What is the standard specimen size for a UCS test?

A cylindrical specimen with a height-to-diameter ratio of 2.5 to 3.0, and a diameter preferably not less than NX core size (approx. 54 mm). (Clause 4.1)

What is the specified loading rate for the test?

The stress rate should be kept constant within the range of 0.5 to 1.0 MPa per second until the specimen fails. (Clause 5.3)

How is the unconfined compressive strength calculated?

It is calculated by dividing the maximum load (P) sustained by the specimen by its initial cross-sectional area (A). (Clause 6.1)

How many specimens are required for a reliable result?

The standard suggests testing a minimum of five specimens from a given sample to obtain a representative average value. (Clause 6.2)

IS 9143

: 1979

Method for the determination of unconfined compressive strength of rock materials

CurrentSpecializedTesting MethodGeotechnical · Rock Mechanics

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This standard specifies the method for preparing rock core specimens and determining their unconfined compressive strength (UCS) in a laboratory setting. It details the required apparatus, specimen dimensions, testing procedure, and the method for calculating the strength, which is a fundamental property for rock classification and engineering design.

Method for the determination of unconfined compressive strength of rock materials

Quick Reference Values

Specimen Height to Diameter Ratio (L/D)2.5 to 3.0

Minimum Specimen DiameterNot less than NX core size (approx. 54 mm)

Specimen End Flatness Tolerance0.02 mm

Specimen End Perpendicularity to Axiswithin 0.001 radian

Rate of Stress Application0.5 to 1.0 MPa/s

Key Formulas

σc = P / A — Unconfined Compressive Strength, where P is failure load and A is initial cross-sectional area.

Practical Notes

Specimen end preparation is the most critical factor for accurate results. Ends must be flat, parallel, and perfectly perpendicular to the core axis to prevent stress concentrations and premature, invalid failure.

The height-to-diameter (L/D) ratio of 2.5-3.0 is essential. Shorter specimens overestimate strength due to end confinement, while slender specimens can fail in buckling.

Always record the mode of failure (e.g., shear, axial splitting, conical) as it provides valuable qualitative information on the rock's behavior and brittleness.