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Point Load Index

Test Method

ASTM Designation: D 5731, “Standard Test Method for Determination of the Point Load Strength Index of Rock”

Summary of Test Method

This index test is performed by subjecting a rock specimen to an increasingly concentrated load until failure occurs by splitting the specimen. The concentrated load is applied through coaxial, truncated conical platens. The failure load is used to calculate the point load strength index and to estimate the uniaxial compressive strength.

The uniaxial compression test (see ASTM D 2938) is used to determine compressive strength of rock specimens, but it is a time-consuming and expensive test that requires specimen preparation. When extensive testing is required for preliminary and reconnaissance information, alternative tests such as the point load test can be used in the field or laboratory to reduce the time and cost of compressive strength tests. The point load test allows testing of rock specimens that do not meet the dimensional requirements of the uniaxial compression test.

Sample Size Requirements

Rock samples are grouped on the basis of both rock type and estimated strength. When testing core or block specimens at least ten specimens are selected. When testing irregular-shaped specimens obtained by other means at least 20 specimens are selected. Specimens in the form of core are preferred for a more precise classification.

The specimen’s external dimensions shall not be less than 1.2 inches and not more than 3.3 inches with the preferred dimension about 2 inches. Specimens shall be free from abrupt irregularities that can generate stress concentrations. Size and shape requirements for diametral, axial, block, or irregular lump testing are presented in the test method.

Last Updated September 29, 2005

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