Estimation of compliance tensor and anisotropy index for fractured rock masses using field measured data

Abstract

This paper proposes a method for calculating the fractured rocks compliance matrix using field data. The method is based on the Oda fracture tensor, which combines the mechanical properties of intact rocks and the geometric and mechanical properties of the fractures. The compliance matrix-based anisotropy index EAI is derived to evaluate the anisotropy of the fractured rock mass. The value of EAI ranges from 0 to 1.0986. The closer the EAI value is to 0, the higher the degree of anisotropy. According to the fracture parameters measured in the field, the fracture's normal stiffness and shear stiffness values are derived. Then the compliance matrix of the fracture is determined. In addition, the influence of the confining pressure on the normal and shear stiffness of the fracture was studied. The results show that (1) as the spacing and stiffness ratio of the fracture group increase, the EAI of the corresponding fracture rock increases; (2) as the confining pressure increases, it is necessary to apply greater normal stress to produce the same strain and the elastic modulus increases; (3) the larger the stiffness ratio is, the larger the anisotropy index EAI is at the same confining pressure.