Page 20 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 1

60 WENTWoRTI{-SHEILDS oN THE coNsTRuCfIoN otr' plinuter ol Pier-Trenclrcs.-The small trenches for the piers were next sunk one by one, l,nd owing to the fact that nearly all these trenches had to pass thlough fine silt, rvhich rvas highly charged with water, this part of the work proved to be difficult and expensive, The difficulties, as is usul,lly the case in such work, were instructive. Two types of " timbering " were tried for theso trenches, exemplified by piers Nos. 3 and 6. In the first ca,se, the site of the pier was surrounded by interlocking Bteel piling, driven by a pile-engine erected on a ga,ntry rvhich travelled both along and across the trench. The piles were 38 to 44 feet long, so as to extend from the top to the bottom of the pier-trench. When they had been driven to a depth of about 24 feet, the trench which they enclosed was excavated to a depth of about 20 feet, and supported in the usual way with timber struts and walings. The piles were then further driven to their full depth, and excavation was resumed. Unfortunately the section adopted for the piles was not quite strong enough to withstand the prolonged driving, and a few of them got bent and torn near the cutting edge. At these points, water and sand found. their way in through the rents, which it was almost impossible to repair. In one .. or two piers, these blows " of water and sand were so serious that it became necessary to allow the water to rise to the top of the pier- trench, and to finish the excavation and timbering by divers. fn such cases, the foundation concrete was also put in under water by divers; the trench was then pumped dry, and tLre remainder of the concrete wa,s deposited in the usual wa.v. This work was, of course, slorv antl expensive, though it presented no insurmountable difliculty. frr the alternative method of ,stimberingr,'the pier-trench was surrounderl with interlocking steel piles as before, but the piles were only rr,bout 24 f.eet in length, so that they could easily be driven to their full depth. When the excavation hatl reacheil the feet of these piles, an inner setting of 4-inch timber runners was used while sinking tlre remainder of the trench. The drawback of this arrarrgemerrt wrrs tlr:lt the runners in the lower setting had to be ornitterl, rrrrd clrss Poling substituted wherever the struts occurred, anrl tt, tlrest-r poirrts there rvas always a tendency for water and sand to blow in. .. Comprrrirrg thcso two methods of timberingr,' it may be said that tho fir'st methorl, with the long steel piles, was on the whole the nroro suitrl-rlo uutlor the cilcumstances. It would probably have been t:omPlotr:ly srrcctrssful if a stlonger section of piling had been usetl. lrr fuct, most of the piles were driven successfully through uroto thorr 40 foet of hard sand and gravel; which shows that steel