This standard specifies the laboratory method for determining the strength of a cylindrical rock specimen under triaxial stress conditions. It details the apparatus, specimen preparation, test procedure, and calculation methods required to derive the Mohr-Coulomb strength parameters, namely cohesion (c) and the angle of internal friction (φ), which are fundamental for designing structures in or on rock.
Method for determination of the strength of rock materials in triaxial compression
Quick Reference Values
Minimum specimen diameter45 mm (preferably NX size 54.7 mm)
Specimen length to diameter ratio (L/D)2.0 to 2.5
Specimen end flatness tolerance0.02 mm
Perpendicularity of ends to specimen axis±0.001 radian (or 0.25 mm in 250 mm)
Recommended time to failure5 to 15 minutes
Minimum number of tests for a sample3 to 5
Key Formulas
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion
σ1 = P / Ac — Major principal stress at failure
σ3 = Confining pressure — Minor principal stress
sin(φ) = ( (σ1-σ3)/2 ) / ( c/tan(φ) + (σ1+σ3)/2 ) — Relationship for graphical determination of c and φ
Practical Notes
Specimen preparation is the most critical step; any imperfections in end flatness or parallelism can lead to premature failure and significant underestimation of strength.
A minimum of three, but preferably five, tests at different confining pressures (σ3) are required to accurately define the Mohr-Coulomb failure envelope and determine 'c' and 'φ'.
The rate of axial loading should be controlled to ensure the specimen fails within 5 to 15 minutes, avoiding both impact loading and undrained creep effects.