Shear strength is the parameter that governs bearing capacity, slope stability and lateral earth pressure. IS 2720 covers it via the direct shear test (Part 13 → cohesion c and friction angle φ), unconfined compressive strength (Part 10 → qu/undrained shear of clay) and triaxial tests (Part 11). The right test and drainage condition (drained vs undrained) must match the design problem.
Key Requirements
•Direct shear (Part 13): c and φ from the failure envelope (granular/effective-stress parameters)
•Unconfined compressive strength (Part 10): qu → undrained shear strength of cohesive soil (cu ≈ qu/2)
•Triaxial (Part 11): controlled drainage (UU/CU/CD) for c, φ under the relevant condition
•Match the test & drainage to the problem: undrained for short-term clay; drained/effective for long-term
•Representative undisturbed/remoulded sample at field density/moisture as appropriate
Formulas
τ = c + σ·tan φ ; cu ≈ qu / 2
Mohr–Coulomb shear strength; undrained shear from unconfined compressive strength
✓The classic error is the wrong drainage condition: using drained parameters for a short-term undrained clay problem (or vice-versa) gives an unsafe or uneconomic design.
✓Shear parameters feed bearing capacity (foundations), slope stability and retaining-wall earth pressure — they are the most consequential soil design inputs.
Common Mistakes
⚠Using the wrong drainage condition (UU vs CU vs CD) for the design timescale.
⚠Applying UCS-derived clay strength to a granular soil (UCS is for cohesive soils).
⚠Testing disturbed samples where undisturbed strength governs.