-5-
10
the maximum depth being about 61 feet, the rock being jointed
and decomposed in places.
consists of sand and shingle.
The material overlying the rock
22. The bottom of the valley is flat, varying in level from 10 to 14 and is about 400 feet in width, the sides rising steeply at slopes of about l† to 1 on the eastern and 2 to 1
on the western side.
23.
The sand and shingle being pervious and saturated
with water owing to the river and the close proximity of the sea, points to the advisability of constructing the foundations within steel pile cofferdams to minimise the pumping which would otherwise be required, and our estimate has been prepared on that assumption.
24.
The whole bottom of the valley is subject to flooding during the monsoon and the top of the cofferdams should be at such a level as to obviate flooding of the excavation and when
constructing the lower portion of the dam.
25.
Judging from a surface examination and the records of the prickings which were carried down to rock, it would appear that the great bulk of the excavated material would be suitable
for concrete.
26.
Various designs for the dam were investigated, with the result that the most suitable type for this particular location would take the form of a gravity masonry dan resting on undecomposed though jointed rock, percolation below the dam being dealt with by a cut-off wall carried down at the base of the dam to a considerably lower level, and grouting.
27.
Borsholes cannot be relied upon to give more than an approximate idea of the sub-surface conditions, and the actual depths of foundations can only be determined as excavation proceeds, but in preparing our estimate we have assumed depths which in all probability will prove more than adequate.
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