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GATE 2025 CE (CE1) – Question 38

Geotechnical Engineering · Compaction and one-dimensional consolidation · 2 marks · Multiple choice

In an oedometer apparatus a specimen of fully saturated clay has been consolidated under a vertical pressure of 100 kPa and is at equilibrium state. Immediately on increasing the vertical pressure to 150 kPa, the effective stress $\sigma'$ and excess pore water pressure $\Delta u$ will be

  1. $\sigma' = 50$ kPa, $\Delta u = 100$ kPa
  2. $\sigma' = 100$ kPa, $\Delta u = 50$ kPa
  3. $\sigma' = 150$ kPa, $\Delta u = 50$ kPa
  4. $\sigma' = 100$ kPa, $\Delta u = 150$ kPa

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Show answer and explanation

Correct answer: (B) $\sigma' = 100$ kPa, $\Delta u = 50$ kPa

Explanation

A fully saturated clay has water that cannot escape at once, so the whole increase of 50 kPa is carried by the pore water at the first instant. The excess pore pressure is then $\Delta u = 50$ kPa and the effective stress is still 100 kPa. As the water drains the effective stress rises to 150 kPa.