a = width of rib for 200 feet depth (see Fig. 180). (See also first rules for design, p. 126.) The following are the width of ribs (including front plate, or web), and the thicknesses of plates and ribs by the above rules for a 14-foot shaft: This rule for design B is suitable for all depths and diameters, and gives a very strong plate without undue weight, because of the width of the ribs. In calculating the pressure, it is necessary to consider the height of the source of the water (see Fig. 177). In this case the water, pressing against the lower length of tubbing, comes in a rock that crops out on the top of the hill, and the pressure against the tubbing will be that due to the depth from the top of the hill to the bottom of the tubbing. Faced Tubbing.—Tubbing has been made with internal flanges (see Fig. 181), the vertical joints planed and the segments united with bolts, the entire ring of plates being then placed on a lathe and turned; in this way a whole length of tubbing may be accurately fitted. Bolts unite the horizontal joints. A layer of soft felt soaked in tar,inch thick, is placed in each joint; when the bolts are tightened, this felt is squeezed, so that the plates seem to touch. In putting in tubbing of this kind, wooden sheathing is placed on the horizontal joint on the rebate of the wedging-curb, and also on the top of the highest ring of tubbing, and these two are the only joints that require to be wedged. The plates, being all numbered and marked, can be put together in the pit as quickly as they can be lowered and with very little labour. In the top ring the last plate is made in two pieces (see Fig. 181), and a 2-inch strip of iron is inserted in the middle; the sides of this strip and of the adjoining edges of the plate being parallel, the strip can be pushed in when all the other plates are in position. Bolts pass through this strip and through each of the adjoining flanges, and so it is held securely. The advantage of this kind of tubbing is the rapidity with which it can be placed in position, and its strength, because of the support that each plate gives to the next one through the bolts, and there is no strain on the tubbing as the result of wedging, but simply that due to natural causes. It is probable, however, that this bolted tubbing will be more rigid than wedged tubbing, and therefore less likely to yield to any movement of the ground; on the other hand, if it does crack, the fragments will not fall out.1 Against the advantages of this species of tubbing must be put the extra cost of machining the joints. In order to provide for pumping and other machinery, it is necessary in every long length of tubbing to put in wall-boxes (see elevation and section, Fig. 182) or HHHH rings with internal flanges; crossbeams can be rested in these wall-boxes. The depth should be about 2 feet, and the width of the wall-boxes inside the plate 10 inches. The vertical ribs are shown I foot apart, with intermediate brackets to prevent the flanges being broken by the pressure of the wedging. For very heavy machinery wider wall-boxes must be put in, resting on and securely fastened to the solid ground. FIG. 182.-Wall-box, 5 feet X 2 feet, 12 inches deep. Coffering. Fig. 183 shows a cast-iron wedging-curb on which is built an internal brick wall 9 inches thick laid in cement, and an external brick wall 4 inches thick. Between these walls is poured a mixture of sand and cement, making a water-tight wall. A modification of this is to use stones instead of brick. 1 The writer believes he was the first to introduce this kind of tubbing, at a colliery in North Wales, in the year 1873; it is still perfectly sound. K Another modification is (see Figs. 184 and 185 1) to build a brick wall 14 inches thick, of three rings each 44 inches and two spaces each inch, these spaces being grouted with cement, the middle 4-inch ring having its courses the soil kept the back of the wall quite dry. A wedging-curb is always the foundation of this coffering. Wooden Tubbing.-Wooden tubbing (see Fig. 187) is commonly used in France. It is made of wooden blocks sawn out of the best heart of oak. Each block is 3 feet long and 9 inches square; the joints are cut in radial lines, so that, when placed in the shape of a dodecahedron, the blocks fit together and cannot be pressed inwards. Each block is carefully planed, so that the joints are quite close. The curb is wedged in a different manner to English wedging-curbs. It is made of wooden blocks like the rest of the tubbing; the space between the block and the ground, being say 6 inches or more, is partly occupied by Coffering of Shafts," by W. N. Griffiths, N. E Institute Mining 1 66 Engineers. a piece of wood, say 2 inches thick; between it and the ground the space is filled in with compressed moss; between the second piece of wood and the curb, wedges are placed and driven in, Fig. 184. Bank Fig. 185 FIG. 184.-Brick-and-cement coffering. FIG. 185. Section of coffering. B, back casing of 9-inch dry brickwork put in while sinking; D, puddled clay; E, three rings of brickwork in hydraulic mortar; F, two rings of hydraulic mortar grouted in. which compress the moss; against the ground two or more curbs are wedged, and the space at the back of the blocks is filled up. When all the blocks have been placed in position up to the top, they finish underneath a brick or stone wall going up to the surface. Elevation of stone walling. Plan of stone walling. FIG. 186.- Stone coffering. 1, concrete backing; 2, holes to relieve water-pressure; 3, stone walling. On the top of the last ring of blocks is placed a series of short vertical screws working in nuts like a screw-jack. The screws all round the shaft are tightened simultaneously, so that all the tubbing is tightly pressed down. The space above them is then filled up with blocks carefully cut to fit, the screws being withdrawn in turn. The tubbing is now caulked, like the caulking of a wooden ship the joints are opened with a chisel to a depth of 1 inch, and into this hemp steeped in tar is forced with a caulkingtool. The late Sir W. W. Smythe described an instance of wooden tubbing put in at a depth of 524 feet. The French engineers prefer wooden tubbing, but the late Professor Callon, of Paris, said that in very deep mines iron was to be preferred, owing to the difficulty in getting timber of sufficient strength. It is probable that, if there is any movement of the strata, wooden tubbing would be more easily repaired than cast-iron tubbing. Quicksand. The ordinary operations of sinking are often FIG. 187.-Wooden tubbing. FIG. 188.-Sinking through quicksand by piling. impeded by quicksand, which runs into the shaft like water, so that it is impossible to make it any deeper. This difficulty is sometimes overcome by a system of piling (see Fig. 188). In this case a wooden ring or curb of oak about 9 inches wide, 6 inches thick, and say 30 feet in diameter, is laid on the ground. Outside this and round it, piles, which consist of deal planks sharpened at one end, about 9 inches wide and 3 inches thick, are driven vertically down as deep as possible without breaking the piles. The ground inside the piles is now excavated, and an additional curb is placed inside, say 2 or 3 feet below the first. As the excavation proceeds, other curbs are placed inside the piles to prevent them being squeezed in. When the excavation |