VARIOUS TYPES OF BREAKWATERS. 211 



A block-mound with steep slopes, in addition to the economy 

 which it effects as regards quantity of material, possesses the 

 further advantage of not intensifying the power of the waves in 

 the manner that long rubble foreshores have been shown to do ; 

 it also occupies less ground than a rubble mound, which, in some 

 situations, would be a great desideratum. 



The voids in a pell-mell block mound, and the ruggedness 

 of its slope, materially assist in breaking up waves and checking 

 their run. 



Although there is a large saving in the quantity of material 

 required to form a concrete block mound, as compared with one 

 of rubble stone, the cost per cubic yard of the former will 

 probably be at least five or six times as much as of the latter, 

 so there is not a great deal to be said in favour of block mounds 

 from a pecuniary point of view. 



The pell-mell system of constructing breakwaters has been 

 more largely used in the Mediterranean than elsewhere, and it 

 has, for the most part, proved satisfactory. 



In some positions, however, as for instance at Biarritz, on 

 the west coast of France, facing the Bay of Biscay, at Oran, and 

 also at Port Erin, in the Isle of Man, breakwaters of this con- 

 struction have suffered severely. 



FIG. 51. Section of breakwater with vertical superstructure on rubble base faced with pell-mell 



blocks. Type (e). 



When a good class of rubble is available, a large saving may 

 be effected by using it in combination with blocks in one or 

 other of the modes already described; type (e), Fig. 51, forming a 

 good example of this class of structure for sites where the 

 exposure is not very great. 



In constructing breakwaters of this type two points require 

 special attention, viz. 



(1) To be careful not to expose more rubble at a time than is 



