i88 HARBOUR CONSTRUCTION. 



they would be lifted and conveyed to the site of the breakwater, 

 and there be set in position. This system was successfully 

 adopted by him in constructing river walls in the Liffey at 

 Dublin ; but the operation would be attended with much more 

 difficulty and risk in the open sea. 



No doubt, a breakwater constructed in this manner could be 

 made very stable; but in exposed situations the number of 

 working days would be very small, and the cost of plant would 

 be too large to admit of the system being adopted in any but 

 very large undertakings. 



This mode of constructing breakwaters scarcely belongs to 

 the monolithic system, but, in regard to size of blocks, it is akin 

 to it. 



5. Concrete Bag-work. 



Concrete placed in strong bags made of jute sacking, and 

 deposited while in a soft state, may be conveniently applied in 

 forming under- water foundations and the bases of piers. By 

 this method, inequalities, especially in crossing rocky ground, 

 may be readily adjusted, and a fairly level bed be obtained 

 upon which to raise a superstructure. 



Where bags, or " bag-blocks," as they are sometimes called, of 

 large size are used, the spaces between them may be filled up 

 and levelled with rich concrete deposited in the manner described 

 on pp. 266 et seq., or smaller bags may be used, in which case 

 they should be placed and well packed in by divers. 



Bags of large size are often used in forming " aprons " to 

 protect foundations from disturbance by scour. They conform 

 very closely to the irregularities of the surface, and so bind 

 together the loose stones or rubble upon which they rest. This 

 quality gives bag-work a special value when applied to " aprons/' 

 or in other situations subject to much disturbance. 



The bags are commonly deposited by means of iron skips of 

 special construction, having their bottoms formed of flaps or 

 lids opening outwards and supported by catches, as shown in 

 Fig. 33. 



These skips may be of almost any capacity, but they generally 

 range from about 5 tons to 20 tons, 



In constructing the breakwater at Newhaven (Sussex), bag- 

 blocks weighing 100 tons each were deposited by means of a 



