A numerical study of the mechanical response to excavation and ventilation around tunnels in clay rocks

As an attempt to more accurately model the nonlinear-elastic response and excavation damaged zone (EDZ) behavior induced by excavation and ventilation in clay/shale, we have incorporated and implemented a recently developed stress–strain relationship, called the two-part Hooke's model (TPHM), within the TOUGH–FLAC3D code. TPHM is based on a macroscopic-scale approximation using a natural-strain-based Hooke's law to describe elastic deformation for a fraction of pores subject to a large degree of deformation; an engineering-strain-based Hooke's law is used for the other part. We carried out a series of numerical verification tests on the deformation behavior of small-scale clay and shale samples based on TPHM. A comparison with an elastic model described by the conventional Hooke's law (the single-part Hooke's model, SPHM) indicates that the TPHM can more accurately capture observed mechanical deformation in the low-stress region of clay/shale rocks. Furthermore, based on numerical simulations of elastic response and EDZ behavior around a tunnel opening (in Opalinus Clay) induced by excavation and ventilation, we found a larger radial elastic deformation on the roof and floor of the tunnel, and considerably different stress distributions than were found with SPHM. Although desaturation was influenced by ventilation only and occurred within a narrow zone around the tunnel opening, a suction state in the rock mass developed over quite a long radial distance, which had a substantial effect on the macroscopic behavior of the rock matrix during the desaturation process. Throughout the tunnel, the mode and extension of damage zones calculated by TPHM showed clear differences from those calculated by SPHM. The results based on TPHM are more consistent with observations, indicating the need and importance of incorporating more accurate constitutive models for modeling coupled processes.

► A modeling capability for THM processes in clay was developed.
► Near-field response to excavation and ventilation in a clay tunnel was investigated.
► Numerical results based on TPHM and SPHM model were compared and analyzed.