Page 11 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 1

l'rxroodings.] TIIE " WHITE STAR " DocK AT SoUTHAMPToN. 51 l,lrrr,t the water had found its way from the dock under this wall i r r l,o the trench. It was at once realized that if this were allowed to l,rntinue the sand under the completed wall might get washed rr u rry, and that both the new trench and the completeil concrete rvrr.ll to the north of it might be wrecked. Accordingly the trench I'unps were drawn up, and the water was allowed to rise to within rrlxrtrt 9 feet of low-water level. This had the effect of checking the lrlrws to some extent, and although water continued to enter t,lu'ough the blow-holes, it no longer brought sand with it. Utncrete Caissons,_T}r,e problem now arose, how to complete the r,xrrrrvation of the trench to foundation-level. As it was found im- ;,,,ssible to trace exactly where the water came from and to stop it rrt, its source, it seemed foolhardy to pump out the trench again. ,\ sr,lreme was prepared for removing the trench-timbers one by ,,rrr, irrrd. fiIling with sand, with a view to excavate afresh in open lrrt,ting by drcdgers and grabs. This would have been risky and costJy, and eventually it was decided to abandon this idea and to lorrtinue the excavation by means of reinforced-concrete caissons. 'l'lre :rrrangement, is shown in Figs. 7, Plate 1 Each caisson was l(i feet 9 inches long by 8 feet 3 inches wide and 16 feet deep, lxring thussmall enough to fit into one of thepockets formed by llrrr trench-timbering, The walls of the caissons were of concrete ll'| iuches thick, ancl the reinforcement consisted of one thickness of ,,xprrrded metal, 3-inch mesh, and weighing 16 lbs. per square 1,rlrtl. Each caisson was strengthened by a cross wall which divided il, into two pockets. ft was built on a platform laid on the trench- I.irrrbering just above the level to which the water had been allowed l,o rise, and .immediately over its fnal position, As each caisson rvrrigllsd about 62 tons, none of the available cranes was capable of lowering it, and special apparatus had to be mado to perform I lris operation, Three pairs of steel girders were placed acloss the krlr trench-timbers immediately over each caisson, X'rom these l,lro rnisson was suspended by six sets of eye-bars, 'which were lirstoned at, their lower etlds to eye-bars built in the caisson, and at I,lrrrir upper end to long screwed bars which passed through nuts t'lsl,itr* on the girders. Thus by turning the nuts the whole caisson corrltl be raised or lowered at will. When a caisson was ready to be hrrvtrlctl it was first lifted slightly by screwing up the nuts, causing l,lro srrrews and eye-bars to take its weight, and the stage on which il, lrrr.tl been built was removed. The six nuts lvere then turned riirrrrrlta.neously and the screws rvere lowered out to their full extent. 'l'lrrr 1y1eig]1; was now taken temporarily by six stirrups, which took lroll of the upper eye-bars and thus released the screws. The screws E2