Numerical characterization for rock mass integrating GSI/Hoek-Brown system and synthetic rock mass method

Abstract

Rock mass characterization is a key issue to derive mechanical properties of jointed rock mass. In this research, the strength, modulus and post-peak behaviour of rock masses with varied joint orientations and confining stresses are characterized by integrating an empirical geological strength index (GSI)/Hoek-Brown system and a synthetic rock mass (SRM) numerical method. The SRM model is a 3D distinct element model supporting explicit simulation of discontinuous joints and crack propagation in the rock mass. The rock mass structure as denoted by a discrete fracture network is correlated to the GSI system through a quantitative factor of rock block size. The strength and modulus are then derived from the GSI/Hoek-Brown system and used as references for a series of synthetic rock mass models. A comparative study shows that when the predominant joints are steeply inclined in the rock mass subjected to axial loading, the empirical GSI/Hoek-Brown system provides reliable mechanical properties. When the joints are less favorable to shear sliding, the GSI/Hoek-Brown system underestimates the rock mass strength. Cracking characteristics are also investigated in selected SRM models, where few cracks are simulated when inclined joints are inserted in the rock mass and low confining stresses are applied. This research finally shows the method to construct continuum rock mass models using the numerical parameters of the SRM models. The numerical results suggest that the global failure zone is thin when the post-peak is elasto-brittle, and the failure zone is much wider when the post-peak is elasto-plastic. The involvement of the additional failed zones is responsible for the post-peak ductility. The paper is meant to explore new methods to derive high quality geo-data for jointed rock mass.