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IS 7746:1991 is the Indian Standard (BIS) for the in-situ shear test on rock. 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
! 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.
Standard Test Method for In-Situ Determination of Direct Shear Strength of Rock Discontinuities
Both standards detail procedures for in-situ direct shear testing to determine the strength of rock, primarily focusing on discontinuities.
ISRM (2014)International Society for Rock Mechanics and Rock Engineering (ISRM)
HighCurrent
Suggested Method for the In Situ Assessment of the Shear Strength of Rock Joints
A globally recognized methodology that aligns almost perfectly with the IS code's objective of testing rock joints in-situ.
GOST 24941-81Euro-Asian Council for Standardization, Metrology and Certification (EASC), Russia/CIS
MediumCurrent
Rocks. Method for determination of shear resistance in workings
Covers in-situ shear testing of rock, but with procedural and equipment specifications that may differ from the IS/ASTM approach.
Key Differences
≠IS 7746 primarily describes the Constant Normal Load (CNL) test condition, whereas ASTM D4554 explicitly details procedures for both Constant Normal Load (CNL) and Constant Normal Stiffness (CNS) conditions.
≠The Indian standard provides more prescriptive block dimensions (e.g., 700 x 700 mm), while international standards like ASTM and ISRM offer performance-based criteria, relating the specimen size to the scale of joint asperities (e.g., specimen length > 10x asperity amplitude).
≠The scope of IS 7746 is titled broadly as 'shear test on rock', whereas ASTM D4554 and the ISRM method are more specific, targeting 'rock discontinuities' or 'rock joints', which is the primary, though not exclusive, application of the IS code.
≠The ISRM suggested method provides more quantitative guidance for the shear displacement rate (e.g., 0.1 to 1.0 mm/min), whereas IS 7746 gives a more qualitative guideline to use a 'slow and constant rate'.
Key Similarities
≈All standards share the same fundamental principle: isolating a rock block, applying a controlled normal force, and shearing it laterally to measure force-displacement behavior and derive shear strength parameters.
≈Both IS 7746 and its international counterparts recommend a multi-stage testing procedure on a single specimen, using progressively higher normal stresses to efficiently develop a failure envelope.
≈The general test apparatus, involving hydraulic jacks for load application, reaction frames or pads, and displacement transducers (LVDTs or dial gauges) for measurement, is conceptually identical across all standards.
≈All referenced standards require a consolidation period, where the specimen is allowed to deform under the applied normal load until movement stabilizes before the shear phase begins.
Parameter Comparison
Parameter
IS Value
International
Source
Typical Specimen Shear Area
700 mm x 700 mm is specified as a normal size.
Shear area of at least 500 mm x 500 mm is recommended.
ISRM (2014)
Specimen Size Guideline
Block size can vary from 500x500 mm to 1000x1000 mm depending on rock mass characteristics.
The specimen dimension in the direction of shear should be at least 10 times the amplitude of the largest asperities.
ASTM D4554-20
Normal Stress Control
Procedure is based on maintaining a Constant Normal Load (CNL).
Allows for both Constant Normal Load (CNL) and Constant Normal Stiffness (CNS) conditions.
ASTM D4554-20
Shear Displacement Rate
A constant rate, slow enough to avoid inertia and allow full mobilization of shear resistance.
Typically 0.1 to 1.0 mm/min, adjusted to ensure drained conditions.
ISRM (2014)
Number of Tests per Specimen
A series of 3 to 5 tests with increasing normal load is recommended.
A minimum of three, but preferably five, shear tests at different normal stresses are performed.
ASTM D4554-20
Ratio of Block Height to Length
The height of the test block should be at least half the side length of the block.
The height of the test block is usually between 0.5 and 0.7 times the block length.
ISRM (2014)
Maximum Shear Displacement
Shearing is continued until a residual state is reached, or to a displacement of about 10-15% of the block length.
Continued until the shear stress becomes constant or a displacement of 10% of the specimen length is reached.
ASTM D4554-20
⚠ Verify details from original standards before use
Key Values6
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)
Tables & Referenced Sections
Key Tables
Table 1 - Recommended Minimum Size of the Test Block
Key Clauses
Clause 3 - Apparatus
Clause 4 - Test Procedure
Clause 5 - Calculations and Interpretation of Test Data
Clause 6 - Reporting of Results
Appendix A - Typical Set-up for In-situ Shear Test
What is the main purpose of the in-situ rock shear test?+
To determine the shear strength parameters (cohesion 'c' and friction angle 'φ') of a rock mass or discontinuity under field conditions.
What is the minimum recommended size for a test block?+
70 cm x 70 cm for good quality massive rock. Larger sizes are required for poor quality or blocky rock to ensure the sample is representative (Table 1).
How is the test block loaded?+
A constant normal load is applied perpendicular to the shear plane, followed by a gradually increasing shear load applied parallel to the plane until failure occurs (Clause 4.3).
Why is a concrete or grout cap used on the test block?+
To provide a smooth, regular surface for the uniform application of normal and shear loads from the hydraulic jacks (Clause 4.1.3).