Question

A consolidation undrained (CU) triaxial compression test is conducted on a normally consolidated clay at a confining pressure of 100 kPa. The deviator stress at failure is 80 kPa, and the pore-water pressure measured at failure is 50 kPa. The effective angle of internal friction (in degrees, up to one decimal place) of the soil is

• A. 26.38

Answer 1

The correct option is A 26.38
$Confiningpressure={\sigma }_3=100KPa$

$Deviatorstress={\sigma }_1-{\sigma }_3=80KPa$

${\sigma }_1=80+{\sigma }_3$

= 180 KPa

Pore pressure = u = 50 KPa

${\overline{\sigma }}_1={\sigma }_1-u$

= 180 - 50

= 130 KPa

${\overline{\sigma }}_3={\sigma }_3-u$

= 100 - 50

= 50 KPa

For NC soil C' = 0

${\overline{\sigma }}_1={\overline{\sigma }}_3\left(\frac{1+sin\varphi }{1-sin\varphi }\right)$

$130=50\left(\frac{1+sin\varphi }{1-sin\varphi }\right)$

$sin\varphi =\frac{8}{18}$

$\varphi ={26.38}^{\circ }$