A consolidated-drained triaxial test was conducted on a normally consolidated clay with a chamber pressure, 3 = 172 kN / m2. The deviator stress at

A consolidated-drained triaxial test was conducted on a normally consolidated clay with a chamber pressure, σ3 = 172 kN / m2. The deviator stress at failure, (Δσd)f = 227 kN/m2. Determine:
a. The angle of friction, ϕ'
b. The angle θ that the failure plane makes with the major principal plane
c. The normal stress, σ'f, and the shear stress, τf, on the failure plane