276 HARBOUR CONSTRUCTION. 



say, 20 feet or more below low water, it will be very little 

 affected, even by the heaviest seas, until the superstructure has 

 been placed upon it (p. 201). 



Seeing, therefore, that settlement cannot be avoided, our aim 

 should be to ensure its being as uniform as possible. In this 

 respect staging offers facilities which do not pertain to the 

 system of over-end setting. 



By the former method the work can be gradually raised over 

 a considerable length of foundation, which is thus more evenly 

 loaded than is possible where over-end setting is resorted to, 

 and it is further relieved from the weight of the setting-machine, 

 which in some of the more modern examples is excessive, and 

 such as to prejudicially affect new work founded upon a yielding 

 foundation, such as we are now considering. 



When unequal settlement occurs, the superstructure is often 

 fissured, blocks are loosened, and the lower courses not unfre- 

 quently fall away from those overlying them, in which case 

 serious damage ensues, as has already been pointed out (pp. 

 181 et seq.). 



Breakwaters built with sloping courses readily accommodate 

 themselves to settlement by an exceedingly small opening of the 

 joints, which we will further consider when speaking of bond 

 (p. 283). 



General uniform settlement is of comparatively little impor- 

 tance. A settlement of about 3 feet 6 inches approximately ^ 

 the height of the mound took place in the Holyhead break- 

 water after the superstructure had been built, but the movement 

 was so gradual and regular that the work was not affected by it. 



At Alderney the settlement amounted to about - Q ^ ne 

 height of the mound, or to a maximum of about 6 feet. This 

 cracked the superstructure, and otherwise injured the work. 

 The mound of this breakwater was, however, of exceptional 

 height, and the superstructure was particularly liable to produce 

 unequal settlement, owing not only to its design, but also to the 

 manner in which it was built. 



With respect to this, Mr. Vernon-Harcourt, M. Inst. C.E., 

 states 



"As, however, it was very important to raise a certain length to 

 the level of low water each fortnight during the working season, so 

 that the masons might commence on the cemented courses as low as 



