An Energy-Based Method to Determine Rock Brittleness by Considering Rock Damage

Accurate evaluations of rock brittleness are very significant in the engineering geology and geotechnical engineering fields. Most previous studies have adopted the stress–strain relationship to propose a series of indices for rock brittleness estimations but have seldom considered rock damage. Rock damage can be viewed as an energy dissipation process during rock deformation, which is closely related to rock brittleness. In this study, a new brittleness index (BI23) was proposed by considering rock damage, and the rock damage was calculated by the energy-based method. Then, the newly proposed rock brittleness index was validated by analyzing the variations in rock brittleness under increasing confining pressures and temperatures. The results indicate that the rock brittleness estimated by BI23 shows a significant drop in the case of increasing confining pressures and temperatures. To demonstrate its performance and advantages, a comparative study between the BI23 index and some previous indices was conducted by analyzing the stress–strain curves (SSC) of four rock types (e.g., limestone, marlite, feldspar lithic sandstone, and feldspathic quartz sandstone). The comparative study shows that the BI23 is able to produce more stable and consistent rock brittleness even for the same rock type under different tests, which is considered to be a major improvement over previous indices. Finally, the brittleness value distribution patterns of BI23 for normal and extreme conditions are discussed. It is suggested that the scope of rock brittleness evaluations under normal conditions should be defined to be between 0.5 (ductile) and 1 (brittle) in practical applications.