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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 σ' and excess pore water pressure Δu will be

1. σ' = 50 kPa, Δu = 100 kPa
2. σ' = 100 kPa, Δu = 50 kPa
3. σ' = 150 kPa, Δu = 50 kPa
4. σ' = 100 kPa, Δu = 150 kPa

Correct answer: σ' = 100 kPa, Δu = 50 kPa

Solution

When the vertical pressure is increased from 100 kPa to 150 kPa, the total stress increases by 50 kPa. Since the clay is fully saturated, this increase in total stress results in an equivalent increase in pore water pressure, leading to an excess pore water pressure of 50 kPa. The effective stress is then calculated as the total stress minus the pore water pressure, resulting in an effective stress of 100 kPa.

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