200
DOCK ENGINEERING.
Fig. 133 shows a section of a timber dam (a description of it will be
found on p. 105, ante) which has been floated over the site (at Liverpool),
weighted, sunk, and piled. The bottom edge of the dam has been made
watertight by means of a tipped bank of clay puddle, which is prevented
from slipping away under the softening influence of water by barges sunk
on the outer side. Within the enclosed area thus provided, work may
High Water
Puddled
Clay
Level
Roacti
Rock
Puddled
Clay
Roach
Fig. 133. — Construction within Temporary Dam.
li
Fig. 134. —Dam and Quay Wall at Ardrossan.
proceed as usual. Pumping power, however, is in this case a more essential
feature, not only on account of emptying the dam in the first instance, but
also for dealing with leakage, which is sure to be continuous, and the pos-
sibility of an inburst of water. Inbursts are most likely to occur in faulty
ground, the water being forced, under the great head, through a pervious
stratum in the dock bottom. It is, therefore, advisable to dredge the site