824 



MINING FOE COAL 



cribbing must sustain the sum of the two water-pressures, or 62 fathoms ; since tho 

 stratum a meets the alluvial cover at d, the fountain-head of all the water that occurs 

 in sinking. Thus we perceive, that though no water-feeder of any magnitude should 

 present itself till the shaft had been sunk 100 fathoms ; if this water required to be 

 stopped up or tubbed off through the breadth of a stratum only 3 feet thick, the 

 tubbing would need to have a strength to resist 100 fathoms of water-pressure. For 

 though the water at first oozes merely in discontinuous particles through the open 

 pores of the sands and sandstones, yet it soon fills them up, like a myriad of tubes, 

 which transfer to the bottom the total weight of the hydrostatic column of . . gg 

 100 fathoms ; and experience shows, as we have already stated, that what- 

 ever water occurs in coal-pits, or in mines, generally speaking, proceeds 

 from the surface of the ground. Hence, if the cover be an impervious bed 

 of clay, very little water will be met with among the strata, in comparison 

 with what would be found under sand. 



1497 



1408 



When several fathoms ot tne strata must be tubbed, in order to stop up the water- 

 flow, the shaft must be widened regularly to admit the kind of tubbing that is to be 

 inserted ; the greatest width being needed for plank-tubbing, and the least for iron- 

 tubbing. Fig. 1498 represents a shaft excavated for plank-tubbing, where a, a, a, a are 

 the impervious strata, b, b the porous beds water-logged, and c, c the bottom of tho 

 excavation, made level and perfectly smooth with mason chisels. The same precau- 

 tions are taken in working off the upper part of the excavation d, d. In this opera- 

 tion, three kinds of cribs are employed ; called wedging, spiking, and main cribs. 

 Besides the stout plank for making the tub, a quantity of well-seasoned and clean 

 reeded deal is required for forming the joints ; called sheeting deal by the workmen. 

 This sheeting deal is always applied in pieces laid endwise, with the end of the fibres 

 towards the area of the pit. Since much of the security from water depends on the 

 tightness of the tub at its jointing with the rock, several plans have been contrived to 

 effect this object ; the most approved being represented in fig. 1499. To make room 

 for the lower wedging crib, the recess is excavated a few inches wider, as at c ; and 

 from b to c, sheeting deals are laid all round the circle, or a thin stratum of oakum 

 is introduced. On this the wedging crib d is applied, and neatly jointed in the radius- 

 line of the pit, each segment being drawn exactly to the circle : and at each of its 

 elements sheeting deal is inserted. This wedging crib must be 10 inches in the bod, 

 and 6 inches deep. The vacuity e, at the back of tho crib, about 2 and a half inches 

 wide, is filled with pieces of dry clean reeded deal, inserted endwise ; which is regularly 

 wedged with one set of wedges all round, and then with a second and a third set of 

 wedges, in the same regular style, to keep the crib in a truly circular posture. By 

 this process, well executed, no water can pass downwards by the back of the crib. 

 The next operation is to fix spiking cribs f, to tho rock, about 10 or 12 feet from tho 

 lower crib, according to the length of tho planks to be used for the tubs. They must 

 be set fair to the sweep of tho shaft, as on them its true circular figure depends. The 

 tubbing deals, k, must now be fixed. They are 3 inches thick, 6 broad, and planed on 

 *11 sides, with the joints accurately worked to the proper bevel for the circle of the 

 pit. The main cribs, g g, are then to be placed as counterforts, for the support and 

 strength of the tubbing. The upper ends of the first set of tub-planks being cut 

 square and level all round, tho second spiking crib, I, is fixed, and another set of 

 tubbing deals put round like the former, having sheeting deal inserted betwixt the 

 ends of the two sets at/. When this is wedged, the cribs, h h, are placed. 



Oak cribbing is made with pieces of the best oak, from 3 to 4 feet long, 10 inches 

 in the bed, and 7 or 8 inches deep. 



The third mode of tubbing, by means of iron cylinders cast in segments, now 

 supersedes the wooden tubbing, from the great reduction in the price of iron, 

 and its superior strength and durability. Each segment is adjusted piece to piece 

 in the circular recess of the pit cut out for their reception. The flange for tho 

 wedging joint is best turned inwards. In late improvements of this plan, executed 

 by Mr. Buddie, where the pressure amounted to several hundred feet, tho segments 

 were 6 feet long, 2 feet broad, and an inch thick, counterforted with ribs or raiM'd 

 work on the back ; the lip of the flange was strong, and supported by brackets. 



