This standard outlines the method for determining the point load strength index of rock, a key parameter for classifying rock material strength. It details the required apparatus, specimen preparation, testing procedure, and calculations to derive the size-corrected point load index (Is(50)), which is often used to estimate the Unconfined Compressive Strength (UCS).
Specifies the method for determining the point load strength index of rock specimens.
Quick Reference Values
Recommended core specimen diameter (D)50 mm (approx.)
Minimum specimen length to diameter ratio (L/D) for diametral test1.1
Minimum specimen length to width ratio (L/W) for block test0.5
Loading rate to cause failurewithin 10 to 60 seconds
Validity check - fracture distance from platen tip> 0.5 * D
Standard platen cone angle60 degrees
Standard platen tip radius5 mm
Key Formulas
Is = P / D² — Uncorrected point load strength index (Diametral test)
Is = P / We² — Uncorrected point load strength index (Axial, Block, Lump tests)
We = √(4A/π) — Equivalent core diameter from area A
Is(50) = F * Is — Size-corrected point load strength index
F = (De/50)^0.45 — Size correction factor
Practical Notes
The point load test is a valuable index test for field use due to the portability of the equipment, allowing for rapid rock strength classification on site.
A test is considered valid only if the rock specimen breaks with a fracture plane passing through both loading points. Off-center fractures invalidate the result.
Accurate application of the size correction factor to obtain Is(50) is crucial for comparing strength results from specimens of different sizes and shapes.