Explicit numerical models for the prediction of plastic and weakening rockmass behaviour around a circular tunnel in isotropic and anisotropic stress conditions

The Geological Strength Index, GSI, is an assessment system for rockmass structure conditions (blockiness and discontinuity condition) based on the visual examination of exposed rockmass surfaces. The Generalized Hoek-Brown failure criterion is widely used with GSI to estimate the strength and stiffness of jointed rockmasses in traditional continuum numerical modelling. Modern numerical analysis tools allow networks of discrete structure to be represented with assigned properties based on site investigation data. In this paper, models with defined explicit structure are shown to yield similar results to implicit GSI-based models and provide more detailed information on the spatial variability of tunnel response when reasonable rockmass input parameters are estimated and calibrated. Once plastic explicit models are calibrated to give similar response to plastic implicit GSI-based models, explicit models using weakening joint and intact rock elements are used to determine appropriate post-yield strength and dilation parameters for use in implicit GSI-based modelling.