3D Modelling of Excavation-Induced Anisotropic Responses of Deep Drifts at the Meuse/Haute-Marne URL

Abstract

Understanding the excavation-induced fractured zone (EFZ) around drifts is paramount in the context of the deep geological disposal for nuclear waste since fractures can introduce pathways for the migration of radionuclides. Drifts in the Meuse/Haute-Marne Underground Research Laboratory (URL) have been essentially excavated following the two main directions of major and minor horizontal stresses. Field observations on the two drifts GCS (parallel to major horizontal stress direction) and GED (parallel to minor horizontal stress direction) in the URL show anisotropic shapes of EFZ around drifts through both orientations and anisotropic convergences. These anisotropic responses resulted from the inherent and/or induced anisotropies of the host rock as well as the anisotropic stress field. This study focuses on 3D numerical modelling of excavation-induced anisotropic responses including shape and extent of EFZ, and short-term convergences of drifts. The main assumption is that the failure of claystone material is due to fracturing along weakness planes (ubiquitous joints) and the failure of the rock matrix. The ubiquitous joint failure is represented by perfectly plastic models for both tensile and shear yield functions. Their orientation is determined from the stress state based on the fracture mechanics, which includes tensile, longitudinal splitting and shear (conjugate planes) cracks. The rock matrix is assumed to be elastoplastic with hardening, softening and residual behaviours. Confining pressure dependency for the post-peak behaviour with a brittle–ductile transition is taken into account for the rock matrix. The proposed model is implemented into a commercial numerical software FLAC^3D. The main features of the implemented model are shown by the simulation of laboratory triaxial compression tests, as well as field observation within the URL. In particular, comparisons between 3D simulations of GCS and GED drifts with in situ observations shows promising results, which demonstrates advances of present model with respect to existing models.

Highlights

•3D modelling of excavation induced anisotropic mechanical responses of COx claystone.

•A coupling between weakness planes and elastoplastic rock matrix for rock fracturing.

•Stress state controlling the weakness plane occurring and its orientation.

•Galleries within Andra URL following both minor and major horizontal stress orientations are considered.

•Convergence and damage zone are rather well reproduced by the proposed model.

Appendix A: Details on the Conceptual Model of Weakness Planes and Numerical Implementation

Definition of terms plotted in Fig. 2b are recalled below:

σ_t is the prescribed tensile strength, if different to the Hoek and Brown tensile strength, ${{\sigma }_{\text{t}}}^{\text{HB}}=\frac{s{ \sigma }_{\text{c}}}{m}$.

O: intersection between tensile and shear yield functions.

T: tangent to the Hoek and Brown envelope ${\mathcal{C}}_{\text{HB}}$ at point O.

L₁: bisects the area bounded by BOT

L₂: vertical line passing through the confining stress of 1 atm

${\mathcal{D}}_{1}$: domain bordered by σ₃-axis, BO segment and the half-line OL₁. ${\mathcal{D}}_{1}$ is associated to the tensile failure, with a fracture plane normal to the tensile direction

${\mathcal{D}}_{2}$: domain delimited by OL₁, OA and AL₂. ${\mathcal{D}}_{2}$ is associated to the shear failure, with fracture plane oriented normal to σ₃ (i.e. parallel to σ₁-direction, the maximum compressive stress)

${\mathcal{D}}_{3}$: domain delimited by AL₂ and ${A\mathcal{C}}_{\text{HB}}$. ${\mathcal{D}}_{3}$ is associated to the shear failure, with conjugate fractures with an angle α_f of $\pm \left(\frac{\pi }{4}-\frac{{\varphi }_{\text{wp}}}{2}\right)$ with respect to σ₁-direction, the maximum compressive stress and φ_wp is the internal friction angle of the weak planes.

Finally, line L₁ represents the diagonal between the surfaces F_s^m = 0 and F_t^m = 0 in the (σ₁ − σ₃, σ₃)-plane and divides the complementary domain of elastic area (F_t^m > 0 and/or F_s^m > 0) into two distinct subdomains: ${\mathcal{D}}_{1}$ and ${\mathcal{D}}_{2}$. In subdomain ${\mathcal{D}}_{2}$ (respectively, subdomain ${\mathcal{D}}_{1}$), projection will be performed by using F_s^m and G_s^m (respectively, F_t^m and G_t^m) and their associated partial derivatives for shear failure (respectively, tensile failure). It is also the same procedure for the weakness planes.