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(c) A loose sand layer is treated by sand piles which are installed by vibration method. Before the installation of sand piles, the sand layer has a thickness of 13 m and an average void ratio of 0.80. After the installation of sand piles, the ground subsides by 0.8 m. The sand piles formed from imported sand have a diameter of 0.8 m and a centre-to-centre spacing of 2.2 m in a square pattern. The length of the sand piles is equal to the final thickness of the sand layer. Determine the average void ratio of the sand layer after treatment. (8 Marks) (d) For the sand layer described in Q1(c), the minimum and maximum void ratios of the sand determined in the laboratory are 0.41 and 0.90, respectively. Calculate the relative density of the sand layer before and after installation of sand piles. Which in-situ tests are commonly used to evaluate the relative density of sand after ground improvement? (5 Marks)
Appendix Relationships between average degree of consolidation and time factor: 0,1 T, = Hai T For U, <52.6%, T, U, 2 For U, 52.6%, T, = -0.933 log(1-U)-0.085 U =1-(1-U)(1-0) Un =1 -8T U = 1 -exp F(N) C, T d? N. F (N.)= In(NL)-0.75+)In(N.) N k d = 2(a+b)/a, N, = d.1d, N, = d, 1 d. d. = equivalent diameter of a unit cell d = equivalent diameter of vertical drain de = diameter of smear zone k, k, = radial permeability of undisturbed soil and smeared soil, respectively