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.

Automated summary

The macro-indentation test is commonly used for evaluating rock mechanical properties, and various indentation indices have been proposed. In this study, previous studies on indentation indices were summarized, and indentation tests were conducted on 47 granite specimens to calculate 26 indices. The indices generally concentrate around the mean value and have different dispersion degrees. Force indices and energy indices show high correlation, while force indices and modulus indices have little correlation. Most modulus indices are correlated, while most dimensionless indices are independent. For highly correlated indices, those with low dispersion are preferred. Proper indices for estimating rock strength, representing rock brittleness, and drillability are suggested based on their distribution characteristics, correlations, and prediction accuracy. The minimum testing number to achieve the required accuracy is also proposed for these selected indices.