ENCLOSING DAMS. 



61 



that it acted as an arch. 1 It may, however, be noted that this 

 would only hold good providing the workmanship was perfect ; 

 that is, that all the piles were in close contact, and that the dam 

 retained its perfect segmental form; it appears, however, that 

 the dam became very irregular in form, and bearing in mind the 

 large radius and small versed line, but little support could have 

 been derived from its circular design. 



Alexandra Dock, Hull. 2 This coffer-dam (Fig. 45), enclosing 

 the lock entrance to the Alexandra Dock, Hull, was 461 feet total 

 length. It was constructed of two rows of whole-timber piles 

 13 inches square, spaced 6 feet apart, with clay puddle between, 

 and shored at the rear in the manner shown by Fig. 45. 



i r /t H bolts 



SCALE 

 10 20 



40 FEET 



FIG. 45. 



The piles were from 50 to 60 feet long, and were driven 

 about 33 feet into the ground, the main piles reaching down to 

 54 feet below high water. 



Inside and outside walings 13 inches square were spaced 

 about 5 feet apart, centre to centre, and tied together with 2-inch 

 through-bolts. 



Five sluice-valves or openings 2 feet square were made to 

 allow the tide to flow in and out till the completion of the dam. 



The connections between the ends of the dam and the em- 

 1 M.P.I.C.E., voL li. p. 152. 2 Ibid., vol. xcii. p. 146. 



