ENCLOSING DAMS. 



49 



Immediately behind each pile, a mass of rubble stones was 

 roughly built, to give further stability, and to divide the pressure 

 over a large surface of earth filling. 



PLAN 

 FIG. 35. 



Coffer-dam at the Middle Level. 1 For closing this dam, when 

 the outward rush of water, as the tide fell, was very great, Sir 

 J. Hawkshaw adopted a system of sliding panels. The method 

 of constructing the dam (Figs. 36 to 39) being as follows : 



Temporary piles were first screwed into the ground, on which 

 a platform was erected to carry the pile-driving engines. Two 

 rows of close sheet-piling 25 feet apart were then driven trans- 

 versely down the slope of the drain on each side and extending 



FIG. 36. 



from the centre of the banks to within about 20 feet of the 

 bottom, leaving a central space of 88 feet in length to be com- 

 pleted in the following manner : 



Piles of whole timbers were driven on each side so that these 

 would be in pairs 7 feet 6 inches apart from centre to centre. 

 The piles in each pair were placed back to back, a space of 7 

 inches being left between them, preserved by a central piece 

 7 inches by 3 inches. 



The piles were driven separately; one with the distance- 



1 M.P.I.C.E, vol. xxii. p. 499. 



