VARIOUS TYPES OF BREAKWATERS. 203 



Where the mound is isolated, as at Plymouth and Cherbourg, 

 it becomes necessary unless the gap between the shore and the 

 breakwater can be bridged by the staging to convey and dis- 

 charge the material by means of hopper-barges. I shall, how- 

 ever, have more to say on this subject later on. 



In constructing the Plymouth breakwater, only rubble of 

 large size was used in the mound, the smaller stone having been 

 separated from it and cast aside. At Holyhead and Portland, 

 large and small were used indiscriminately. 



The question as to whether the rubble forming a mound 

 should all be of large size, or whether it should be a mixture of 

 different sizes, just as it comes from the quarry, is one upon 

 which opinions differ. 



The objections which have been advanced against the admix- 

 ture of small material are chiefly based upon the idea that the 

 smaller stones become rounded and form a carriage to facilitate 

 the movement of the larger ones ; and also that by their erosive 

 action the larger blocks are worn away, as also the face of any 

 superstructure that may be placed upon the mound. 



No doubt small material in undue proportions is open to 

 these objections, and is, further, likely to give rise to increased 

 settlement. On the other hand, a moderate admixture of small 

 material, by filling up all interstices, must conduce to the 

 stability of a mound. 



Sometimes the material is sorted, the smallest of it being 

 placed in the centre and at the bottom of the mound, while the 

 roughest is reserved for the sea-face and upper portion, where 

 wave-action is most felt. This system was adopted in con- 

 structing some of the Mediterranean breakwaters. Mounds thus 

 formed, not being homogeneous, are liable to settle unequally 

 and cause trouble when superstructures are placed upon them. 



Much of the trouble which was experienced at Plymouth, 

 through the instability of the mound, was attributed to the fact 

 of all the small stone having been excluded from it, whereby 

 large interstices were left, into and through which the waves 

 played. 



In order to mitigate this evil as far as possible, the small 

 material, which had been discarded, was afterwards spread over 

 the mound, in the hope that it might, by the action of the 

 waves, be carried in amongst the larger stones and bind them 

 together. 



