Last reviewed 16 Sept 2026 · 6 min read
Shear strength
Shear strength is the maximum resistance a soil offers to shearing stress on a plane. Bearing capacity, slope stability, earth pressure on walls and pile capacity all depend on it. Soil fails in shear, not by crushing of grains.
Shear resistance comes from:
- Friction between particles (sliding and interlocking), which grows with effective normal stress, and
- Cohesion — bonding/attraction between particles in clays (and apparent cohesion from suction or cementation).
Mohr–Coulomb failure criterion
Total stress:
Effective stress (Terzaghi):
, = cohesion intercept; , = angle of shearing resistance (internal friction). Shear strength is fundamentally governed by effective stress.
Typical parameters:
| Soil | ||
|---|---|---|
| Loose sand | 0 | 28°–32° |
| Dense sand | 0 | 35°–45° |
| Normally consolidated clay | ≈ 0 | 20°–30° |
| Over-consolidated clay | small positive | 20°–30° |
| Saturated clay, undrained (φ = 0 analysis) | 0 |
Mohr circle and failure plane
At failure the Mohr circle of stress touches the strength envelope. The failure plane makes an angle with the major principal plane:
For :
Laboratory tests
Direct shear (box) test
Soil in a split box is sheared along a forced horizontal plane under a normal load. Plot shear stress at failure against normal stress → and .
- Advantages: simple, quick, good for sands; residual strength by reversals.
- Limitations: failure plane predetermined; non-uniform stress distribution; drainage cannot be controlled and pore pressure cannot be measured; area changes during shear; principal stresses unknown.
Triaxial compression test
A cylindrical specimen (commonly 38 mm diameter, 76 mm high) in a rubber membrane is subjected to cell pressure , then an axial deviator stress until failure. Drainage and pore pressure are controlled and measured.
| Test | Consolidation stage | Shearing stage | Parameters | Field use |
|---|---|---|---|---|
| UU (Unconsolidated Undrained, "quick") | Undrained | Undrained | , ≈ 0 for saturated clay | End-of-construction stability of clays; quick loading |
| CU (Consolidated Undrained) | Drained | Undrained (pore pressure measured) | , and , | Rapid drawdown; staged construction |
| CD (Consolidated Drained, "slow") | Drained | Drained (slow) | , | Long-term stability; sands |
Advantages over direct shear: failure on the weakest plane, drainage control, pore pressure measurement, known principal stresses, full stress–strain behaviour.
Unconfined compression test (UCS)
A special triaxial test with , for saturated clays:
= unconfined compressive strength. Quick; used for consistency of clays (very soft < 25 kPa ; soft 25–50; medium 50–100; stiff 100–200; very stiff 200–400; hard > 400). Also gives sensitivity when repeated on remoulded soil.
Vane shear test
A four-bladed vane pushed into soft clay (in the laboratory or field) and rotated; the torque at failure gives undrained strength. For a vane of diameter and height with both ends shearing:
Ideal for soft, sensitive clays that are difficult to sample.