Shear Strength & the Mohr-Coulomb Envelope

Worked: a drained test on sand

A consolidated-drained triaxial test on a clean sand runs at a confining pressure of 20 psi and fails at a deviator stress of 40 psi. σ₃′ = 20 psi and σ₁′ = 20 + 40 = 60 psi. sin φ′ = (60 − 20)/(60 + 20) = 0.5, so φ′ = 30°, and the failure plane is inclined 60° to the major principal plane.

Separately, a clay sample with q_u = 2,400 psf has s_u = 1,200 psf.