210 



HARBOUR CONSTRUCTION. 



being faced with pell-mell blocks, as at Marseilles and 

 Boulogne (Fig. 51). 



(/) A low rubble mound with vertical superstructure, having 



a wave-breaker of pell-mell blocks on the weather side, 



as at Madras and Mormugao (Fig. 39, p. 195). 



The slopes assumed by rubble in breakwaters of the mound 



type are, cceteris paribus, proportionate to the size and specific 



gravity of the stones of which the mound is composed or faced. 



The same may be said of mounds formed of concrete blocks. 



OFFI NQ 



H. W. 0. S. T. 



FIG. 49. Mound breakwater. Pell-mell blocks backed with rubble. Type (c). 



FIG. 50. Ditto. Pell-mell blocks with superstructure. Type (<2), 



Inasmuch, however, as concrete is a costly material, it should be 

 used as sparingly as possible, having due regard to the stability 

 of the work. 



In all cases, therefore, where pell-mell block mounds are 

 used, the slope of the mound should be steep, and the blocks 

 proportionately large. 



Large blocks are as easily made as smaller ones, and the only 

 addition to their cost is due to the heavier, and therefore more 

 expensive, plant that is required for handling them ; but even 

 this, in the case of large works, would probably be more than 

 compensated in other ways. 



