Question

A consolidated undrained \(\left( {\overline {CU} } \right)\) 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 ________

Answer 1

Confining pressure = σ₃ = 100 kPa

Deviator stress = σ₁ – σ₃ = 80 kPa

σ₁ = 80 + σ₃ = 180 kPa

Pore pressure = u = 50 kPa

\({\bar \sigma _1} = {\sigma _1} - u\)

= 180 – 50 = 130 kPa

\({\bar \sigma _3} = {\sigma _3} - u\)

= 100 – 50 = 50 kPa

For NC soil C’ = 0

\(\begin{array}{l} {{\bar \sigma }_1} = {{\bar \sigma }_3}\left( {\frac{{1 + \sin \phi }}{{1 - \sin \phi }}} \right)\\ 130 = 50\left( {\frac{{1 + \sin \phi }}{{1 - \sin \phi }}} \right)\\ \sin \phi = \frac{8}{{18}} \end{array}\)

ϕ = 26.4°