54 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the dam was close planked and thoroughly caulked. The head 

 of pressure at high water of ordinary spring tides was 44 feet. 

 This dam proved satisfactory in every way, and so little ironwork 

 was used in framing the trusses that the whole of the timber was 

 fit for permanent purposes after the dam was removed. 



Dublin Coffer-dam. 1 This dam (Fig. 42) was considered to 

 be of very light construction, bearing in mind the purpose for 

 which it was used, viz. the removal of an old quay wall, and 

 the construction of a new wall at a much greater depth. 



" The strata consisted of 2 feet of soft mud, 4 feet of fine 

 sand, 3 feet of vegetable deposit overlaying a thick bed of 

 alluvial clay, ranging in thickness from 4 to 11 feet, and a bed 

 of strong gravel, interspersed with fine sand and boulders, and 

 in some places brown clay." 



The design consisted of two rows of piles with clay-puddle 

 filling. In the outer row, the main piles of Memel timber, 32 

 feet long by 12 inches square, were driven as gauge-piles at 

 distances of 12 feet from centre to centre. In the inner row, 

 the piles were 40 to 42 feet long, spaced to correspond with the 

 piles in the outer row ; the space for the puddle between the 

 two rows of piles being 4 feet 6 inches. The spaces between 

 the main piles in both the inner and outer rows were filled in 

 with sheet-piles 12 inches wide by 6 inches thick. Those in 

 the outer row were driven 12 feet into the ground, and reached 

 up to the level of high water, the total length being 28 feet. 

 The sheet-piles in the inner row were of similar length, but 

 were driven down to the level of the foundation of the new 

 wall, 24 feet below low water, their heads reaching to 4 feet 

 above low- water level. 



The sheet-piling was driven in line with the inner faces of 

 the gauge-piles, so as to offer no obstruction to the settlement 

 of the puddle, and the lower edge of the 3-inch horizontal 

 planking with which the dam was sheeted on the inner row 

 was levelled off with the same object. 



Two IJ-inch tie-rods of wrought iron were passed through 

 two half- timber wales on the outside and inside of the main 

 piles, provided with cast-iron washers 8 inches square and 2J 

 inches thick at each end, and screwed up with hexagonal nuts. 

 The tie-rods were placed one at 1 foot below high- water mark, 

 and the other at 2 feet above low water. 



1 M.P.J.CK vol. li. p. 137. 



