212 HARBOUR CONSTRUCTION. 



necessary, having in view the danger of its being dispersed 

 before it could be protected by blocks ; and (2) not to use rubble 

 of so small a size, on the seaward face, as to admit of its being 

 drawn out between the blocks by the action of the waves. 



In very exposed situations, it is obvious that in breakwaters 

 of the (e) type, considerable risk would be incurred, during con- 

 struction, by carrying up the rubble into the sphere of wave- 

 action. 



Mixed rubble may be safely used in the lower section of the 

 mound ; and by carrying this well forward as the work advances, 

 the larger class of rubble may be selected and used as a coating, 

 without entailing inconvenience or additional cost. 



A mound breakwater with a rubble core checks the passage 

 of sand, mud, and undulations into a harbour much more effec- 

 tually than a mere mound of blocks can do. 



When a pell-mell mound is placed upon a rubble base, the 

 latter should be of sufficient width at the top to allow of a level 

 benching extending beyond the toe of the block slope. This may 

 vary in width from 10 to 20 feet, or more, according to circum- 

 stances, and should always be composed of, or be protected by, 

 rubble of large size. 



In a pell-mell mound formed entirely of concrete blocks, 

 only about f or J of its cube will be solid, the remaining J or \ 

 being represented by voids between the blocks. 



The size of blocks for pell-mell work suited to any particular 

 site can only be decided by analogy, and by a careful study of 

 all the conditions affecting such site. The facts in the following 

 table, pp. 214, 215, relating to a few existing works, may there- 

 fore be of use in considering this point. 



During recent years, experience has shown that blocks 

 weight and other conditions being equal laid in stepped courses 

 are more stable than when deposited pell-mell. 



In some of the more recent works, especially those carried out 

 by Italian engineers in the Mediterranean, stepped block facing 

 has therefore been adopted in preference to pell-rnell work. The 

 new breakwater at Cette, in the south of France, has also been 

 constructed in this way. 



A type section illustrating this mode of construction is given 

 in Fig. 52, p. 213. 



The placing of blocks in stepped courses is naturally some- 

 what more expensive than merely throwing them into the sea 



