GATE 2020 CE (CE2) – Question 58
A 5 m vertical wall supports saturated clay of unit weight 18 kN/m³, cohesion 20 kPa and friction angle zero. To prevent a tension zone, the wall is raised and dry sand placed above the clay. Sand unit weight is 16 kN/m³ and friction angle 30°. The wall back is smooth and backfill horizontal. Find minimum added wall height (m), to one decimal.
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Correct answer: 2.4 to 2.6
Explanation
For φ = 0 clay, $K_a=1$ and lateral pressure at its top is q − 2c. Avoiding tension requires $q\ge2c=40$ kPa. Added sand produces q = 16h, hence **h = 2.5 m**.