GATE 2016 CE (CE1) – Question 52
An undisturbed soil sample was taken from the middle of a clay layer (i.e., 1.5 m below GL), as shown in figure. The water table was at the top of clay layer. Laboratory test results are as follows: Natural water content of clay : 25% Preconsolidation pressure of clay : 60 kPa Compression index of clay : 0.50 Recompression index of clay : 0.05 Specific gravity of clay : 2.70 Bulk unit weight of sand : 17 kN/m3 A compacted fill of 2.5 m height with unit weight of 20 kN/m3 is placed at the ground level. Assuming unit weight of water as10 kN/m3, the ultimate consolidation settlement (expressed in mm) of the clay layer is ______.
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Correct answer: 36 to 38
Explanation
Saturation gives initial void ratio e0=wG=0.25·2.70=0.675. At the clay mid-depth, effective stress is17+0.5[((2.70+0.675)/1.675)·10−10]=22.075kPa. Fill adds2.5·20=50kPa, crossing60kPa. Settlement is1000/(1.675)[0.05log10(60/22.075)+0.50log10(72.075/60)]=**36.73mm**; use recompression below60 and virgin compression above it.