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 2–5 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 the statistical analysis, minimum number of indentation test was determined for random size samples. It is suggested that indentation testing will be carried out using uniform fragment sizes to reduce the required number of indentation tests.