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Point Load Strength Index Is(50) Calculator

The point-load strength index is an indirect index from a controlled rock-specimen test. It is not uniaxial compressive strength, tensile strength, rock-mass strength, a design value, or proof of material suitability. GeoMiner calculates the arithmetic for one specimen after the user has determined that the specimen and failure satisfy the governing test procedure. The uncorrected index Is equals failure load P divided by equivalent core diameter squared De squared. When load is entered in newtons and equivalent area in square millimetres, the result is numerically in megapascals because one newton per square millimetre equals one megapascal. For a diametral core test, equivalent diameter De is the core diameter D and De squared is D squared. For axial core, block, and irregular-lump geometry, De squared equals four times specimen width W times platen separation D divided by pi. The calculator accepts failure load in kilonewtons, newtons, or pounds-force and dimensions in millimetres, centimetres, or inches, then converts them to newtons and millimetres before calculation. Point-load index depends on specimen size. The published correction factor F equals De divided by 50 millimetres, raised to the power 0.45. Corrected Is 50 equals F multiplied by Is. A 50 mm equivalent diameter has a correction factor of one. Smaller and larger specimens are adjusted mathematically, but correction does not repair invalid geometry, poor platen contact, an unacceptable failure surface, damaged apparatus, a recording error, or unrepresentative sampling. The cited RMR89 table provides point-load Is 50 intact-strength bands. A value greater than 10 MPa corresponds to rating 15. Exactly 10 through 4 MPa corresponds to rating 12. Values from 2 to below 4 MPa correspond to rating 7, and values from 1 to below 2 MPa correspond to rating 4. Below 1 MPa the cited table uses the UCS method instead; GeoMiner does not extrapolate a point-load rating into that lower range. The band is only one input to RMR89. RQD, discontinuity spacing, discontinuity condition, groundwater, and application-specific orientation evidence remain separate. A point-load result from one intact specimen does not represent the fractured in-situ rock mass. The failure must occur in the prescribed relationship to the loading points rather than follow a pre-existing discontinuity that invalidates the intended test.

  • Rock UCS calculator — Calculate direct cylindrical-core UCS from a valid uniaxial compression result without inferring rock-mass strength.
  • RMR89 calculator — Continue with the complete RMR89 classification while keeping every other field rating separate.
  • Rock mass classification guide — Separate intact strength, RQD, discontinuity evidence, groundwater, stress, and design.
  • Core recovery and RQD calculator — Calculate the directional core-quality input independently from intact strength.
  • GeoMiner methodology — Review source versioning, evidence, uncertainty, privacy, and consequential-use limits.

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • undefined. Publishes the uncorrected point-load equation, equivalent-core-diameter geometry, 50 mm size correction, specimen rejection guidance, and the need for formation-specific UCS correlation.
  • undefined. Explains that point-load testing measures an index corrected to Is(50), and that UCS transformations carry substantial uncertainty and should be calibrated to local paired testing.
  • undefined. Lists USBR 6535 for estimating compressive strength by point load and directs users to ASTM D5731 for indirect laboratory and field point-load testing.
  • undefined. Reproduces the RMR intact-strength point-load Is(50) bands of greater than 10, 4 to 10, 2 to 4, and 1 to 2 MPa, with UCS preferred below that range.

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Common questions

What is point-load strength index Is(50)?
Is(50) is the point-load strength index corrected to an equivalent specimen diameter of 50 mm. It is an index from a specific test procedure, not uniaxial compressive strength and not rock-mass strength.
How is equivalent core diameter calculated?
For a diametral core test, equivalent diameter De equals the core diameter. For an axial, block, or irregular-lump test, De squared equals 4WD divided by pi, where W is specimen width and D is platen separation at failure.
Can Is(50) be converted directly to UCS?
Only with a justified local correlation. Published multipliers vary widely by rock type, condition, direction, and dataset. GeoMiner does not present a generic multiplier as a UCS result.
Can one specimen be reported as the rock strength?
No. One result is single-specimen arithmetic. The governing procedure defines specimen validity, sample size, direction, rejection rules, statistics, and reporting. Separate material domains, moisture conditions, and anisotropy directions.

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