A Simplified Approach for Modeling the Long-Term Behavior of the Fractured Zone Around Excavated Galleries in Callovo–Oxfordian (COx) Claystone

Abstract:

The excavation of deep underground structures in clay formations, such as the Callovo–Oxfordian (COx) claystone, significantly alters the mechanical properties of the rock due to in situ stresses. When the ratio between the applied stresses and the rock strength becomes critical, fracturing may develop around tunnels, generating a damaged zone whose extent and properties control the long-term stability of the structures. Observations conducted at the Meuse/Haute-Marne Underground Research Laboratory have shown the decisive influence of several factors on gallery convergence: the initial stress state, the orientation of the galleries relative to the principal stresses, excavation methods and rates, as well as the intrinsic anisotropy of the rock.

Modeling this fractured zone as an equivalent continuous medium has been widely studied because of its simplicity for engineering applications. In the literature, the rock mass is often represented using viscoelastic or elasto-viscoplastic models that explicitly distinguish between intact rock and fractured rock. In this work, we propose a simpler approach in which the rock mass is represented as a homogeneous elasto-viscoplastic medium, without distinguishing between intact and fractured rock.

The adopted elasto-viscoplastic model incorporates triple anisotropy. Elasticity is described using a transversely isotropic behavior. An anisotropic Mohr–Coulomb criterion is used to represent plasticity, while an anisotropic creep law of the Lemaitre type is employed to describe time-dependent behavior. Anisotropy is introduced through the definition of an equivalent stress tensor, transformed according to the principal orientation of the observed, fracture deformation induced, anisotropy.

The model parameters are calibrated using laboratory tests at the sample scale and then adjusted based on in situ measurements. Application to the GED and GCS galleries, excavated respectively in the directions of the minor and major principal horizontal stresses, demonstrates the ability of this simplified model to reproduce the observed anisotropic convergences.

Short bio:

Akram is a Civil and Public Works Engineer. In the fall of 2023, he joined École Nationale des Ponts et Chaussées to pursue a research Master’s degree in Geomechanics and Geotechnics (MSROE). During this program, he joined Navier Laboratory as a research intern, where he worked on synthetic rocks to investigate scale linkage in mechanical behavior and gas migration. He is currently pursuing a PhD focusing on the behavior of the excavation-induced fractured zone in clayey rock formations for deep geological radioactive waste storage.