Sand Drains
10.17 As mentioned previously, consolidation of the very soft clay under the fill could be accelerated by forming vertical pervious drains within the clay layer. These would be connected to the ground-water level above, enabling excess voids water to be squeezed out more rapidly than if it all had to find its way up through the clay. Sand drains, formed by boring holes and filling with sand have been used for this purpose in many countries. Required spacing of sand drains would depend, inter alia, on their diameter. If tenders were called, it would be possible to find the most economical drain size, but estimates here are given only for 12 in. dia., which is expected to provide an adequate guide for these.
10.18 2 ft. of settlement caused by consolidation of the fill could be expected to take place in about 2 years and this added to calculated settlements in the clay is shown on Fig. 2 for various triangular drain spacings together with estimated unit costs.
SQ. FT.
/$
FT. OR
12
FIG. 2: SETTLEMENT & COST/SAND-DRAIN SPACING
10
FINAL SETTLEMENT
8
6
4
2
4
6
EQUIVALENT PAPER
DRAIN
SPACING
5-YEAR SETTLEMENT
2-YEAR SETTLEMENT
1-YEAR
SETTLEMENT
UNIT COST
8
10
SPACING
-
41
12
14
16
FT.
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