Compressed Air Work And Diving 1909

EAST RIVER TUNNELS. 91 then lowered by removing some of the packing uncler- neath them, and when they had again taken the weight of the caisson, some of the packing was removecl from underneath the remaining brackets, and the caisson was again lowered, until the brackets once more rested on the packings. As the cutting- edgre coulcl not be allowed to rest on the rock, which was touched a little way below the surface of the ground, a similar sort of arrangement was aclopted inside the working chamber for holding up the caisson and lowering it. Brackets were bolted in pairs to the diaphragms or brackets forming the shoe, and every alternate one was packed up underneath with timber. Beneath the other brackets packings were built up, with a space between the tops and the brackets rather less than the amount the caisson required to be lowered. The other packings on which the weight of the caisson rested were then knocked out by excavating below them, and the pressure lowered a pound or so, and this allowed the caisson to sink gradually down until it rested on the other packings. The packings had to be kept a little high, as there was always some settlement after they had taken the weight of the caisson. The rock was excavated by means of drilling and biasting, and for this purpose air pressure was taken clown to the working chamber at an initial pressure of 100 Ibs. to the square inch. The rock was loaded into ordinary skips and lifted out by a crane outside the caisson. A hook hanging from the jib of the crane was hookecl on to the bottom of the skip as it left the air-lock, and this enabled it to be tippecl automatically. As the work was starteel from both sides the river, eight shields were required. These were specially designeel for the work by Mr Moir. The design is