248 HARBOUR CONSTRUCTION. 



control, and can be more easily placed in position when deposit- 

 ing material. 



Barges, somewhat similar to those employed for depositing 

 rubble, are used for laying concrete bag- work foundations. The 

 bases of the breakwaters at Aberdeen (north pier), Fraserburg, and 

 Newhaven (Sussex) were formed in this manner (pp. 188 et seq.). 



Barges are also employed for depositing the blocks used in 

 constructing breakwaters of the pell-mell type. So long as the 

 surface of the mound is sufficiently low to allow of the barges 

 floating over it, the blocks may either be dropped through the 

 hopper bottom in which case the depth must be sufficient to 

 admit of the blocks so dropped clearing the bottom of the barge 

 or they may be launched from the barge decks, which should 

 be formed to a suitable inclination, and provided with convenient 

 appliances for holding the blocks in place and releasing them 

 when required. 



Sometimes, instead of the blocks being dropped or launched 

 as just described, they are suspended from a beam resting upon 

 two barges or pontoons, and are thus floated into position. 1 

 When conveying blocks in this way, their weight is greatly 

 reduced by allowing them to remain immersed. 



When the mound becomes too high to admit of barges passing 

 over it, the blocks must either be cast in situ, or be placed in 

 position by means of floating shears. A drawing of one of the 

 25-ton barges, fitted with shears, which was used for this 

 purpose in constructing the breakwater at Alexandria, is given 

 in Fig. 65. 



It is not always easy to satisfactorily compare the relative 

 advantages of staging and barges; but the following are some 

 of the principal points which require consideration : 



(1) First cost and maintenance. 



(2) Cost of quay or stage whereat to load the barges. 



(3) The provision of adequate shelter for all floating plant. 



(4) Working expenses of floating plant, also of cranes and 

 other appliances for loading the barges. 



(5) Sea-risk. 



(6) Relative amount of work which can be done by the two 

 systems in a given time. 



1 Blocks weighing from 200 to 300 tons were conveyed and deposited in this 

 way in constructing the South Gare breakwater, at the mouth of the Kiver Teea 

 (Min. Proc. Imt. C.E., vol. xc, p. 352). 



