ENCLOSING DAMS. 



45 



Each section, 12 feet 6 inches long, 7 feet wide and 4 feet 

 6 inches deep, weighed about 30 cwt. 



The caissons were sunk by excavating the ground within 

 them, and weighting with iron blocks of about 9 cwt. each, cast 



T.H. 



to the shape of the rings, and piled upon timbers which rested 

 upon the flanges and formed the strengthening struts. 



Timber Dams used on the Thames. 1 In putting in the dam 

 shown by Figs. 30, 31, with the view of saving time, the ground 



PLAN 

 FIG. 31. 



was not dredged before the piles were driven ; the driving was 

 in consequence a slow and difficult operation. In many cases 

 it was found impossible to force the piles down, and many had 

 to be drawn and replaced. Generally, when drawn, the piles 

 were found to have cast their shoes, and their points were 

 bruised into a mass of tangled shreds. The failure usually 

 occurred whilst the point of the pile was passing through a bed 

 1 M.P.I.C.E., vol. xxxi. p. 3. 



