Elastic deformation modulus for estimating convergence when tunnelling through squeezing ground

Squeezing in tunnelling is commonly assessed using the linearly elastic-perfectly plastic Mohr-Coulomb (MC) model. Weak rocks and fault materials, however, exhibit confining stress-dependent and strain-hardening behaviour prior to failure; in other words, the higher the confining stress and the lower the shear strain the stiffer the rock behaviour. As the MC model assumes a strain- and stress-independent Young's modulus, the selection of an appropriate 'operational' value, E-MC, remains a major problem in tunnel studies using this model. Although E-MC has a significant effect on the deformation predictions (they are inversely proportional to this value under small strain theory), there is no widely accepted or well-validated approach to its selection. This paper shows, using the results of triaxial compression tests on weak rocks and fault materials from the Gotthard base tunnel and five other projects, and performing a theoretical analysis of the ground response to tunnel excavation, that E-MC can be taken as equal to the secant modulus E-50 (rather than equal to the unloading-reloading modulus) extrapolated from standard triaxial compression test results to the in situ stress level. This is particularly useful for practical purposes as it allows standard computational methods to be used with sufficient accuracy, rendering more refined models unnecessary, at least at the preliminary design stage.