Response of an Anisotropic Rock Mass under Polyaxial Stress State

The strength of rock mass probably is the first engineering parameter that a geotechnical engineer may be keen to establish before the start of any mining and civil engineering project. The strength of a rock mass at site is generally influenced by joint geometry and the stress state that it experiences. Field studies show that stress conditions around underground structures will be generally true triaxial or polyaxial, i.e., ${\sigma }_1>{\sigma }_2>{\sigma }_3$ . The rock mass behavior in such field conditions can effectively be simulated in laboratory by conducting large-scale physical modeling. In this study the most commonly occurring joint configurations (three joint sets) were created on rock mass models in the laboratory and tested under polyaxial stress compression. The model rock was a sand-lime block with low strength and medium deformability. The joint configurations were varied to assess the influence of joint geometry on the stress-strain curve, failure mechanism, and strength anisotropy. Similarly, on each type of geometry, ${\sigma }_2∕{\sigma }_3$ ratio was also varied to study the effect of intermediate principal stress on engineering behavior of rock mass. The maximum enhancement of 310% in strength due to an increase in the ${\sigma }_2∕{\sigma }_3$ ratio was observed, which corresponded to joint inclination, $\theta =60°$ . Based on testing results, the polyaxial strength criterion was evolved similar to the von-Mises failure envelope.