Point Load Test for Rock Strength: ASTM D5731 Guide

Why Use the Point Load Test?

The Point Load Test (PLT) is a rapid, low-cost index test for estimating rock strength that can be performed on irregular rock fragments, drill core, or cut blocks — with minimal specimen preparation. It is particularly useful for site investigation programs where large numbers of samples need to be screened quickly.

Results are reported as the Point Load Strength Index Is(50), which is then converted to an estimated Uniaxial Compressive Strength (UCS) using empirical correlations. The test is standardized under ASTM D5731 and ISRM Suggested Methods.

Equipment

• Point Load Test Apparatus: A loading frame with two conical hardened steel platens (60° cone angle, 5 mm tip radius). Typically hydraulic or screw-driven with a pressure gauge. Portable units are available for field testing.
• Load Measurement: Bourdon gauge or digital load cell reading the failure load P in kN.
• Dimension Measurement: Calipers for measuring specimen diameter D or equivalent diameter De.

Test Modes

• Diametral test: Core loaded across the diameter. Simplest — D = core diameter.
• Axial test: Core loaded along the axis. L/D ratio 0.3–1.0.
• Block or irregular lump test: Irregular pieces; equivalent diameter calculated from dimensions.

Calculating Is(50) and UCS

Raw Is is size-corrected to a reference equivalent diameter of 50 mm:

Is(50) = F × (P / De²)  where F = (De/50)^0.45

UCS is estimated as: UCS ≈ K × Is(50) where K typically ranges from 20–25 for strong rocks and 15–20 for weaker formations.

Rock Strength Classification