/o NOTES ON DOCKS AND DOCK CONSTRUCTION. 



at a distance of about 16 feet 6 inches inside the second ring of 

 the dam, and tied back with whole-timber baulks. The space 

 between this additional sheeting and the dam was then filled in 

 with sand. 



The effect of the vibration caused by the pumps working on 

 a stage connected with the dam was considered to be injurious; 

 therefore a new stage was erected in a position clear of the dam. 



The water was again pumped out, and the excavation for the 

 lock was proceeding, when a large mass of earth broke off inside 

 the dam at a distance of 38 feet from the inner ring of piles, in 

 the line of a small grip which had been made for drainage. The 

 sluice-valves were immediately opened, the water from them 

 being allowed to fall on a bed of fascines laid for the purpose at 

 the spot. The ground, however, between the place where the 

 mass of earth had slipped and the dam soon fell in, leaving the 

 piles bare for a depth of 30 feet, and causing a breach in the 

 dam. An interval of two minutes elapsed from the time the dam 

 first began to bend until the breach opened, and in about three 

 minutes more the space inside the dam was filled with water. 



It became evident from these failures that the dam needed 

 further strengthening ; this was done chiefly by depositing 

 more sand both inside and on the outside, and driving a fresh 

 row of close sheet-piles on the outside, at a distance of 29 feet 

 6 inches from the inner row of piles, to keep the sand in place. 

 Sand-bag buttresses, with layers of fascines, were also con- 

 structed inside the dam, when there was sufficient space between 

 the dam and the brickwork. 



Outside it always proved difficult to keep the sand from 

 being disturbed by the wash of the waves. Three groynes of 

 pile-work driven on the most exposed side had a useful effect, 

 but the waves still continued to disturb the mound of sand, and 

 it was some time before it could be raised to the water level and 

 covered with fascines. 



Much trouble was experienced in winter from floating ice, 

 raking piles were driven outside the dam 10 feet apart, but they 

 frequently broke when huge masses of ice came against them, 

 triangular chocks were spiked at intervals under the outer lower 

 waling, so that the ice might the more readily slide up the 

 waling, and not lift it. When the lake was frozen, the ice was 

 always kept broken round the dam; the result of these pre- 

 cautionary measures was that no damage occurred to the dam. 



