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
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)
Practical Notes
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.