140 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



which was set within timber caissons about 67 feet long and 

 13 feet wide ; these caissons were floated at high tide over the 

 piles, and allowed to settle down upon them as the tide receded. 

 The sides of these caissons were removed when the mass of 

 be'ton within was set, and they are used repeatedly in the con- 

 struction of this part of the wall. The blocks of be'ton are 

 surmounted by a wall in masonry, but this was constructed at 

 low tides, before the caisson that served as a coffer-dam was 

 removed; a facing of brick covers the whole front of the 



masonry that is exposed to the action of water, and a clear 

 space of 12 inches left between the successive blocks of be'ton is 

 filled in with cement. 



This type of quay wall has given very excellent results, and 

 has shown itself capable of resisting the thrust of the water 

 and the mud deposited, and as it is very economical in construc- 

 tion, costing less than 75 per yard run, French engineers have 

 not hesitated to adopt a similar type in other special works that 

 have had to be constructed in this and other ports. 



Training-walls on the Lower Seine, These embankments 

 or training- walls are composed chiefly of chalk, with a facing 

 of concrete, or a stone-pitched face set dry (Figs. 119, 120). 

 The concrete was deposited in place during periods of low 

 water, and protected from the action of the rising tides by 



1 En<jinnnij, vol. xlviii. p. 415. 



