GATE 2021 CE (CE1) – Question 54

Geotechnical Engineering · Compaction and one-dimensional consolidation · 2 marks · Numerical answer

Sand extends 15 m below ground; an 8 m saturated clay layer follows. Sand unit weights are 17.5 kN/m³ above GWT and 18.5 below; saturated clay is 17. GWT is presently at 5 m depth and historically fell to 10 m. With γw=9.81, find OCR at clay midpoint to two decimals.

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Correct answer: 1.18 to 1.26

Explanation

At depth 19 m, present effective stress is $5(17.5)+10(18.5)+4(17)-14(9.81)=203.16$ kPa. With the historical 10 m GWT it was $10(17.5)+5(18.5)+4(17)-9(9.81)=247.21$ kPa. OCR is the maximum past effective stress divided by present effective stress: $247.21/203.16=\mathbf{1.22}$.