This standard specifies the laboratory procedure for determining the direct shear strength of rock joints under constant normal load. It details the required apparatus, specimen preparation, testing procedure, and the method for calculating and presenting key shear strength parameters like cohesion and angle of friction.
rock joints-direct shear strength-laboratory method of determination
Quick Reference Values
Minimum Specimen Side Length50 mm
Minimum Specimen Size relative to Asperities10 times the amplitude of major asperities
Typical Shear Displacement Rate0.1 to 1.0 mm/min
Specimen Encapsulation MaterialPlaster of Paris, cement mortar, or resin
Key Formulas
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion for shear strength
Shear Stiffness (Ks) = Δτ / Δu — Change in shear stress over shear displacement
Practical Notes
The orientation of the joint in the shear box must be carefully controlled to match the in-situ orientation for relevant results.
Disturbance of the joint surface or infill material during sampling and preparation is a major source of error and must be minimized.
The applied normal stress levels should be selected to represent the range of expected in-situ stress conditions acting on the joint.