Compressed Air Work And Diving 1909

46 COMPRESSED AIR JUORE. be stopped at the line of excavation. When the caisson is out of position or level, the clearing away round the cutting edge must always be made with reference to the final position which the caisson will occupy. It sometimes happens, as for instance in sloping ground, that the caisson will move a little, even though correctly pitched. Under such circumstances it will be a good plan to use one or two light struts as a preventive measure to prevent the caisson getting further out of position. As a general rule with very large caissons an inch or two out of position is not of great moment, and under such circumstances it is best not to risk any injury to the caisson by using too much force. Another matter to which attention must be paid is the protection of the cutting edge, which will very easily get bent if it comes into contact with some harel substance. Some caissons are fitted with one or two big girders with a wicle bottom flange. A bank of earth, or of the strata being passed through, is left under each of these, and this is gradually cut away, or the girders are allowecl to erush through them so that in this way they act as a brake on the descent of the caisson and prevent injury to the cutting edge. Brackets bolted to the sides of the working chamber and packed underneath with timber may also be usecl. In harel rock the cutting edge should not be allowed to take any of the weight, and the rock should be well clearecl all round and behind the cutting edge before each lowering of the caisson. The caisson will be held up by skin friction and is lowerecl by reclucing the pressure of the air. If there is a large range of ticle, and the brake girders are unable to take the difference in weight due to the alteration in the water level, some of the kentledge must be in a movable form so that at low ticle some of it may be taken off. In caissons with double