This standard specifies the laboratory method for determining the peak and residual direct shear strength of rock joints under constant normal stress. The results, including cohesion and friction angle, are fundamental for assessing the stability of rock slopes, foundations, and underground structures.
Method of determination for direct shear strength of rock joints
Quick Reference Values
Minimum specimen dimension50 mm
Minimum ratio of specimen width to max particle size in gouge10:1
Typical shear displacement rate0.1 to 1.0 mm/min
Minimum number of specimens per test series5
Minimum seating loadaround 50 N
Key Formulas
τ = Ps / A — Shear stress
σn = Pn / A — Normal stress
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion for shear strength
Practical Notes
Ensuring the shear direction is correctly aligned with the joint's asperity orientation is critical for accurate results.
The condition of the joint (e.g., roughness, presence of infill material, weathering) must be carefully documented as it heavily influences strength parameters.
A sufficient seating load must be applied before the test to ensure the sample is properly bedded in the shear box.