Contribution of the bounding surface plasticity to the simulation of gallery excavation in plastic clays

Abstract

Excavation of a gallery induces perturbations of the surrounding in-situ stress ( σ) field and pore pressure ( u w) field, combined with the generation of displacements ( u). In the framework of the underground storage for high level radioactive waste, an accurate assessment of these perturbations is important. In the past, analytical solutions ( σ and u fields) have been derived, based on classical continuum mechanics (total stress analysis) and on simple constitutive models (elastic, Tresca, Mohr–Coulomb). More recently, these solutions have been extended to the undrained response ( σ, u and u w fields) of saturated porous media, still considering simple mechanical models. However, these solutions underestimate the displacements and the variations of pore pressure compared to the in-situ values monitored for the Boom clay, obtained during the excavation of the experimental Test-Drift (TD) gallery at Mol, Belgium. Here, we investigate whether such discrepancy could result from the sharp elastic–plastic transition on which classical plasticity models (of which the Cam-clay) are based. To take into account a smooth transition from elastic to plastic states, two bounding surface (BM) models based on the Cam-clay model are presented. The first one is a simplified version of the Dafalias–Kaliakin bounding surface model, and the second one is based on the Al Tabbaa's bubble model which has been generalised here. In the following, available in-situ data are briefly presented. Eventually, undrained hydromechanical simulations of the Test-Drift excavation are performed, considering the classical Cam-clay and the previously mentioned bounding surface models. As a main result, the radial convergence obtained with the bounding surface models is quantitatively much closer to in-situ measurements than results obtained with the Cam-clay model. Also, the extent of the hydraulic disturbance predicted from the retained bounding surface models is qualitatively closer to in-situ data than the value predicted from the Cam-clay one. In the future, these conclusions will be checked against new in-situ data which will be available for Boom clay in late 1999 within the CLIPEX (CLay Instrumentation Programme for the EXtension of an underground research laboratory) European project.