Method of determination of point load strength index of rocks
1998 Edition

IS 8764 mandates that specimens for point load testing adhere to specific sizes and forms depending on the test type. These include diametral, axial, block, and irregular lump specimens, each detailed with dimensions in Figure 3 of the standard. For axial tests, core specimens should maintain a length-to-diameter ratio between 0.3 and 1.0. Diametral specimens are typically disc-shaped, blocks are rectangular, and irregular lumps are used as received. Loading platens must be conical with a 60° angle and a 5 mm radius spherical truncation to ensure consistent load application.

The point load strength index (Is(50)) is calculated by dividing the failure load (P) by the square of the core diameter (D), normalized to a standard 50 mm core size. Mathematically, it is expressed as Is(50) = P / D² × (D / 50)², which simplifies to P / 2500 when D equals 50 mm. This normalization allows strength comparisons across specimens of varying sizes by adjusting the load according to the equivalent standard core diameter.

IS 8764 requires that displacement gauges used in point load testing be calibrated using proving rings to generate calibration charts that ensure accurate displacement measurements. The measurement system must resist hydraulic shocks and vibrations to maintain precision during repeated testing cycles. A zero displacement verification feature is necessary to confirm no initial offset when platens contact specimens. Distance between platens should be measured with a direct reading scale, maintaining an accuracy of ±2% relative to the measured distance.

Anisotropic rocks, such as shaly or schistose types, exhibit varying strength depending on the orientation relative to their planes of weakness. Testing must consider directions of maximum and minimum strength by orienting cores perpendicular and parallel to these planes, respectively. For the least strength, load is applied along a weakness plane; for the greatest strength, load is applied perpendicular to it. The strength anisotropy index, defined as the ratio of point load strength perpendicular to parallel to the fabric, quantifies this variation. Proper orientation and reporting of test directions are essential for accurate characterization.

IS 8764 provides a linear correlation between the uniaxial compressive strength (qc) and the corrected point load strength index (Is(50)) for unweathered rocks, expressed as qc = 15 × Is(50). This relationship allows engineers to estimate the compressive strength of rock from point load tests efficiently. The correlation is valid only for fresh, unweathered rock specimens, and anisotropic rocks require testing in multiple orientations to capture strength variability.