Module 11
Shear strength of soils
Why and how soil fails in shear: friction and interlocking, the Mohr–Coulomb criterion on effective stress, peak, critical and residual states, and drained versus undrained strength.
What this module covers
- Explain shear strength as friction, mobilised on the effective stress
- State and apply the Mohr–Coulomb failure criterion τf = c′ + σ′·tanφ′
- Use the Mohr circle and derive the principal-stress form σ′1f = σ′3·Kp + 2c′√Kp
- Locate the failure plane at 45 + φ′/2 to the major principal plane
- Distinguish peak, critical-state and residual strength
- Distinguish drained (c′, φ′) from undrained (su, φu = 0) strength and know when each governs
Lessons
Friction on the effective stress, interlocking and dilation, and the peak–critical–residual sequence.
Start lesson →The failure line, the Mohr circle tangent to it, and the principal-stress form.
Start lesson →Why the same clay has two strengths, and when each governs.
Start lesson →
Module checkpoint
Check what you have taken in
3 questions
Question 1
A drained triaxial test on sand (c′ = 0, φ′ = 35°) is run at σ′3 = 60 kPa. What is the axial effective stress at failure, in kPa? (Kp = tan²(45 + 35/2).)
Question 2
A rise in porewater pressure, with no change in total stress, does what to the available shear strength?
Question 3
For a cohesionless soil at failure with σ′1 = 180 kPa and σ′3 = 60 kPa, what is the mobilised (here, peak) friction angle, in degrees? (sinφ′ = (σ′1 − σ′3)/(σ′1 + σ′3).)