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IS 8764:1998 is the Indian Standard (BIS) for method of determination of point load strength index of rocks. This standard specifies the apparatus, procedure, and calculation method for determining the point load strength index of rock. It serves as an indirect method to estimate the Uniaxial Compressive Strength (UCS) for rock classification and is widely used in geotechnical site investigations for infrastructure projects.
Method of determination of point load strength index of rocks
! The correlation factor 'k' to estimate UCS from Is(50) is highly variable (15-50) and rock-specific. Use the commonly suggested range of 20-25 with caution and calibrate with actual UCS tests for a specific project.
! Ensure that the failure is a valid split through the rock mass and not along a pre-existing weakness plane. Invalid breaks should be noted and the results may be discarded.
! This test is portable and cost-effective, making it ideal for preliminary characterization of rock strength on-site, especially for large quantities of rock core.
Consolidated list per BIS. For the text of each amendment, refer to the BIS portal link above.
rock
International Equivalents
Similar International Standards
ASTM D5731-16ASTM International, USA
HighCurrent
Standard Test Method for Determination of the Point Load Strength Index of Rock and Application to Rock Strength Classifications
Defines the apparatus, procedure, and calculation for determining the point load strength index, nearly identical in principle.
ISRM (1985)International Society for Rock Mechanics (ISRM)
HighCurrent
Suggested Method for Determining Point Load Strength
Provides a widely accepted methodology that forms the basis for many national standards, including IS 8764.
Key Differences
≠The method for correcting the point load index to a standard 50 mm diameter equivalent differs significantly. IS 8764 uses a graphical method (a correction chart), whereas ASTM D5731 specifies a formula: Is(50) = Is * (De/50)^0.45.
≠For the axial test on core specimens, IS 8764 specifies a length-to-diameter ratio (L/D) between 1.1 and 1.5, while ASTM D5731 requires a ratio between 0.3 and 1.0.
≠ASTM D5731 provides more detailed criteria for test validity, including specific rules for rejecting tests based on fracture patterns (e.g., failure not occurring through the loading points), and gives clearer guidance on the minimum number of tests (10) and discarding outliers.
Key Similarities
≈The fundamental principle of the test is identical: applying a concentrated load through conical platens until a rock specimen fails, with the index calculated as Is = P/De², where P is the failure load and De is the equivalent core diameter.
≈The specifications for the testing apparatus are nearly identical, particularly the conical platens, which are required to have a 60° cone angle and a 5 mm tip radius in both standards.
≈Both standards recommend a loading rate that results in a specimen failure time of 10 to 60 seconds to minimize effects from loading rate and creep.
≈All primary test configurations are recognized in both standards, including diametral tests on core, axial tests on core, and tests on irregular lumps.
Parameter Comparison
Parameter
IS Value
International
Source
Platen Tip Radius
5 mm
5 ± 1 mm
ASTM D5731-16
Platen Cone Angle
60°
60°
ASTM D5731-16
Recommended Time to Failure
10 to 60 seconds
10 to 60 seconds
ASTM D5731-16
Size Correction Method for Is(50)
Graphical Chart (Fig. 3)
Formula: F = (De/50)^0.45
ASTM D5731-16
Axial Test Specimen L/D Ratio
1.1 to 1.5
0.3 to 1.0
ASTM D5731-16
Reference Diameter for Normalization
50 mm
50 mm
ASTM D5731-16
⚠ Verify details from original standards before use
Key Values4
Quick Reference Values
Standard specimen diameter for corrected index (Is(50))50 mm
Recommended specimen Length/Diameter (L/D) ratio for diametral test>= 1.0
Minimum distance from platen to specimen end0.5 * D
Required time to failure during loading10 to 60 s
Key Formulas
Is = P / De² — Uncorrected point load strength index (where De is equivalent core diameter)
Is(50) = F x Is — Size-corrected point load strength index
F = (De/50)^0.45 — Size Correction Factor F
σc = k * Is(50) — Estimation of Uniaxial Compressive Strength (k is correlation factor, often 20-25)
Tables & Referenced Sections
Key Tables
Fig. 5 - Size Correction Factor Chart
Key Clauses
Clause 4 - Apparatus
Clause 5 - Rock Specimen
Clause 6 - Procedure
Clause 7 - Calculation of Point Load Strength Index
It is used to determine the point load strength index (Is) of rock, which is then used to estimate the Uniaxial Compressive Strength (UCS) for rock mass classification. (Clause 1.1)
To what standard diameter is the index corrected?+
The point load strength index is corrected to a standard equivalent core diameter of 50 mm, denoted as Is(50). (Clause 7.2.1)
What types of rock samples can be tested?+
The test can be performed on rock core (diametral or axial test), blocks, or irregular lumps. (Clause 5)
How quickly should the load be applied?+
The load should be applied steadily such that failure occurs within 10 to 60 seconds. (Clause 6.2)