Method of determination for direct shear strength of rock joints
1989 Edition

Per the standard, the required apparatus includes a shear box to contain the specimen and apply shear force; a specimen carrier to securely hold the rock joint sample within the box; encapsulating materials to fill voids around the specimen preventing movement; and a low friction mechanism to minimize resistance between moving components. The setup ensures precise measurement of both peak and residual shear strengths under specified normal stresses.

Specimen preparation involves selecting rock blocks or cores with minimal disturbance, ensuring the test surface is flat, smooth, and at least 2500 mm² in area. Protective waterproof packing and binding wire or metal bands should be used during transportation to maintain specimen integrity. For mounting, specimens are placed in carriers with orientation preserved, and an encapsulating material such as Plaster of Paris is poured around the halves, leaving a 5 mm clear zone on either side of the shear plane free of encapsulation to avoid interference. After setting, the specimen is ready for testing.

Normal loads are applied using hydraulic, pneumatic, or mechanical systems, uniformly distributed and centered on the shear plane, maintained within ±2% of the target. Shear loads are applied via hydraulic jacks or mechanical drives uniformly along the specimen’s half-face, with low-friction devices ensuring equipment resistance remains under 1% of the maximum shear force. Shear displacement is controlled incrementally or continuously, with precise measurement using micrometer dial gauges or displacement transducers meeting accuracy and travel specifications, while calibration data is recorded.

During testing, shear stress and normal displacement are plotted against shear displacement to identify the maximum shear stress (peak strength) and the steady-state shear stress after large displacement (residual strength). These values are tabulated alongside corresponding normal stresses and displacements. Subsequently, plots of peak and residual shear strength versus normal stress allow determination of shear strength parameters such as cohesion and friction angles. Peak strength represents the maximum resistance before failure, while residual strength reflects post-failure behavior important for stability analysis.

The test report should encompass a schematic and detailed description of the equipment used; methods for specimen collection, packing, transportation, storage, and mounting; a clear outline of the testing procedure; a description and measurement of the shear plane including photographic documentation if applicable; index tests on rock, filling, and debris materials; graphical presentation of peak and residual shear strengths versus normal stress; and derived shear strength parameters. Additionally, the report may include non-linear peak strength curves correlated as per relevant standards.