A new brittleness index based on crack characteristic stress and its engineering applications

The inherent brittleness of the surrounding rock poses a threat to the safety of underground engineering construction. One of the most straightforward methods for quantifying the degree of rock brittleness is using a rock brittleness index. Over the past several decades, a dozen rock brittleness indices based on uniaxial and conventional triaxial tests have been proposed and used. However, these indices do not reflect the impact of the intermediate principal stress σ2, which may render them unsuitable for rocks subjected to true triaxial stress. To address this limitation and explore the influence of σ2 on rock brittleness, a series of true triaxial tests were conducted. Crack characteristic stresses were introduced to investigate the different stages of deformation in the stress–strain curve of rocks. The relationship between σ2 and brittle–ductile damage of rocks was analyzed by introducing the dimensionless parameters Rci and Rcd. It is shown that a higher Rci indicates that the rock stores more elastic energy that is released in the post-peak stage, while a higher Rcd indicates that more microscopic tensile cracks are generated within the rock and indicates a shorter duration of interpenetration of tensile cracks. Thus, high values of Rci and Rcd indicate an increased likelihood of brittle failure occurring in the rock. Building upon these findings, a novel pre-peak brittleness index, Bi, is proposed. This index effectively quantifies the influence of σ2 on rock brittleness. Importantly, it overcomes the limitation posed by existing indices that are not suitable for rocks subjected to true triaxial stresses. An application of Bi to the Yebatan underground powerhouse was conducted using numerical simulation. The results indicate that the potential for brittle damage can be significantly reduced by increasing the angle between the axis of the underground cavern and the initial maximum principal stress in an appropriate manner.