146 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



The largest blocks in this instance were 12 feet long, 6 feet 

 6 inches wide, by 5 feet high, and weighed 25 tons. They were 

 transported from the block-yard on tramways worked by ropes 

 driven by steam power, and were allowed to run down a slip 

 into the sea, until the upper surface was slightly below the 

 surface. 



A raft consisting of two boxes or pontoons, placed at such a 

 distance apart as to admit a block between them, and connected 

 at the top by a frame of timber, was then floated over the 

 submerged block. 



By admitting water into the pontoons, the raft was lowered 

 over the block, which was then made fast to the timbers by 

 chains. On the water being pumped out, the raft rose with the 

 block suspended ready for removal. 



When placed over the proper position in the work, the block 

 was gradually lowered by again letting the water into the 

 pontoons, and guided in its descent by a diver ; when accurately 

 placed, the chains were removed and the raft floated away. 



Quay Walls at Dublin. 1 A good example of a deep-water 

 quay wall constructed without a dam may be found in the 

 work carried out by Mr. B. B. Stoney, M.A.M.I.C.E., in improving 

 the basin accommodation at Dublin. 



The method adopted consisted in the use of artificial blocks 

 of such magnitude as rapidly to bring the wharf wall above 

 low-water mark, and so to avoid the use of coffer-dams, staging, 

 and all the incidental expenses connected with the usuaJ 

 methods of getting in foundations under water. 



The special appliances required were a powerful floating 

 shears (Figs. 125, 126) for lifting the blocks off the building 

 wharf and conveying them to their destination, and a diving- 

 bell for excavating and levelling the ground (Figs. 127, 128). 



The position of the works was extremely well sheltered, and 

 interruptions due to bad weather were consequently reduced to 

 a minimum. 



The blocks were built upon a wharf about J mile from the 

 site of the work (Figs. 129, 130). 



The blocks as constructed on the wharf were 27 feet in 

 height, 21 feet 4 inches wide at the base, 12 feet 9 inches 

 at the top, and 12 feet long in the direction of the wall. Each 

 block contained nearly 5000 cubic feet of material, and weighed 



1 M.P.I.C.K, vol. xxxvii. p. 332 ; nnd Engineering. 



