Direct Shear Test for Soils: ASTM D3080 Procedure and Equipment

Purpose of the Direct Shear Test

The direct shear test determines the shear strength of soils by measuring the shear stress required to cause failure along a predetermined horizontal plane. Results — cohesion (c) and internal friction angle (φ) — are the primary parameters in slope stability analysis, retaining wall design, bearing capacity calculations, and pile design.

The test is standardized under ASTM D3080 (consolidated-drained, CD) and EN ISO 17892-10.

Equipment

• Shear Box: Square (60×60 mm or 100×100 mm) or circular metal box split horizontally at mid-height. The upper half is fixed; the lower half moves horizontally under the applied shear force.
• Normal Load System: Deadweight hanger or pneumatic system applies a constant vertical (normal) stress to the specimen during shearing.
• Horizontal Drive: Motor-driven or hydraulic screw drive applies shear displacement at a constant rate (typically 0.5–1.0 mm/min for CD testing).
• Load Cells and LVDTs: For measuring shear force and horizontal/vertical displacement.
• Data Logger: Modern units record load–displacement curves automatically.

Procedure (ASTM D3080)

1. Prepare specimens at natural or remolded conditions. Place in shear box and apply porous stones and filter paper top and bottom.
2. Apply normal load. Allow consolidation until vertical displacement stabilizes (typically 24 hours for clay).
3. Shear the specimen by moving the lower box at the specified constant displacement rate.
4. Record shear force and horizontal displacement continuously. Continue until 10–15% horizontal strain or after peak shear stress is passed.
5. Repeat at minimum three different normal stresses.
6. Plot shear stress vs horizontal displacement for each test. Identify peak and residual shear strengths.
7. Plot Mohr-Coulomb failure envelope: τ = c + σ tan(φ). Read c and φ from the line.

Peak vs Residual Strength

Dense sands and overconsolidated clays exhibit a peak shear strength followed by a lower residual strength after large displacement. Slope stability analysis uses residual strength for pre-existing failure planes. Foundation bearing capacity uses peak strength.