186 HARBOUR CONSTRUCTION. 



A convenient way of depositing the concrete is to fill it into 

 skips having hinged flap bottoms. The skips are then lifted 

 by a crane and swung over the site where the concrete has to 

 be lodged. They should be lowered to within a short distance 

 of the bottom, only sufficient room being left for the flaps to 

 swing open. On the catch being released which is usually 

 done by means of a small chain from above the contents of the 

 skip are discharged. The skip should then be withdrawn from 

 the water slowly and steadily, every precaution being taken to 

 avoid disturbance of the cement. 



The practice, sometimes adopted, of sending a diver down 

 into the frame to level the concrete is objectionable, and is 

 calculated to do more harm than good, inasmuch as the cement, 

 if stirred up in the water, either loses its setting properties, or 

 is washed out and lost. 



With the exercise of due care in regard to the position of 

 the skips, and the occasional use of a sounding-pole, concrete 

 may be deposited with sufficient regularity within the frame to 

 obviate the necessity for any manipulation until its surface has 

 been raised above the water-level. 



Sometimes mass concrete is deposited under water by passing 

 it through a shoot or trunk. In order to exclude the water, 

 the shoot should be kept full of concrete by filling it in con- 

 tinuously at the top. It should extend nearly to the bottom, 

 and it ought to be slowly moved about so as to distribute 

 the concrete. The system of depositing concrete by means 

 of skips is, however, generally preferred. A system which 

 I have found successful, and which I believe I may claim 

 to have originated, is that of lowering concrete through water 

 in bags of convenient size, mouth downwards. Those I have 

 used contained about 2J cubic feet of concrete apiece, but, if 

 desired, they could, of course, be made to contain much more. 



The system will be more fully described when treating of 

 rock foundations (pp. 267 et seq.). 



In order to avoid the formation of cleavage surfaces, it is of 

 importance that the deposit of concrete should be as continuous 

 as possible. 



Very good work may be done on the monolithic system, but 

 so much depends upon the care with which it is executed, that 

 ifc entails a considerable element of risk. It has been largely 

 used both above and below water, and is generally applicable 



