The direct shear test determines the drained shear strength parameters of soil — cohesion (c′) and angle of internal friction (φ′) — by shearing a confined specimen along a fixed horizontal plane under a known normal stress.
What the Test Measures
Three or more specimens are sheared under different normal stresses. Plotting peak shear stress against normal stress gives the Mohr–Coulomb envelope: its intercept is cohesion, its slope the friction angle. These parameters feed bearing capacity, slope stability and retaining wall design.
Apparatus Required
- Shear box machine with motorised drive (60 mm square or 100 mm square box, split horizontally)
- Load hanger and weights or pneumatic system for normal stress
- Proving ring or load cell for shear force; dial gauges/LVDTs for horizontal and vertical displacement
- Porous plates, grid plates and specimen cutter
Test Procedure
- Trim the specimen into the box between porous plates; apply the target normal stress and allow full consolidation (log-time plot).
- Select a shearing rate slow enough for drained conditions — from t50; clays commonly shear at 0.0025–0.01 mm/min, sands at up to 1 mm/min.
- Shear while recording force and displacements until peak and residual values are established (typically 10–12 mm travel).
- Repeat on fresh specimens at two or more further normal stresses.
Calculation & Reporting
Shear stress = shear force / corrected area. Report peak and residual stress for each normal stress, the fitted c′ and φ′, consolidation data and shearing rate.
Acceptance Criteria
Typical drained friction angles: loose sand 28–32°, dense sand 35–45°, silts 26–32°, normally consolidated clays 20–28° with c′ near zero. Results outside expected ranges usually indicate too-fast shearing or disturbed specimens.
Frequently Asked Questions
Direct shear or triaxial — which test should I specify?
Direct shear is faster and simpler for sands and interface strength, but forces the failure plane and gives no pore-pressure data. Specify triaxial testing where undrained strength, stress paths or accurate clay parameters are needed.
Why must shearing be slow for clays?
The test assumes drained conditions. Shearing faster than pore water can escape generates excess pressures, understating the true effective-stress parameters.
Recommended Apparatus
NL Scientific manufactures the Digital Controller Direct Residual Shear Box for this method. Browse the full Soil Testing Equipment range or request a quotation from our engineers.
