, and the width of the flanges also in proportion to the cube root of the pressure, we shall get a plate more calculated to resist the variety of strains than if the thickness only is increased directly as the pressure. Thus a rule might be made as follows :- x = thickness of plate in inches for a depth of 200 feet. P = depth in feet. diameter of shaft in feet. thickness of plate for depths greater or less than 200 feet. width of ribs for depths greater or less than 200 feet. |