$4\mathrm{.}7$ A half$-$closed clay layer is $8$ m thick and it can be assumed that $c_v=c_h.$ Vertical sand drains $300$ mm in

diameter, spaced at $3$ m centres in a square pattern, are to be used to increase the rate of consolidation

of the clay under the increased vertical stress due to the construction of an embankment. Without sand

drains, the degree of consolidation at the time the embankment is due to come into use has been calcu$-$

lated as $25\%.$ What degree of consolidation would be reached with the sand drains at the same time?