Anisotropic constitutive model of pentagonal prism columnar jointed rock mass

The pentagonal prism is the dominant structural shape in the columnar jointed rock mass (CJRM) structure, and its anisotropic mechanical properties are crucial for engineering safety. In this paper, we propose an anisotropic constitutive model to accurately describe the anisotropic behavior of the CJRM. First, a generalized model of a pentagonal prism column with variable angle θ was established according to the basic structural features of the CJRM. Based on this model, the constitutive relationship space was derived in three dimensions and then improved through analysis of the influence of the inner disturbed belt and coordinate deflection. Subsequently, considering the CJRM at the Baihetan Hydropower Station in China, we determined the optimal value of θ using one set of in situ test results, thereby obtaining the constitutive relationship for this structure. Finally, the optimal constitutive model of the pentagonal prism was verified by the remaining test results, and its anisotropic characteristics were compared with the existing quadrangular and hexagonal prism constitutive models. The results revealed that the calculated elastic parameters of the pentagonal prism model are in good agreement with the test results and that the curve of the horizontal elastic modulus is approximately circular, which reflects the quasi-transverse isotropy of the proposed model. We found that the calculated results of the three models exhibit different deviations from the test results, owing to the different structural characteristics, and that the deviation in the pentagonal prism model is the smallest.