Page 27 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 1
P. 27
l'r,r,norling8.] THE '(WHITE STAR" DOCK AT SOUTHANIPTON, 67 In orrlsr to test the sufficiency of theee assumptions, similar celculations using llrxlnloconstsntgweremade for other walls at Southampton which bad been l,,rrll, rrruler similer conditions. For instonce, in the case of the Empress Dock ,'',ll, prrbof which had moved Bome yeers previously, tho cslculations gavo tho l,,ll,rs ing results :- Per Llneal l'oot Por Llneal Fmb ol WBll, ol Wall. 1'on8. Tons. Lrtcral prossure of backing . 27.6 ,, ,, due to surcharge . 2.0 Tol,al oubward forces . 29.6 Ilesistance duo to watcr-pressure 9.7 ,, ,, Blope of foundations 1.1 friction 18.4 Total inward forces 24.2 Giving a deffcit from safety of - j'4 I l, rvrs thus evident that tho White Star Dock wall (portion A) wes considerably n,rfr,r tlnn the Emprees Dock wall, snd the fact thet the former hs8 not Bhown any ,,;'prociable sign of mpvement proves that the values assumed for the eymbols n lrrr sufrcient. I'rolnbly the principal reason why the White Stor Dock wall has proved. more rlnlrlo than the older ono is that a much greater Blope hr8 been given to the bsse ,'l l,he wall. It hae been found from experience at Southampton that a sloping l,qo assists greatly in preventing forward movement. Mony engineers discsrd llrix slope on the ground that, even if the bsse is sloped, the eorth on which it rort,n tende to slide on o horizontal plene below the baee. But experieuce seeme t,' rhorv that, with a clay foundation, the coefhcient of friction at e plane below I lur base is greater than at the plane of the base iteeU. This is probably beceuse tlro surface of the clay gets softened wittr r'eter, and the undersid€ of the nrrll becomes slippery from the formation of laitarwe on the concrete. The ,,r,lhcient of friction at the bsse of tho Southampton walls has been taken aa rl"J. Lower down in the clay it would undoubtedly be more than thie. ln order to aecertsin whether the wall complied with the second condition of ^tnlrility, the maximum vertical pressure between thewall and the material on ulrir:h it rested wes ascertsined in tho usual wey by equating tho.overturning ,,'uplo (produced by the horizontal forces) with tho righting couplo (produced by I lur two vertical forcee, nomely, tho weight of the well &nd tho reaction from the lirrrurrrl uuderneath it). In the case of the well for portion A of the dock, it was tlrrrx fourrd tbrat there wae a maximum vertical pressure of 4'8 tons por squero lrrrrl, under the toe of the rvall. To debermine how much preesure the material rrlrlrrr l,he toe was capable of sustaining, Rankine's formula was used:- _ p (toh+,,,/,)(1+*H ), ru lr,rr! ,r douotes tho aafo pressure on foundation per unit of area. 'll) t, weight of m&torial in front of too per unit of volume. h, ,, height of ,) ,, ,, q" angle of repose of ,, ,, weight of water above same per unit of volume. h' height of water (at low water). n2
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