What it measures
- In direct shear, two halves of a box impose a defined sliding plane while a normal load is applied.
- Shear stress against displacement shows peak resistance and, after repeated movement, may reveal residual strength.
Typical sequence
- Prepare a specimen of known dimensions, place it in the box, and record its condition, density, and moisture.
- Apply the selected normal stress and allow consolidation where required by the method.
- Move the box halves at the specified rate and record shear force, displacement, and vertical deformation.
- Repeat at several normal stresses to construct a strength envelope.
Residual strength and related methods
- During back-and-forth cycles or large displacement, strength may stabilize at a residual level; interpret each cycle, not just the initial peak.
- Ring shear can maintain the shear area over large displacement, although it commonly uses remoulded material.
- A laboratory vane is useful for a rapid strength assessment in soft clay, following its own procedure.
Limitations
- The shear plane is imposed by the box and stress distribution is not uniform; consider these limitations when applying parameters to the ground.
- Shear rate must allow the required drainage condition. Report stresses, rate, corrected area, and strength criterion according to the standard.
Strength envelope
- Effective normal stress: σ′n = (N − u·A)/A, with normal load N, pore pressure u, and shear area A. If u is not measured or the test is drained, state the condition used.
- Shear stress: τ = T/A, where T is shear force. For a rectangular box, corrected A decreases with displacement; use the current geometry required by the method.
- Fit results (σ′n, τf) from several tests with τf = c′ + σ′n·tan φ′ to obtain effective cohesion c′ and friction angle φ′. Do not force an intercept when the project criterion requires another fit.
- State whether τf is peak or residual. Do not combine results from different drainage, density, preparation, or accumulated-displacement conditions.
Educational reference guide. Check the current standard, calculation assumptions, and project specification before applying a procedure or acceptance criterion.