[42 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



ringed, and driven in bays of 15 feet, with a key or wedge-pile 

 in each bay for the purpose of tightening up the joints. 



The pile heads were driven down and trimmed to low-water 

 level and secured to longitudinal walings 12 inches by 6 inches 

 (Figs. 121, 122). 



The casing was divided into sections 50 feet in length by a 

 temporary cross-end of timber, which was removed after the 

 concrete was deposited. 



On the completion of a section of the casing, the interior was 

 excavated to the required depth by a Priestman's grab-dredger. 



Owing to the very soft muddy character of the material over 



FIG. 121. 



FIG. 122. 



a portion of the bottom, extending over about 150 feet in length, 

 the foundation required to be materially strengthened ; this was 

 effected by driving, after the excavation had been removed, pitch- 

 pine bearing-piles 10 inches square, and spaced 4 feet centre 

 to centre. These piles were 31 feet long, and were driven until 

 the heads were 8 feet above the bottom of the wall (Fig. 121). 



The concrete was made in the proportion of 1 part Portland 

 cement, 1J part clean sand, and 4J parts of clean ballast, or 

 stones broken to pass a 2-inch gauge. The mixing was done by 

 apparatus erected on a travelling-stage following the pile-drivers; 

 when thoroughly mixed, it was delivered into a hopper-box 



