Figs. 111, 112.
Fig. 113.
The process of fixing the crib is as follows;—The diameter of the shaft is made about 30 inches larger than that of the inside of the tubbing. When a bed of rock, which may be considered sufficiently hard and close to separate the feeders above and below it, is reached, the shaft is contracted to the diameter of the tubbing, and a smooth horizontal face is made on which to place the wedging crib. The wedging crib, which usually consists of segments about 4 feet long by 6 inches high by 14 inches wide, is then placed on the bed. To give the crib a firm and secure position, it is tightly wedged with wood, both behind and between the joints; the tubbing is then built upon it to the next wedging crib, which rests upon a bell-shaped section of rock. When the tubbing nearly reaches this crib, the rock is removed piece by piece, and the top ring of tubbing is placed close up against the crib. It will thus be seen that the fixing of each crib is a costly process, often causing considerable delay.
In some cases, where it has been difficult to find suitable foundations for intermediate wedging cribs, the whole of the water-bearing rocks have been sunk through without attempting to stop the feeders separately, and no tubbing has been placed in the shaft till the wedging crib could be fixed below the lowest feeder. This process is more expeditious where there are small quantities of water; but where the water is excessive greater delay is caused by contending with it than from putting in numerous sets of tubbing to stop the feeders separately. The tubbing used in England has almost invariably been of cast-iron; on the Continent, till recently, tubbing of wood has chiefly been used. Illustrations of both descriptions are shown by [Figs. 111 and 112].
Figs. 114, 115.