Determination of deformation modulus and characterization of anisotropic behavior of blocky rock masses

Abstract

The anisotropy in the deformational behavior of blocky rock masses has been comprehensively investigated. The uniaxial deformation modulus was selected as the key parameter. This modulus is generally anisotropic and depends on the loading direction, as well as on the properties of the intact rock, joints, and joint setting. Representative volumes of blocky rock masses were numerically simulated using the discrete element method and were loaded uniaxially in various directions. Subsequently, the failure mode and the deformation modulus were studied for different loading directions and various relative joint settings. A new nonlinear, stress- dependent stiffness matrix for joints was introduced, incorporating the surface conditions of the joints in terms of the Joint Roughness Coefficient (JRC) and the properties of the intact rock materials in terms of the Uniaxial Compressive Strength (UCS). The results of the assessments are presented in the form of rose diagrams, showing variations in the deformation modulus of the blocky rock mass that depend on the joint’s JRC, the intact rock’s UCS, and the structure of the rock mass in term of the relative joint angle. Also, the expected degree of anisotropy for various joint surface conditions and uniaxial compressive strengths of intact rock were introduced. In the Geological Strength Index (GSI) table, results are classified such that assigning a value to the JRC for each class of joint surface conditions allows for the corresponding deformation modulus and degree of anisotropy. According to this chart, it is deduced that the effect of joint roughness on the deformation modulus of blocky rock masses is greater than that of the intact rock UCS. The results support the hypothesis that a blocky rock mass has a critical strain that is independent of the loading angle (θ) and the orientation of the third joint set (α).

Keywords

• deformation modulus
• blocky rock mass
• anisotropy
• joint stiffness matrix
• degree of anisotropy
• failure mode