This standard specifies the method for conducting in-situ shear tests on rock masses. It is used to determine the shear strength parameters (cohesion 'c' and angle of internal friction 'φ') along a specific plane, such as a rock joint, fault, or bedding plane, under various normal stresses. These parameters are fundamental inputs for the stability analysis and design of foundations for heavy structures like dams, bridges, and powerhouses founded on rock.
Code of practice for the in-situ shear test on rock
Quick Reference Values
Minimum test block size (good rock)70 cm x 70 cm
Minimum test block height35 cm
Typical number of normal stress increments5 to 6
Rate of shear loading1.5 to 2.5 kg/cm²/min
Minimum curing period for grout/concrete7 days
Minimum reaction load safety factor1.5 times the maximum test load
Key Formulas
τ = S / A — Shear Stress (S=Shear Force, A=Area)
σn = N / A — Normal Stress (N=Normal Force, A=Area)
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion to find cohesion (c) and friction angle (φ)
Practical Notes
The test plane must be carefully selected to be representative of the discontinuity or rock mass feature being investigated. Poor selection leads to non-representative and misleading results.
Ensuring the reaction system is sufficiently stiff and does not yield during the test is critical for accurate load application and measurement.
The test block should be kept moist throughout the preparation and testing period to maintain its natural moisture condition, unless dry conditions are being simulated.