a) The soil profile on a site consisted of 4 m thick homogeneous clay deposit overlying an impermeable rock. The investigation results indicated that the clay had a saturated density of 1.6 Mg/m3, an average initial void ratio of 1.8 and the preconsolidation pressure at the mid depth of the clay layer was 20 kPa. A piezometer standpipe was installed at the mid depth of the clay layer. The water table always remained at the same elevation as the original ground surface. Following the site investigation, a sand fill, which was large in lateral extent, was placed on the ground surface. The height of the sand fill was 1 m and the total density of the sand fill was 2.0 Mg/m3 above and below the water table. Two years after the placement of the sand fill, the ground settlement was 30 cm and the water level in the piezometer standpipe was at 0.6 m above the water table. Ignore the buoyancy effect on the applied pressure from the sand fill in your calculation. Treat the clay as one layer; subdivisions are not required.
(i) Calculate the coefficient of consolidation and the compression index of the clay. The recompression index of the clay can be taken as 10% of its compression index.
(ii) Three (3) years after the placement of the sand fill, a soil sample was taken from the mid depth of the clay layer and a consolidation was performed on the sample.
(ii-a) Estimate the preconsolidation pressure of the soil sample and calculate the consolidation settlement 3 years after the placement of the sand fill.
(ii-b) Calculate the water level in the piezometer standpipe with respect to the water table 3 years after the placement of the sand fill.
b) Briefly explain the difference between piezometer and tensiometer with respect to the working principle, special design consideration and limitation of the instrument.