Page 29 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 3 Correspondence

144 conRESpoNDENcE oN TIIE coNsrRucrroN oF [Minuter ot Mr. witling. project those curves forward for another 20 years, Mr. \Yilding thought he would be conveying the best indication tbat was possible now of what should be anticipatcd and provided for. Thc Auttor. The Autrrot, in reply, pointed out that he had already dealt with somo of the questions raised, e,g., rvhy the open excnvation had boen tnket.r down to the 30-feet level before stn,ting trench-work, nnrl rvhy the dock had been flooded beforo the trench-work was frrlly completed. I{e had also dealt with the question of using 8-to-l concrete, which had proved successful at Southampton. With rognrd to the enclosing bank rouud portion A of the rvorks, tho puddle trench suggested by Mr. Copeland would have been very oxperrsivo and slow, as the gravel bed through which most of the rvlter penetrated extended down to 35 feet belorv high-water level. I)robably Mr. Ilall Scott's suggestion thnt steel sheet piling miglrt hnvo been driven along the bank was a better one. ft was vely rloubtful, however, whether Mr. Scott's proposal to enclose the whole of the works bya coffer-damwould have beeneconomical. A long length of that dam would have hacl to be placed on ground which was obout 22 feet below low water-not at low-rvnter lovel ns Mr, Scott assumed-and such a dam would be costly nnd dillicult to construct. Several correspondents had challenged the calcula- tions for the sbability of the wall, given in the Appendix. fn mnking these calculations the Author had assumed the rveight of tho concrete to be 140 lbs. per cubic foot, which agreed fairly rvell with tho weights of actual samples. The nature of the backing vnricd considerably, and no doubt its lower strata were saturated with w:r,ter. fn order to allow for this, its weight had becn nssurnerl to be 120 lbs. per cubic foot, and its slope of repose 26" (2 to l). 'Ihe slope of repose of the material underneath the wall hod beon ossumed to be 15o only, as it had seemed quite likely tlrlt the cnrshing-effect of the wall would reduce it to this figure. The rosist{rnce olfered by the slope had been anived at by multiplying the totnl weight on the base by the rate of slope (]). The resistance off'ered by the mnterial in front of the toe had been neglected. 'I'his, porhnps, was an error on the safe side, but in the cnse of tho mnteriol being softened by water the resistance would be very smull. Mr. Olivo thought the long toe might have been omittod, but if thnt hrrd boen done the pressure on the clay under the wall, which wnc now estimnted to be 4'8 tons per square foot, would hsvo lnon considerably higher, and the wall might have failed in 'With conso(lllone,o. regard to the design and construction of tho wall for poltion A, the Author had studied the question of a hollorv rvu,ll, but so frrr hc lnd not succeeded in designing a satis-