270 HARBOUR CONSTRUCTION. 



Where the depth of water and other conditions are such as 

 to admit of inequalities of rock being made up by the deposit of 

 rubble, the same methods of founding blocks may be adopted as 

 in the case of work founded upon a rubble mound, which we will 

 shortly consider. I would, however, here draw attention to a 

 danger which attends the levelling of rock inequalities with 

 rubble. I refer to unequal settlement arising from unyielding 

 peaks of rock approaching too near to the surface of the rubble. 

 This cannot be too carefully guarded against. The Manora 

 breakwater at Kurrachee sustained serious damage from this 

 cause. 1 



Founding upon Sand. Sand, being practically incompressible, 

 forms an excellent foundation, so long as it is not allowed to 

 escape. When, however, a rubble mound breakwater, or one 

 composed of blocks thrown in pell-mell, is placed upon a sandy 

 bottom, the rubble or blocks, as the case may be, will soon 

 subside to a certain extent, owing to the wash of the water 

 about them excavating the sand and drawing it away. They 

 thus gradually settle down, find their bearing, and become stable. 



In placing a vertical breakwater upon a sand foundation, it 

 is necessary, in the first instance, to protect the sand from scour. 

 This is usually done by depositing a thick layer of rubble stone 

 over the site to be occupied by the breakwater, and extending it 

 well to seaward, so as to form a wide apron. It should also be 

 extended though not necessarily so far on the sheltered side, 

 this being requisite, not only for the purpose of checking scour 

 set up by currents and wave-action during construction, but also 

 for protecting the sand from the excavating action of water 

 thrown over the pier during storms. 



If the site is much exposed, this apron may itself require 

 protection along the seaward face of the breakwater as the work 

 progresses, and this may readily be given either by depositing 

 blocks or large bags of concrete. 



At Ymuiden, where the breakwaters are of the vertical type, 

 and founded upon sand, the covering layer of rubble is, for the 

 most part, about 3 feet 6 inches thick ; but in some places it is 



years. Attention has, however, recently been directed by Mr. W. B. Kinipple, 

 M. Inst. C.E.,to its value when applied to breakwater construction, especially to the 

 grouting of rubble in foundations, etc. ; and he has done good service by developing 

 the system and bringing it into more general use. 

 1 Min. Proc. Inst. C.E., vol. xliii. pi. ii. 



