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IS 13047:1991 is the Indian Standard (BIS) for method for determination of the strength of rock materials in triaxial compression. This standard specifies the laboratory method for determining the strength of a cylindrical rock specimen under triaxial stress conditions. It details the apparatus, specimen preparation, test procedure, and calculation methods required to derive the Mohr-Coulomb strength parameters, namely cohesion (c) and the angle of internal friction (φ), which are fundamental for designing structures in or on rock.
Method for determination of the strength of rock materials in triaxial compression
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Specimen preparation is the most critical step; any imperfections in end flatness or parallelism can lead to premature failure and significant underestimation of strength.
! A minimum of three, but preferably five, tests at different confining pressures (σ3) are required to accurately define the Mohr-Coulomb failure envelope and determine 'c' and 'φ'.
! The rate of axial loading should be controlled to ensure the specimen fails within 5 to 15 minutes, avoiding both impact loading and undrained creep effects.
Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures
Method C of this standard directly covers triaxial compressive strength of rock.
ISRM-SMG-2007International Society for Rock Mechanics (ISRM)
HighCurrent
The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974-2006 (specifically, 'Suggested Method for Determining the Triaxial Compressive Strength of Rock Materials', Kovari et al. 1978)
Provides foundational international recommendations for the triaxial testing procedure.
ASTM D2664-95aASTM International, USA
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Standard Test Method for Triaxial Compressive Strength of Undrained Rock Core Specimens Without Pore Pressure Measurements
A direct historical equivalent, very similar in scope and vintage to the IS code.
Key Differences
≠IS 13047 specifies a constant axial stress loading rate of 0.5 to 1.0 MPa/s, similar to the older ISRM method. Modern standards like ASTM D7012 prefer controlling the test by time to failure (5-15 minutes) or a constant strain rate (e.g., 10⁻⁵ /s), which is considered more representative of material response.
≠The tolerance for perpendicularity of the specimen ends to the core axis is stricter in the IS code (within 0.001 radians or ≈0.06°) compared to ASTM D7012 (within 0.25°).
≠ASTM D7012 is a much more comprehensive document, including procedures for high temperatures, pore pressure control (Method D), and detailed calculations for elastic moduli, which are outside the scope of the more focused IS 13047.
≠IS 13047 requires the recording of axial load at every 50 kN interval, whereas ASTM D7012 requires a continuous record or at least 10 data points throughout the test, with more points near failure, allowing for a more detailed stress-strain curve.
Key Similarities
≈All standards are based on the same fundamental test principle: applying a constant hydrostatic confining pressure to a cylindrical rock specimen and then increasing the axial load until failure occurs.
≈The recommended specimen geometry is highly consistent, with a length-to-diameter (L/D) ratio of 2.0 to 2.5 specified in both IS 13047 and ASTM D7012.
≈The required tolerance for the flatness of the specimen ends is very similar across standards (e.g., 0.02 mm for IS 13047 and 0.025 mm for ASTM D7012).
≈All standards mandate the use of a flexible, impermeable membrane (jacket) to separate the specimen from the confining fluid and prevent its penetration into the rock.
≈The primary objective of the test is the same: to determine the strength of the rock under different confining pressures, typically to construct a Mohr failure envelope and determine the cohesion (c) and angle of internal friction (φ).
Parameter Comparison
Parameter
IS Value
International
Source
Specimen L/D Ratio
2.0 to 2.5
2.0 to 2.5
ASTM D7012-14(2020)e1
Axial Loading Rate
Stress rate of 0.5 to 1.0 MPa/s
Controlled to produce failure in 5 to 15 minutes (strain rate control preferred)
ASTM D7012-14(2020)e1
End Flatness Tolerance
Within 0.02 mm
Within 0.025 mm (0.001 in.)
ASTM D7012-14(2020)e1
End Perpendicularity to Axis
Within 0.001 radian (≈0.06°)
Within 0.25°
ASTM D7012-14(2020)e1
Minimum Specimen Diameter
45 mm
47 mm (1 7/8 in.)
ASTM D7012-14(2020)e1
Recommended Specimen Size
NX size core (≈54 mm)
NX size core (54.7 mm) is recommended
ASTM D7012-14(2020)e1
Axial Loading Rate (Historical Match)
Stress rate of 0.5 to 1.0 MPa/s
Stress rate of 0.5 to 1.0 MPa/s
ISRM Suggested Method (1978)
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Minimum specimen diameter45 mm (preferably NX size 54.7 mm)
Specimen length to diameter ratio (L/D)2.0 to 2.5
Specimen end flatness tolerance0.02 mm
Perpendicularity of ends to specimen axis±0.001 radian (or 0.25 mm in 250 mm)
Recommended time to failure5 to 15 minutes
Minimum number of tests for a sample3 to 5
Key Formulas
τ = c + σn tan(φ) — Mohr-Coulomb failure criterion
σ1 = P / Ac — Major principal stress at failure
σ3 = Confining pressure — Minor principal stress
sin(φ) = ( (σ1-σ3)/2 ) / ( c/tan(φ) + (σ1+σ3)/2 ) — Relationship for graphical determination of c and φ
What is the purpose of the triaxial test on rock?+
To determine the shear strength parameters (cohesion 'c' and angle of internal friction 'φ') under various confining pressures, which simulates the in-situ stress state of the rock mass.
What is the standard L/D ratio for the rock specimen?+
The length to diameter (L/D) ratio should be between 2.0 and 2.5. (Clause 5.1)
How many tests are needed to determine the strength parameters?+
A minimum of three, preferably five, successful tests on specimens from the same sample at different confining pressures are required to define the failure envelope. (Clause 6.4)
What is the minimum recommended specimen diameter?+
The diameter shall not be less than 45 mm, and preferably not less than NX core size (54.7 mm). (Clause 5.1)