62 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



bankment were to be made good by a seal of clay keyed into 

 the clay backing of the embankment, but on the western side 

 some runaway trucks deposited their loads of chalk in the trench 

 which was being formed round the dam ; and, as this chalk was 

 not fully removed, owing to its low level, the water leaked 

 through in several places near the junction. 



The leakage was stopped by sinking a trench outside the 

 dam, removing all the chalk, and filling the trench with fresh 

 puddle. This difficult operation was also carried out at the 

 western junction for the purpose of overcoming a similar 

 leakage. 



Pola Dock. 1 In driving the sheet-piling to form the casing 

 for the concrete walls of the Pola Dock it was desirable that the 

 inner surface should be accurate and even, so as to render the 

 face of the beton wall uniformly smooth. 



This, in deep water and with timber of light specific gravity, 

 was somewhat difficult of accomplishment. To obviate the 

 difficulty, an apparatus termed a " Spider " was devised by the 

 engineer, Mr. H. E. Towle (Fig. 46). Two timbers were formed 

 by sawing a log 35 feet long and 12 inches square so that the 

 larger ends were 8 inches by 12 inches, and the smaller ends 

 4 inches by 12 inches. These were placed side by side 

 horizontally, with the larger ends in the same direction, and 

 spaced 12 inches apart, and parallel to each other, to form the 

 side pieces. At about 8 feet from the thicker ends was fitted a 

 solid oak block 30 inches thick and about 6 feet in length, a 

 width of 9 inches on either side of the lower end of the block was 

 cut away, leaving a thickness of 12 inches in the centre, which 

 was fitted between the parallel side pieces at an angle with the 

 vertical of 33, and the whole securely bolted together. Two in- 

 clined side pieces or cleats of timber 8 inches by 12 inches, and 

 17 feet long, were secured to the oak block and to the side 

 timbers at their thicker ends, the whole forming a throat or clip 

 in which to receive the piles, the side pieces and sloping oak 

 block forming a guide for the point of the pile to the 12-inch 

 space between the horizontal timbers below. Two vertical 

 timbers, termed hangers, 8 inches by 12 inches, and about 20 

 feet in length, were inserted between the side pieces in the rear 

 of the oak block and spaced about 20 feet apart, and hinged at 

 the lower ends by stout iron bolts passing through the si<le 

 1 M.r.l.C.K,vu}. xxxii. p. CO. 



