Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/468

450 MINING [SECURING EXCAVATIONS. supported by a side prop or leg (fig. 40), and very often by two legs. The forms of joint between the cap and leg are numerous Sf2%d^ /, B Fig. 39. Fig. 40. (fig. 41), depending to a great extent upon the nature of the pres sure, whether coming upon the top or sides. With round timber the top of the leg is sometimes hollowed as shown in fig. 42 A, but occasionally the joint is flat and , . . a thick nail, or nog, "** ** is put in (fig. 42 B) Fi S- 41 - Fig- 42. to prevent the effects of side pressure, or, better, a piece of thick plank is nailed under the cap (fig. 43). Where the floor of a level is soft and weak, a sole-piece or sill be comes necessary, and, if the sides or roof are likely to fall in, a lining of poles or planks is used (fig. 43). In some very heavy ground in the Corn- stock lode a special system of timbering is adopted (fig. 44). 44 If the ground is loose, so that the roof or sides, or both, will run in unless immediately supported, the method of working called spilling or poling is pursued. It consists in supporting the weak parts by boards or poles kept in advance of the last frame set up. The poles or boards (laths) are driven forward by blows from a sledge, and the ground is then worked away with the pick; as soon as a sufficient advance has been made a new frame is set up to support the ends of the poles or boards and the process is re- Fig. 45. Fig. 46. peated (figs. 45 and 46). In running ground it is necessary to have the laths fitting closely together, and the working face also must be supported by breast-boards kept in place by little struts resting against the frame. These are removed and advanced one by one after the laths in the roof and side have been driven beyond them. Iron On account of the high price of timber, iron is sometimes em- supports, ployed in its place. One method in use in the Harz consists in bending a rail into the form shown in fig. 47 and making it support other rails laid longi tudinally, against which flattish stones are placed ; the vacant spaces are then filled with rubbish. Masonry. Masonry has long been used for supporting the sides of mining excavations. The materials necessary are stone, ordinary bricks, or slag-bricks, and they may be built up alone (dry walling) or with the aid of Fig. 47. mortar or hydraulic cement. The bottom of a level is occasionally lined with concrete to carry a large stream of water, which other wise might run into lower workings through cracks and crevices. Dry walling is not uncommon, and it may be combined with the use of timber (or iron) as shown in fig. 69, in which a level is maintained between two walls keeping back a mass of rubbish. Figs. 48 and 49 show methods of securing a drift by arches when a lode has been removed. The timbering required for shafts varies according to the nature Timber ing for shafts. Fig. 48. Fig. 49. of the ground and the size of the excavation. A mere lining of planks set on their edges (fig. 50) suffices for small shafts, corner pieces being nailed to keep the successive frames together. In some of the salt-mines of Cheshire the shafts are lined with 4 -inch planks united by mortice and tenon joints. The usual method of securing shafts is by sets or frames. Each set consists of four pieces, Fig. 50. two longer ones called icall-plates and two shorter ones called end- pieces. They are joined by simply halving the timber as shown in Fig. 51. Fig. 52. fig. 51. A more complicated joint (fig. 52) is often preferred. The separate frames are kept apart by corner pieces (studdles, Cornwall ; jogs, Flintshire), and loose ground is prevented from falling in by boards or poles outside the frames. As shafts are frequently used for the several purposes of pumping, hoisting, and affording means of ingress and egress by ladders, it becomes necessary in such cases to divide them into compartments. Pieces of timber parallel to the end-pieces (bunions or dividings) are fixed across the shaft, and serve to stay the wall-plates and carry the guides as well as to support planks (casing boards) which are nailed to them so as to form a continuous partition or brattice. The magnificent timbering of some of the shafts on the Comstock lode is described by Mr James D. Hague as follows : J "The timbering consists of framed sets or cribs of square timber, placed horizontally, 4 feet apart, and separated by uprights or posts introduced between them. Cross- timbers for the partitions between the compartments form a part of every set. The whole is covered Fig. 55. on the outside by a lagging of 3-inch plank placed vertically. " Figs. 53, 54, and 55, copied from Mr Hague s plates, illustrate this method 1 United States Geological Exploration of the Fortieth Parallel, vol. Hi.,

" Mining Industry," p. 103.