SETTLEMENT AND BOND. 277 



possible at each spring tide, and also in order that the blocks under 

 water might be secured by the weight of the masonry on them, it has 

 been customary to found the superstructure in suitable lengths, 

 leaving a break at the close of each fortnight's work; and each 

 season's work was terminated by a carefully protected scar. Accord- 

 ingly, the superstructure was composed of a series of lengths, formed 

 at different periods, between which unequal settlement of the base 

 necessarily occurred ; and a still more marked division was formed at 

 the close of each season's work. The junctions formed so many weak 

 points in the superstructure, where serious disruptions tended more or 

 less to occur. Above low water these cracks could be filled up, 

 unless extending far into the interior of the superstructure ; but 

 below low water no means could be used for filling up the crevices, 

 and the waves, rushing in, caused a compression of the air inside 

 them. This resulted in the forcing out of face-stones about low-water 

 mark, which were also liable to drop a little, owing to the lowering of 

 the base." l 



In the Colombo breakwater, where the superstructure is 

 composed of sloping blocks, the settlement amounted to about 

 y 1 ^ the height of the mound, measured from the sea-bed to 

 the base of the superstructure ; nevertheless, the superstructure 

 followed it down, as at Holyhead, without any appreciable 

 disturbance of the work. 



The settlement in Mormugao breakwater, owing to the 

 bottom ]being of mud, was somewhat greater than at Colombo, 

 being about J the height of the mound ; but in this case also 

 the sloping blockwork followed it down without sustaining any 

 injury. 



The Manora breakwater, also of sloping blocks, settled to the 

 extent of about 3 feet over the greater part of its length ; but in 

 some places, owing to peaks of rock protruding, the settlement 

 was irregular, and the work in this instance was damaged. 



In order to compensate for settlement, when the sloping 

 block system is adopted, an estimate of its probable amount may 

 be made, and each successive length of the work may be kept 

 correspondingly high. In horizontally coursed work, however, 

 settlement in sections is much more difficult to deal with, and, 

 as will be readily understood, it is likely to be productive of 

 much mischief. 



When a breakwater composed of a rubble mound and vertical 

 superstructure is projected from a steep rocky shore, difficulty 

 1 Harcourt, "Harbours and Docks," p. 114. 





