Testwork: Unconfined compressive strength

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Testwork: Unconfined compressive strength

Unconfined compressive strength is a standard measurement of rock competency and is often used to determine rock mass rating (RMR) and slope stability for pit walls.

The test is a 'per-specimen' test where individual specimens of prepared rock are hand-picked by the test operator and placed into the apparatus. Typically one carefully prepared specimen is chosen to represent a sample and the result reported includes a subjective comment like "good fracture" or "invalid-fracture failure".

Sample Requirements

The test requires 1 cylindrical piece of large diameter (typically PQ) drill core, minimum length 150 mm.

Test Inputs

Not applicable.

Test Outputs

SAGMILLING.COM uses only metric units of MPa. If the laboratory reports UCS in other units (such as psi), convert the results before entering.

The laboratory will report the following information:

  • N° specimens: The number of specimens tested (typically 1)
  • min The value, MPa, of the specimen with the lowest result (if applicable)
  • max: The value, MPa, of the specimen with the highest result (if applicable)
  • stdev: The standard deviation of value of all the specimens (if applicable)
  • ucs: The arithmetic average of the UCS in MPa of all the specimens

Extra field available for modelling

  • synthetic indicates whether this is a real test result, or just a synthetic one that should only be used for modelling. If this column contains a value of '1' (boolean=true) for a test, then that test is understood to not be a real test result and is therefore not shown on the testwork comparison charts. Synthetic values are available when running circuit model simulations and do show up in the list of model results.


Unconfined compressive strength is not used directly in any SAGMILLING.COM models. Literature suggests it can occasionally be used as a proxy for the Bond crushing work index if a suitable calibration sample set exists for an ore.

The UCS reflects the energy requirement to break rocks in the coarse size range, from >100 mm down to about 10 mm - 20 mm.