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IS 12634:1989 is the Indian Standard (BIS) for method of determination for direct shear strength of rock joints. This standard specifies the laboratory method for determining the peak and residual direct shear strength of rock joints under constant normal stress. The results, including cohesion and friction angle, are fundamental for assessing the stability of rock slopes, foundations, and underground structures.
Method of determination for direct shear strength of rock joints
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Ensuring the shear direction is correctly aligned with the joint's asperity orientation is critical for accurate results.
! The condition of the joint (e.g., roughness, presence of infill material, weathering) must be carefully documented as it heavily influences strength parameters.
! A sufficient seating load must be applied before the test to ensure the sample is properly bedded in the shear box.
Standard Test Method for Performing Laboratory Direct Shear Strength Tests of Rock Specimens Under Constant Normal Force
Directly equivalent method for determining the shear strength of discontinuities in rock.
ISRM Suggested Method (2007)International Society for Rock Mechanics and Rock Engineering (ISRM)
HighCurrent
Suggested method for the laboratory determination of the shear strength of rock joints
A globally recognized foundational method upon which many national standards are based.
EN ISO 17892-10:2018European Committee for Standardization (CEN) / ISO
LowCurrent
Geotechnical investigation and testing — Laboratory testing of soil — Part 10: Direct shear tests
Specifies direct shear testing, but is intended for soils, not rock joints, with different apparatus and sample considerations.
Key Differences
≠IS 12634 provides a prescriptive range for the shear rate (0.1 to 1.0 mm/min), whereas ASTM D5607 specifies a performance-based rate calculated to ensure drained conditions, typically based on consolidation time (t50).
≠The IS code specifies minimum specimen dimensions (50mm x 50mm), while ASTM D5607 and ISRM add a crucial requirement that the specimen length should be at least 10 times the average asperity height of the joint surface.
≠ASTM D5607 is more stringent on the properties of the encapsulating material (e.g., epoxy, gypsum), requiring its compressive strength to exceed the maximum applied normal stress, a detail not explicitly quantified in IS 12634.
≠The pre-shear consolidation phase is more explicitly detailed in ASTM and ISRM methods, requiring monitoring of vertical displacement until it stabilizes before shearing commences. IS 12634 is less descriptive about this stabilization criterion.
Key Similarities
≈All standards are based on the same fundamental principle: applying a constant normal load to a rock joint specimen and shearing it at a controlled rate to determine its shear strength.
≈The basic design of the direct shear apparatus, consisting of a split shear box, a mechanism for applying normal load, and a system for applying shear force, is conceptually identical across all standards.
≈All methods use the test results to plot shear stress versus normal stress to determine the Mohr-Coulomb failure parameters: cohesion (c) and the angle of friction (φ).
≈The requirement to test a series of specimens (typically at least three) under different normal stress levels to adequately define the failure envelope is a common feature.
Parameter Comparison
Parameter
IS Value
International
Source
Minimum Specimen Side Length
50 mm
≥ 50 mm AND ≥ 10 times the mean asperity size.
ASTM D5607-16
Shear Displacement Rate
Prescriptive range: 0.1 to 1.0 mm/min.
Performance-based: slow enough to ensure drained conditions (e.g., total shear time > 10 x t50 from consolidation).
ASTM D5607-16
Recommended Number of Test Specimens
At least three specimens under different normal stresses.
A minimum of three, but preferably five or more test points, especially for non-linear envelopes.
ASTM D5607-16
Encapsulation Material
Plaster of Paris or dental plaster.
High-strength gypsum, epoxies, or alloys, with compressive strength > applied normal stress.
ASTM D5607-16
Test Termination Criteria
Shear stress drops well below peak or shear displacement is 10% of specimen length.
Shear stress reaches a steady residual value or shear displacement is at least 10% of specimen diameter.
ASTM D5607-16
Shear Box Gap
A small gap of about 1 mm to be maintained.
A gap of 1-2 mm is set between the two halves of the shear box.
ISRM Suggested Method (2007)
⚠ Verify details from original standards before use
Key Values5
Quick Reference Values
Minimum specimen dimension50 mm
Minimum ratio of specimen width to max particle size in gouge10:1
Typical shear displacement rate0.1 to 1.0 mm/min
Minimum number of specimens per test series5
Minimum seating loadaround 50 N
Key Formulas
τ = Ps / A — Shear stress
σn = Pn / A — Normal stress
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion for shear strength
What are the primary results from an IS 12634 test?+
The primary results are the peak and residual shear strength parameters: cohesion (c) and angle of internal friction (φ) for the rock joint.
Is this test for intact rock or rock joints?+
This test is exclusively for determining the shear strength of discontinuities like joints, fractures, and bedding planes, not for intact rock material.
What is the purpose of testing at multiple normal stresses?+
Testing at a series of different normal stresses (typically 5 or more) allows for the construction of a Mohr-Coulomb failure envelope, from which c and φ are derived (Clause 7.2).
How is the specimen typically prepared?+
A rock sample containing a natural joint is obtained and encased in a mould using plaster of paris or a resin, leaving the joint plane exposed and aligned with the shear plane of the test apparatus (Clause 5).