Compressed Air Work And Diving 1909

CHAPTER V. Pneumatic Caissons and Cylinders. Compressed air is employecl in the sinking of cylinders and caissons, for the foundations of bridges, clock walls, and other structures in water-bearing strata. It is also employed for the shafts of sub-aqueous tunnels and for mine shafts. The principle of the caisson is that of the diving beil. The caisson or cylinder is open at the bottom, and the water is kept from entering by pumping in compressed air at a pressure equal to that due to the head of water at the level of the bottom of the caisson. The caisson may be comparecl to a box, open at the bottom, closed at the top above high water level, and fillecl with compressed air. Under actual working conditions the caisson usually has a strong roof about 9 ft. from the bottom, dividing off the lower part of the caisson from the upper, and forming what is known as the working chamber. From the roof extend one or more shafts up to above high water. These are closed at the top by the air-locks which allow ingress and egress of men and material without any loss of air. The caisson is made to sink by the material being éxcavated all over the bottom of the working chamber