154 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



The first operation, where the water was not sufficiently deep, 

 was to dredge two parallel trenches to 17 feet below low water, 

 for the foundations of the walls. The erection of a timber 

 staging followed, extending over the whole width of the 

 work, for carrying the tramways, travelling-cranes, and pile- 

 engines. 



Cast-iron guide-piles, placed 7 feet apart in the line of the 

 quay walls, were then driven until the heads were near the low- 

 water mark ; they were then connected transversely by wrought- 

 iron tie-rods which extended through the pier. 



When the proper depth had been made by dredging, and 

 the piling driven, a bed of concrete 3 feet thick and 20 feet 

 wide was deposited in the trenches to form a base for the wall, 

 and to ensure a large bearing surface. 



Granite slabs from 18 inches to 2 feet thick were then placed 

 in position. At the commencement of the work, they were 

 dropped into the grooves formed by the flanges of the cast-iron 

 piles, the bottom slab resting on the concrete base, and on a 

 projecting web cast on the piles (Figs. 134, 135). 



This arrangement exposed the outer face of the piles to 

 the action of the sea-water ; to remedy this, in the latter part 

 of the work, the reverse plan was adopted, and the groove 

 made in the stones which therefore overlapped the iron piles, 

 and formed a continuous facing (Figs. 131, 132). The 

 grooves were then filled up with cement which enclosed the 

 iron, and effectually preserved it from the action of the 

 salt water. In each compartment between the piles, 16 feet 

 in height and 7 feet in width, there were only three stones. 

 Behind this facing, hydraulic concrete was lowered through the 

 water in large boxes having movable bottoms to form the 

 backing and body of the wall. To confine this mass at the back, 

 loose rubble stones were deposited and carried up simultaneously 

 with it. The hearting between the two walls consisted of the 

 same description of materials, namely, rubble stones carried up 

 to the level of low water. 



The entire mass, piles and stone facing, concrete backing, 

 and hearting, was allowed to consolidate for some time, after 

 which the heads of the cast-iron piles and the facing blocks 

 were capped at the level of low water by a granite string 

 course ; the upper portion of the wall was then continued with 

 free stone ashlar face and rubble backing. The remainder of 



