Size effect in strength assessment by indentation testing on rock fragments

• UCS of limestone rocks was predicted by indentation testing on rock fragments.
• Fragment area has negligible effect, while the thickness has significant influence on conventional indentation indices.
• Normalizing size functions are presented to decrease the size dependency.
• Empirical equations were developed to estimate UCS from CTFn and ISn.
• Verification of suggested equations shows strong agreement between the estimated and measured UCS.

The uniaxial compressive strength (UCS) of limestone rocks was estimated by indentation testing on small rock fragments. The effects of rock fragment dimensions (particularly area and thickness) on indentation indices were studied. The investigation shows a size dependency of the conventional indentation parameters. The results indicate the fragment area normal to loading direction has little effect on the indentation indices, while the sample thickness has major influence on the results. To reduce size dependency, the results of indentation test were normalized by a thickness function.The proposed empirical equations were verified against independent data pertaining to other limestone rocks not used in developing the correlations, which showed agreement between the estimated and measured UCS. Statistical analysis was used to determine the minimum number of required indentation tests in relation to the project importance. This study shows that the uniaxial compressive strength of rocks can be estimated reasonably well from indentation testing of small rock fragments.