PRESSURE AGAINST WALLS. in 



the stability will be much increased by the hydrostatic pressure 

 on the tops of the offsets should the wall rest on an impermeable 

 foundation. 1 It is generally assumed in theoretical investiga- 

 tions that the weight of earthwork superimposed vertically over 

 the offsets should be included in the weight of the wall in 

 estimating the moment of stability, but the author (Sir B. Baker) 

 found no justification in practice for this assumption. He had 

 invariably observed that when a retaining- wall moved by settle- 

 ment or otherwise, it drops away from the filling, and cavities 

 are formed. Settlement of but -^ of an inch, after the backing 

 had become thoroughly consolidated, would suffice to relieve the 

 offsets of all vertical pressure from the superimposed earth, and 

 the latter cannot therefore be properly considered as contributing 

 to the moment of stability. 



" A wall with deep offsets at the back is not a desirable form 

 when the foundations are bad, and when consequently the 

 pressure over the foundations should be as uniform as possible, 

 so that a settlement should take the form of a uniform sinking, 

 and not a tilting forward of the coping by reason of the toe 

 sinking quicker than the back of the wall/' 



It is important, especially when dealing with a foundation of 

 at all a yielding character, to gradually carry up the walls by 

 regular lifts, so as to distribute the weight as uniformly as 

 possible over the whole area of the foundation. 



All works naturally subside to some extent as they become 

 consolidated ; therefore, if a portion is carried up very far in 

 advance, it comes to its bearings before the adjoining portion is 

 completed, with the result that the latter being worked to the 

 gauge of the older portion, in its turn subsides and shows an 

 unsightly crack at the point of junction. This will apply more 

 to works executed with lime-mortar than when Portland cement 

 is used, but at the same time it applies to a greater or less 

 extent to all works. 



Depth of Water alongside. In determining the depth to be 

 provided alongside a wharf wall at extreme low water, and 

 consequently the total height of the wall, it is necessary to take 

 into consideration, not only the maximum load-draught of the 

 largest class of vessels to be accommodated, but also the exposure 

 of the wall, and to what extent the water level is likely to be 

 affected by high winds, which set up a send or vertical motion 



1 Rankine's " Manual of Civil Engineering," p. 402. 



