STABILITY OF WALLS. 107 



of labour that everyday experience justifies, afford a very close 

 measure of the actual lateral pressure of the materials retained. 



A wall may fail by settling bodily owing to unstable founda- 

 tions, turning outwards at the top due to the same cause, or to 

 the resulting direction of the thrust due to the vertical pressure 

 of the wall and the horizontal effort of the backing cutting the 

 base too near the toe, or to sliding forward bodily, owing to the 

 1 >;ick pressure overcoming the frictional resistance at the base. 

 A wall efficient in every way as regards dimensions, foundations, 

 and backing, may move in consequence of the full pressure being 

 brought upon it when the work is in a green state, and therefore 

 in its weakest condition. Hence arises the desirability, especially 

 in dock works, of using a quick setting material for the mortar, 

 such as Portland cement, or allowing ample time for the work 

 to consolidate and set before the full pressure of the backing is 

 brought against it. 



Wharf and dock walls generally are subjected to conditions 

 altogether different from those affecting ordinary retaining- walls, 

 and have, therefore, to be dealt with exceptionally. Heavy traffic 

 often setting up strong vibration, the erection of buildings and 

 the depositing of heavy merchandise upon the wharf have to be 

 taken into consideration in determining the section of wall to be 

 adopted ; in addition, there is the contingency of the dock having 

 at some time or other to be emptied for cleaning or repairs, in 

 which case additional strain would be thrown upon the walls, 

 as they would then probably be called upon to bear an abnormal 

 pressure, bearing in mind that it is practically impossible to 

 prevent in all cases the accumulation of water at the back of a 

 dock wall. 



Mr. Giles, M.I.C.E., 1 is of opinion that for dock walls quality 

 is of secondary importance compared with quantity; the desider- 

 atum being weight to resist the lateral pressure, and that a wall 

 should be constructed strong enough to carry a head of water 

 equal to its height. A frequent error is in not making the base 

 wide enough and in not putting a sufficient toe. 



It may be necessary in cases when the backing is excep- 

 tionally bad, and is likely to assume a semi-fluid condition, to 

 provide for a still greater pressure, to be arrived at by substituting 

 the weight per cubic foot of the saturated material forming the 

 backing for the weight of water. 



1 M.P.I.C.R, vol. lv. pp. 52, 53; vol. Ixv. p. 202. 



