Link points to Internet Archive / others. Not hosted by InfraLens. Details
IS 12608:1989 is the Indian Standard (BIS) for rock joints-direct shear strength-laboratory method of determination. 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
ASTM D5607 / D5607M - 16(2021)ASTM International, USA
HighCurrent
Standard Test Method for Performing Laboratory Direct Shear Strength Tests of Rock Specimens Under Constant Normal Force
Both standards detail laboratory procedures for determining the direct shear strength of rock discontinuities/joints.
ISRM (1981)International Society for Rock Mechanics (ISRM), International
HighCurrent
Suggested Method for the Laboratory Determination of the Shear Strength of Rock Joints
Provides the seminal methodology for lab direct shear testing of rock joints, on which IS 12608 is heavily based.
Key Differences
≠The rate of shear displacement in IS 12608 is faster, aiming for failure in 5-15 minutes, whereas ASTM D5607 specifies a slower rate, with failure occurring after a minimum of 15 minutes to ensure fully drained conditions.
≠ASTM D5607 mandates and provides a formula for correcting the shear area as displacement occurs. IS 12608 acknowledges the need for correction but is less prescriptive about the specific method.
≠ASTM D5607 provides more rigorous criteria for minimum specimen size, relating it to the rock's grain size and asperity amplitude, while IS 12608 specifies nominal dimensions (e.g., 150mm x 100mm).
≠ASTM D5607 includes specific requirements for moisture conditioning of the specimen (e.g., soaking) prior to testing, a step which is not explicitly detailed in IS 12608.
Key Similarities
≈Both standards use the same core principle: applying a constant normal stress to a specimen containing a single discontinuity and shearing it to failure to measure shear strength parameters.
≈Both require the rock specimen to be cast or encapsulated (e.g., in plaster, cement, or resin) into two shear box halves to facilitate load application.
≈The general test apparatus, comprising a direct shear box with systems to apply and measure normal/shear loads and displacements, is conceptually identical in both standards.
≈The primary outputs are consistent: plots of shear stress vs. shear displacement and normal displacement vs. shear displacement, which are used to derive the failure envelope and strength parameters (cohesion and friction angle).
Parameter Comparison
Parameter
IS Value
International
Source
Target Time to Failure
5 to 15 minutes
Greater than 15 minutes to ensure drained conditions
ASTM D5607
Rate of Shear Displacement
0.1 to 1.0 mm/min
Typically 0.025 to 0.25 mm/min (or slower)
ASTM D5607
Specimen Size Guideline
Nominal 150 mm x 100 mm; Min. 50 mm dia. for cores.
To determine the peak and residual shear strength parameters (cohesion 'c' and friction angle 'φ') of a rock joint for use in stability analysis of rock slopes, foundations, and underground structures.
What is the difference between peak and residual shear strength?+
Peak strength is the maximum shear stress reached, typically as asperities are overcome. Residual strength is the lower, constant shear stress after significant displacement has sheared off the asperities.
Why is the specimen encapsulated?+
To transfer the normal and shear loads uniformly to the irregular rock sample and to ensure it fits securely within the square shear box (Clause 5.2).
What is a typical shear displacement rate?+
The rate should be slow enough to prevent pore pressure build-up. A constant rate between 0.1 mm/min and 1.0 mm/min is typical, as per Clause 7.5.