Code of practice for the in-situ shear test on rock
1991 Edition

The standard prescribes test block dimensions typically at 700 mm by 700 mm by 300 mm (length × width × height), with a minimum size of 450 mm by 450 mm by 200 mm for smoother surfaces. Blocks should be carefully cut to avoid disturbing the weak discontinuity; alignment with natural joints is preferred. If joints are not aligned, line drilling is employed to isolate the block. Minor irregularities are trimmed by hand, and a weak layer (minimum 20 mm thick, e.g., clay) is applied around the base. The block is then encapsulated either in cement concrete (1:2:4), cement mortar (1:2), or enclosed within a steel casing of at least 10 mm thickness with internal dimensions matching the block size. For concrete-rock interfaces, a concrete block matching these dimensions is cast inside a steel casing, and the rock surface is chiseled to limit trough depth to 10 mm or less. This meticulous preparation ensures minimal disturbance and reliable shear strength measurement.

Normal and shear loads are applied using hydraulic jacks arranged so that the resultant forces act through the shear plane's centroid. The total normal load on the shear plane is calculated as Pn = Pna + Psa × sin 15°, where Pna is the direct normal jack force and Psa is the inclined shear jack force. The shear force is Ps = Psa × cos 15°. During testing, the normal load is adjusted after each shear increment by reducing the component Psa × sin 15° to maintain an approximately constant normal stress. Additional normal load reduction accounts for changes in block width. The shear jack is inclined at 15° with wedges to ensure proper force direction. Load increments are applied gradually, with shear displacement rates controlled around 0.1 mm per meter over approximately 10 minutes. For clay-filled discontinuities, loading rates are slower to allow pore pressure dissipation, typically requiring total time to peak shear strength to exceed six times the primary consolidation time.

Displacement measurements require dial gauges with a least count of 0.01 mm and a travel capacity of 50 mm. Four dial gauges are used to measure normal (vertical) displacement, two for shear (horizontal) displacement, and two for lateral (perpendicular horizontal) displacement. These gauges are mounted on a datum bar supported by stands placed away from the test block and anchoring points. Since the test block surface may be uneven, glass plates are affixed to provide smooth, stable reference surfaces for the gauges. Displacement readings are averaged to obtain mean normal and shear displacements, while lateral displacement data assist in evaluating specimen behavior and adjusting contact area calculations, ensuring precise displacement tracking essential for interpreting consolidation and shear responses.

Peak shear strength is identified as the maximum shear stress observed on the shear stress versus shear displacement curve. Tight joints typically exhibit a distinct peak followed by a sudden drop, whereas clay-filled joints may not show a clear peak. After reaching peak strength, shearing continues under constant normal stress with measurements taken at small shear displacement intervals until shear force stabilizes, defining the residual shear strength. By plotting peak and residual shear strengths against corresponding normal stresses for multiple tests, shear strength parameters such as cohesion intercept (c'), residual friction angle (φr), and apparent friction angle (φa) are derived. Additionally, the asperity angle (i), reflecting joint roughness, can be estimated from the slope of the normal displacement versus shear displacement curve.

For clay-filled discontinuities, a consolidation curve is plotted to determine the primary consolidation time (t100). The test procedure requires that the time to reach peak shear strength exceeds six times t100 to allow effective pore pressure dissipation. The shear force must be applied slowly, with the shear stress versus displacement curve typically lacking a distinct peak and the normal versus shear displacement curve displaying a gradual slope. Conversely, unfilled (tight) discontinuities generally show a pronounced peak in shear stress versus displacement and a steeper slope in normal versus shear displacement curves. For these, the normal load-displacement data can be used to estimate the deformation modulus of the rock mass. These differences ensure that testing accounts for the mechanical behavior influenced by infill materials and pore pressures, providing realistic shear strength evaluations.