Numerical Validation of Empirical Rock Mass Modulus Estimation

By: Colin Hume

Abstract

The rock mass modulus of deformation (Erm) is an important constitutive parameter used in numerical analysis problems within the field of rock engineering for underground mining and tunnelling.  Determining this parameter directly by in-situ test methods is inherently difficult, time consuming and expensive, and these challenges are further exasperated when dealing with tunnels in weaker, softer rock masses where errors in modulus estimation have a profound impact on closure predictions.  Empirical relationships based on rock mass classification schemes have been developed to determine Erm in response to these issues.  The presentation will review the empirical relationship provided by Hoek and Diederichs (2006) based on Geological Strength Index (GSI). Within its range of applicability there is a large variation in measured values compared to the predicted relationship. A range of quality (GSI) rockmasses including very poor, blocky and relatively competent are analyzed using discretely fractured numerical models with particular focus on tunnel response. Tunnel closure in these simulations is used to compare to predictions based on modulus estimates.  The results of these simulations are discussed.