Size Effect in Strength Assessment by Indentation Testing on Rock Fragments

Size DependencyIn this study, uniaxial compressive strength (UCS) of limestone rocks was estimated by indentation testing on small rock fragments. Here, the effects of rock fragment dimensions (particularly area and thickness) on indentation indices were studied. The investigation shows 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

A New Method for Correlating Rock Strength to Indentation Tests

In this Paper, UCS of limestone rock was determined by indentation testing on rock fragments. The size of the fragments was in the range of 2 to 5 mm, which is within the size range of drill cuttings. Here, scatter of the data was investigated by calculating the coefficient of variation that showed the size dependency of the conventional indentation parameters; Indentation Modulus (IM) and Critical Transition Force (CTF). Thus, it is recommended that the results of indentation tests should be normalized by the fragment size and presented in the form of Normalized Indentation Modulus (IMn) and Normalized Critical Transition Force (CTFn). Regression analysis was carried out to show the relationship between the IMn or CTFn and the UCS. Linear and exponential regression provided reasonable correlation coefficients of higher than 0.74 and 0.85, respectively. The proposed empirical equations for estimation of the intact rock UCS from the normalized indentation indices were verified using independent data from limestone rocks not used in developing the correlations. The outcome was a strong agreement (R~1.00) between the estimated and measured UCS. Based on statistical analysis, minimum number of indentation test was determined for random size samples. It is suggested that indentation testing be carried out using uniform fragment sizes to reduce the required number of indentation tests