Question

A layer of clay soil has saturated unit weight of 20 kN/m$^3$. Ground water table is located at ground surface and unit weight of water is 10 kN/m$^3$. If the over consolidation ratio of soil at 5 m below ground level is 2, what would be the maximum effective stress to which the soil has been subjected in its stress history?

• A. 200 kPa
• B. 150 kPa
• C. 100 kPa
• D. 50 kPa

Hint:

Effective stress = total stress $-$ pore water pressure. Multiply with OCR to find maximum past stress in overconsolidated soils.

Answer 1

The Correct Option is C

Given:
Saturated unit weight of soil ($\gamma_{\text{sat}}$) = 20 kN/m$^3$
Unit weight of water ($\gamma_w$) = 10 kN/m$^3$
Depth = 5 m
Over Consolidation Ratio (OCR) = 2
Effective stress at 5 m depth:
$\sigma' = (\gamma_{\text{sat}} - \gamma_w) \cdot h$
$\sigma' = (20 - 10) \cdot 5 = 10 \cdot 5 = 50$ kPa
Maximum past effective stress = OCR $\times$ current effective stress
$= 2 \times 50 = 100$ kPa