Page 23 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 3 Correspondence

138 coRREspoNDENcE oN THE coNsrRucrroN oF [Minures of Mr. Leitch. set, tho oxtra cost of tho plant would not have been higher per cubic yard; tvhile, with Inrger blocks, tho cost of rnoultling and settirrg, por cubic yanl, rvoultl lurve boen leduced. It woull be interestirrg to lea,rrr how tho Oregorr tirrrber piles of the jetty rvere guarded :rgnitrst tho rlthtcks of nur,r'ilo wol.lus a,nd insects, trrd why tinrber Itnrl beotr uscrl. Applrerrtly stcol piles ol tylirrtlers lvould havo becn rttoro rlttroblo. Tho stl,bility of conclote buttlesses, such a,s thoso doscliborl, soorrrorl likely to tloperrd in u corrsiderable degree on thc lifo of tho shoot lrilirrg ru.ourrd theru. Mr. olivo. Mr'. W. T. Or,rvl; considerod that sufticienb data wero nob given in Appcndix I for full criticisrn of the wnlls of section A ; he hld, lrowover, furkorr what he considered to be the weakest point of tho wtll, nrunely, rrt the level 56 feet below the cope, arrd btd found tlurt tho single resultant was well within the stipulated 0'375, tlro grc:r,test permissible deviatiorr of the centre of pressuro fi'orn tho centre of ligure, at the given bed or level, as lnid down by the lrte Professor Rankine. The calculation did not tl,ke into tccourrt the water-pressure on the face of tho wall, which undoubtetlly helped to sustain it ; but he was of opinion tha,t it would be very unsafe to rely upon this, inasmuch rs there rnust be a cert:rin level of saturation at the back of the rvall, due to wntor coruing from undemeath, nnd, by its upward pressur.e, virtrurlly reducing the weight and consequently the stability of the wall, so neutnr,lizing to an indefinite extent the face-prossure cltirncd (seo Appendix I). Had the walle been built entirely of blocks, this objection would have been intensified, as the w&ter. woull lind its way through the wall itself. In his calculatiorrs he hatl r$surnetl tho following woights :-concreto 140 lbs. per. cubic foot, nnd earth-lncking 120 lbs. per cubic foot, oredit being taken for ths 392 cubic feet of ?arth on top of the two off'sets at the back rveighing 2l tons, e