38 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



arrangement of tongues and grooves will form in some measure 

 a guide to the piles in their descent, and although they may 

 still be irregular on plan at the foot, the timbers will be more 

 uniformly in contact. 



An excessive thickness of clay puddle is a mistake ; it throws 

 an undue strain on the tie-bolts and fastenings. It has been 

 proved by experience that a comparatively thin lining of clay 

 well compressed, is all that is required to keep out the water. 



Dams subject to a considerable rise and fall of the tide are 

 seldom in a state of rest owing to the varying pressures. A 

 settlement of the puddle is frequently the result, producing leaks 

 of a more or less serious character at the through-bolts. The 

 clay, as it settles, leaves a vacuity under the bolt, through which 

 the water finds its way. 



Various devices are resorted to with a view of meeting this 

 difficulty. In some instances, through-bolts have been dispensed 

 with altogether, retaining only the top tier, and buttress piles 

 driven outside, to take the pressure of the puddle within the 

 double row of piles. 1 This course cannot, however, always 

 be followed ; as in cases where the ground is very bad and 

 insufficient to support the vertical and lateral thrust of the 

 clay, it will then be necessary to resort to through-bolts at 

 the bottom, at or near the ground line, to tie the piles 

 together. 



An efficient method of overcoming the difficulty of through- 

 bolt leakages consists in the use of a thin plate of wrought or 

 cast iron, from twelve to fifteen inches square, with a hole near 

 the upper edge to fit the bolt. When the bolt is put in place, 

 one or more of these plates are slipped on, and distributed over 

 the length of the bolt, so that they hang down, cutting off any 

 cavity that may form beneath on the settlement of the clay, 

 which, however, still adheres to the sides of the plates, and thus 

 stop the flow of water. 2 



In connecting two rows of piles, great care is necessary to 

 ensure, by efficient walings and equal tightness of the through- 

 bolts, a uniform distribution of the strains due to the weight of 

 the clay puddle. If any bolt is looser than others, the accumu- 

 lated strain is thrown upon the adjacent bolts, w T hich may in 

 consequence give way, with disastrous results 



To pass an additional bolt through a dam after the puddle 



1 M.P.I.C.E., vol. li. p. 152 ; vol. xxxi. p. 27. * Ibid., vol. xxxi. p. 27. 



