Excavation induced over pore pressure around drifts in the Callovo-Oxfordian claystone

Abstract

Continuous field monitoring during drifts excavation at the Meuse/Haute-Marne Underground Research Laboratory showed anisotropic hydromechanical responses, which are mainly evidenced in: (1) pore pressure field around the drifts with zones of marked overpressure; (2) the development of a dissymmetrical fractured zone induced by the excavation. These observations suggest that in addition to the initial stress state, the inherent anisotropy of the Callovo-Oxfordian claystone plays a key role in the response of rock formation. Numerical modelling show that inherent elastic anisotropy is not adequate to fully explain the pore pressure evolution during excavation. This paper aims to show and explain qualitatively and quantitatively the main trends of the excavation induced pore pressure evolution around drifts. First, a qualitative analysis is performed, based on a closed-form solution of elastic fields around an elliptical opening (in order to simulate fractured zone shape), embedded in a transversely isotropic rock. Then, a coupled poro-elasto-plastic analysis is performed, by taking into account a perfect poro-elasto-plastic behavior for both the intact rock and the induced fractured zones with different plastic parameters. It is shown that upon the determination of extension and shape of fractured zone, overpressures induced by the excavation can be well reproduced.