128 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



served to increase the thickness of the base, and afforded increased 

 resistance to the thrust of the back piling. 



The concrete blocks (Figs. 96, 98), forming the face of the 

 wall below the low- water level, were made 12 feet in length, 

 7 feet wide at the base, and 5 feet at the top, the height in 

 front 13 feet, and at the back 14 feet, the weight of each being 

 70 tons. 



The blocks were moulded in timber casings ; the concrete 

 being composed of one part Portland cement to two parts of 

 sand mixed dry, and afterwards moistened with a sufficiency 

 of water to form stiff mortar. To this mortar stones broken 

 to pass through a 3-inch gauge were added in such proportion 

 that the mortar was just sufficient to fill the interstices of the 

 mass when well rammed. The ratio of interstices to bulk of 

 stone was determined by a water test as well as by the practical 

 mixing of samples. There was some variation in the propor- 

 tions depending greatly on the nature and size of the stone 

 used. Practically, however, the proportions used were 1 of 

 Portland cement, 2 of sand, and 5 of broken stone. 



Vertical grooves were moulded on each side (Figs. 96, 97, 

 98), and an opening formed through the block near the bottom 

 for housing the chains used for lifting. After being lowered 

 into position, a rope was made fast to one end of the chain, 

 which was then drawn out, the use of the rope being to allow 

 the chain to descend only as fast as it could be withdrawn, so 

 as to prevent fouling in the groove. 



Before the blocks were lowered into place, a mattress, of 

 the same area as the base, consisting of a bag filled with two 

 inches of mortar composed of a slow-setting cement, was placed 

 on the top of the pile heads. In order to place this mattress 

 in its proper position, it was attached to the under side of a 

 wooden frame by a network of merlin ; when sunk, divers cut 

 away the lashings and liberated the frame which floated, leaving 

 the mattress laid out flat in its proper position on the pile heads. 

 The block was then lowered on to the mattress, which, owing 

 to its plastic condition, neutralized any irregularity in the 

 cutting off, and ensured a fair and regular bedding for the block 

 on the pile heads. 



In order to demonstrate the utility of the mattress, an 

 experiment, under conditions as closely as possible those obtain- 

 ing in the actual work, was carried out in such a manner that 



