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4. (3 marks) The soil profile at a site is shown below and consists of 2 m of silty sand underlain by 5 m of soft, normally consolidated sandy clay, then a layer of sandy gravel. The water table lies 1.5 m below the ground surface level. A 3 m-high compacted fill is constructed at the ground. Soil properties are as follows: Silty Sand: bulk unit weight = 17 kN/m, saturated unit weight = 18 kN/m; Soft Sandy Clay: fully saturated (S=100%), initial water content = 27.5%, specific gravity = 2.71, Compression Index (C) = 0.58, Coefficient of Volume Compressibility (my) = 0.00135 kPa? coefficient of consolidation of clay (c) = 2 m/yr; Compacted fill: bulk weight = 19.5 kN/m'. 4.1 Calculate the initial void ratio (e.). 4.2 If the soil is normally consolidated, what will be the pre-consolidation pressure at the mid- height of the clay layer. 4.3 Estimate the final consolidation settlement (se) of the clay layer, using the Compression Index method. 4.4 Estimate the final consolidation settlement (sc) of the clay layer, using the Coefficient of Volume method. Compare this answer to the settlement estimate in 4.3 and discuss the difference. 4.5 Based on the settlement estimate in question 4.3, (a) estimate the time for 50 mm consolidation settlement, in years and days. (b) Estimate the consolidation settlement in 15 months. (c) How long will it take the clay layer to reach 90% consolidation in years.
1.5m Silty Sand SM 2m Soft Sandy Clay CH 5m Sandy Gravel GP LE