Question

The unconfined compressive strength of a sample of clay is found to be $20kN/m^2$. The drained angle of internal friction of the soil is about ${27}^{\circ }$. The magnitude of pore water pressure at failure will be

• A. Zero
• B. 10 kPa
• C. -10 kPa
• D. -12 kPa

Answer 1

The correct option is D -12 kPa

Given data of unconfined compression test

${\sigma }_3=0$ ${\sigma }_1=20kPa$

Let the pore pressure be 'u' kPa.

${\sigma }_3^{\prime }={\sigma }_3-u$
$=0-u$
${\sigma }_1^{\prime }={\sigma }_1-u$
$=20-u$
${\varphi }^{\prime }={27}^{\circ }$

The failure envelope in terms of efficient success is shown below

$sin{\varphi }^{\prime }=\frac{{\sigma }_1-{\sigma }_3}{{\sigma }_1+{\sigma }_3}$

$sin{27}^{\circ }=\frac{(20-u)-(-u)}{(20-u)+(-u)}$

$=\frac{20}{20-2u}$

$u=-12kPa$