Comparison and Selection of Index for Macro-indentation Test of Brittle Rock

The macro-indentation test is widely used to evaluate rock mechanical properties. Due to the complex indentation process and the multi-peak force–penetration curve, many indentation indices are proposed to evaluate rock properties. It is critically important to assess these indices and select the proper index for certain usages (i.e., estimating rock strength, representing rock brittleness, and drillability). In the present study, indentation indices proposed in the previous studies were summarized first. To analyze statistical characteristic of the indentation indices (i.e., distribution characteristic and correlation), the indentation test was conducted on 47 granite specimens. 26 indentation indices were calculated from each force–penetration curve of the indentation test. The indentation indices are generally concentrated at the mean value, but these indices have different dispersion degree. The force indices and energy indices of the same large-scale fracture are highly correlated, while the force indices and modulus indices are little correlated. Most modulus indices are correlated with each other, while most dimensionless indices are independent of each other. For highly correlated indices, the index with small dispersion is the first choice for usage. Finally, proper indices are suggested for estimating rock strength and representing rock brittleness and drillability according to their distribution characteristics, correlations, and prediction accuracy. The minimal testing number to attain the required accuracy is also proposed for these selected indices. Highlights The definition and application of the existing indentation indices are summarized. The indentation test is conducted on 47 granite specimens to evaluate the applicability of the indentation index. The distribution characteristics and correlations of the testing data of the indentation indices are studied. Proper indentation indices are suggested for estimating rock strength and representing rock brittleness and drillability.