96 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



east and west dock walls at certain points, and the whole of 

 the work at the northern end of the lock seemed in jeopardy. 



The continued flow of water highly charged with silt was 

 likely to lead to further injury, whilst 14 feet of excavation 

 remained to be accomplished for the foundations under the sill 

 as designed. To reach the origin of the disturbance, it was 

 clearly necessary to carry the sheeting lower down ; and 

 accordingly, pitch-pine piles were driven so as to enclose the 

 principal blow-holes and disturbed places, and effectually cut off 

 the flow of water into the foundation. The excavations were 

 then made, and the foundations completed without the slightest 

 appearance of water. 



Avonmouth Lock Foundations. 1 The foundations of this lock 

 rest upon a bed of fine grey sand underlying the clay at an 

 almost uniform level throughout its length, and at a depth of 

 about 6 feet under low water of equinoctial spring tides, or about 

 the level of the bed of the Avon near the dock. The depth of 

 the bed of sand was not ascertained, the trial borings not 

 reaching deeper than 16 feet below low water of equinoctial 

 spring tides ; piles were, however, driven to 9 feet below the 

 level of the borings, and as far as pile-driving can be relied upon 

 as indicating the nature of the sub-strata, there appeared to be 

 no change down to that depth. The frequent occurrence of 

 springs in this sand was a source of some trouble and difficulty ; 

 most of them were weak, though two or three discharged a 

 considerable quantity of water ; but whether weak or strong, they 

 never appeared until the last layer of clay was removed, and the 

 sand laid bare. A layer of clay 6 inches thick was sufficient to 

 keep the water down, but when this was once removed or dis- 

 turbed the springs burst out ; and though the smaller ones were 

 stopped after some weight of masonry had been placed on them, 

 it was impossible to overcome the strong ones, and they had 

 to be provided with permanent outlets. One of the strongest 

 occurred in laying the foundations of the outer round head, on 

 the west side, which for a long time persisted in coming up at 

 the same spot, in spite of every effort to shift it to a less 

 dangerous place. After the wall had been built, it continued to 

 wash up a little sand at the toe of the footings, producing large 

 cracks in the masonry, and causing the whole of the round head 

 to settle downwards about 3 inches. The cracks were filled up 

 M.P.I.C.E., vol. Iv.p. 10. 



