;6 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



became rapidly less toward the ends; and very little farther 

 compression was noticed during the use of this dam. Probably 

 more took place where the pressure was directly greater, but it 

 was not shown by any undue straining, and there were no direct 

 means of testing the amount, the greatest visible strain being 

 the sinking of the washer-plates into the lower waling, and even 

 this was not severe. 



When the water had been partially removed, a bank of earth 

 was deposited against the inside of the dam, to about 10 feet 

 above the bottom of the basin, thus coming up to within 8 feet 

 of the lower waling ; this had a top of about 3 to 4 feet in width 

 against the dam, with a slope of about 1 J to 1 ; it was added as 

 a support to the inner row of piles, not that they in any way 

 seemed to require it. 



To ensure perfect accuracy of form, and that no lateral dis- 

 placement might be possible in any of the piles which, if it 

 had occurred, would have been fatal to the very intention of its 

 curved form a temporary third tier of walings was used. This 

 tier was composed of whole timbers, sawn like the rest, to the 

 proper curve. After the gauge-piles had been driven, and the 

 upper walings fixed, these timbers were attached to the gauge- 

 piles, close to the bottom of the basin, by divers, and through 

 them the piles were driven ; by this course they could not escape 

 being correctly driven and the true form of the dam preserved. 



On the completion of each bay, the temporary walings were 

 removed and refixed in advance of the pile driving. 



Limerick Dock. 1 Fig. 54 shows the dam adopted by Mr. 

 J. Hall, B.E., A.M.I.C.E., in reconstructing the Limerick Dock 

 walls. It consisted of two rows of piles 40 feet long and 12 

 inches by 10 inches scantling, placed 5 feet apart and driven 

 10 feet into the ground, and the space between being filled in 

 with puddled clay well rammed. The foot of the dam inside 

 was strengthened by a counter-dam filled with the excavated 

 rubbish, and the whole stayed immediately from the shore. 



Dockyard, Chatham. Fig. 55 shows an embankment formed 

 across the entrance of St. Mary's Creek, Chatham, for the 

 purpose of enclosing the site of the new docks. It waa con- 

 structed of soil, gravel, and loam, taken from the general excava- 

 tions and deposited on the soft upper mud, through which it 

 sunk to the denser mud below. 



1 M.P.I.CE., vol. ciii. p. 2G2. 



