Effect of anisotropy and hydro-mechanical couplings on pore pressure evolution during tunnel excavation in low-permeability ground
Introduction
The Callovo-Oxfordian Claystone (COx) is an argillaceous formation, lying between depths of 420 and 550 m in Meuse/Haute-Marne region in France. The COx claystone has been studied for more than one decade as a potential host formation for high-level and Iintermediate-level long-lived nuclear waste considering its very favorable characteristics like a very low hydraulic conductivity, a small molecular diffusion and a significant retention capacity for radionuclides.1, 2 The French National Radioactive Waste Management Agency (Andra) began the construction of the Meuse/Haute-Marne Underground Research Laboratory (M/HM URL) in 2000 with the main goal of demonstrating the feasibility of geological repository in Callovo-Oxfordian claystone. A network of experimental drifts (with diameters ranging between 4 m and 9 m) has been excavated following the directions of the horizontal principal stresses. At the main level of M/HM URL (i.e., −490 m) the major principal horizontal stress is about 16.0 MPa and is oriented in the direction N155°E 10°, while the minor principal horizontal total stress is very close to the vertical stress ( 12.0 MPa).3 Therefore, the initial stress state is quasi isotropic (in the plane of their section) for the drifts following the direction of the major horizontal principal stress.
Continuous monitoring of the excavated drifts revealed the development of a fractured zone (extensional and shear fractures) induced by the excavation. The fracture distribution depends on the drift orientation with respect to the in-situ stress field 4 and has an important influence on the drifts deformation.2 Fractured zones observed in situ present anisotropic extents in both directions of excavation (Fig. 1). Accordingly, the convergence measurements show an anisotropic closure for both directions and the ratio of anisotropy depends on the drifts’ orientations.2, 5 Moreover, the field monitoring during the excavation showed marked overpressures and an anisotropic pore pressure field around the drifts.
Interpretation of the observed pore pressure variations around excavation is a challenging task, especially in the formations like claystones with a very low permeability (<10–19 m2). Recent studies have been carried out on the pore-pressure evolutions induced by excavation: Analysis of measurements around undergrounds excavations in claystones,4, 6, 7, 8, 9 coupled modeling of the excavation damaged zone based on strain localization analysis,10, 11, 12 coupled modeling of the thermal impact on the hydro-mechanical behavior of the rock mass,13 and unsaturated hydro-mechanical modeling of the excavation.14
In this paper, the pore pressure evolution induced by excavation of drifts has been analyzed by means of a fully coupled hydro-mechanical finite element simulation. Special attention is put on the drifts following the direction of the major principal horizontal stress, where even with a quasi-isotropic initial stress state in the section of the drift, the pore pressure field and the mechanical response are anisotropic. This observation indicates that the intrinsic anisotropy of the material plays a key role in the response of the rock formation. To understand these phenomena, an anisotropic poroelastic analysis of the pore pressure evolution induced by the drift excavation is performed. The principal objective is to simulate the main trends of the pore pressure evolution with a simple model taking into account the elastic anisotropy of the material. Moreover, a failure analysis is performed in an attempt to understand the role of the hydro-mechanical coupling in the development of the fracture network around the drifts.
Section snippets
Overview of in–situ observations
A continuous monitoring around excavation has been performed in an attempt to control and analyze the response of the rock formation. Several sections have been equipped with high precision measurement sensors, in order to follow the hydro-mechanical response induced by the excavation and its evolution in time.5, 6 An overview of the fracture network extension around drifts and the pore pressure evolution is presented in this section.
Theoretical framework
A fully coupled poroelastic analysis is performed using the finite element method to simulate the pore pressure evolution induced by drift excavation. The main goal of this analysis is to reproduce and understand the main trends of the fractures development with a simple model taking into account the influence of the elastic anisotropy of the material. The fundamental bases of poroelasticity theory have been presented in several textbooks and reference papers (e.g. 18, 19). In this analysis, it
Influence of the cross-anisotropy of rock formation on the short-term ground response
As explained above, in-situ measurements show that for drifts following the direction of , even if the initial stress state is quasi-isotropic in the plane of the drift section, the pore pressure evolution and the mechanical response are anisotropic. These observations indicate that the intrinsic anisotropy of the material plays a key role in the response of the rock formation. Indeed, sedimentation has led to a slightly anisotropic behavior of the claystone. As explained by Blümling et al. ,
Time evolution of the pore pressure distribution around drifts
For the sake of simplicity, the analysis of the time evolution of the pore pressure distribution is performed taking into account only one set of parameters, presented in case 7. The pore pressure at different times, in vertical and horizontal directions around the drift and for the two directions of excavation are presented in Fig. 10. It is observed that, for drifts following the direction of , the instantaneous response (after 1 s) presents an overpressure close to the wall in the
Failure initiation analysis around the drifts
In an attempt to highlight the influence of the hydro-mechanical couplings in the initiation of the fractures around drifts, an analysis of the failure conditions is performed based on the Terzaghi effective stress distribution () around the excavations (for cases 6, 7 and 8, presented in Table 3).
The principal stresses around the excavation ( and ) are calculated in the case of an instantaneous excavation by imposing a zero-flow boundary condition at the drifts wall. This
Conclusions
The influence of the anisotropy of the rock mass formation on the pore pressure evolution induced by tunnel excavation has been analyzed by means of a fully coupled poroelastic model. It is shown that the main trends of the pore pressure evolution can be simulated and analyzed based on an efficient and simple poroelastic model under plane strain conditions. It is observed that the anisotropic pore pressure field observed around the drifts can be directly related to the anisotropy of the elastic
References (54)
- et al.
Short- and long-term behaviors of drifts in the Callovo-Oxfordian claystone at the Meuse/Haute-Marne Underground Research Laboratory
J Rock Mech Geotech Eng.
(2013) - et al.
Complete in situ stress determination in an argillite sedimentary formation
J Phys Chem Earth.
(2007) - et al.
Modelling a mine-by test at the Mont Terri rock laboratory, Switzerland
Int J Rock Mech Min Sci.
(2007) - et al.
Shear banding modelling in cross-anisotropic rocks
Int J Solids Struct.
(2015) - et al.
Coupled modeling of excavation Damaged Zone in Boom clay: strain localization in rock and distribution of contact pressure on the gallery's lining
Comp Geotech.
(2015) - et al.
3D numerical modelling thermo-hydromechanical behaviour of underground storages in clay rock
Tunn Undergr Space Technol.
(2012) - et al.
An unsaturated hydro-mechanical modelling of two in-situ experiments in Callovo-Oxfordian argillite
Eng Geol.
(2013) - et al.
Fundamentals of Poroelasticity
Material coefficients of anisotropic poroelasticity
Int J Rock Mech Min Sci.
(1997)Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste
Cem Concr Res.
(2011)
Poromechanical behaviour of hardened cement paste under isotropic loading
Cem Concr Res.
Thermodynamically consistent anisotropic constitutive relations for a poroelastic material saturated by two immiscible fluids
Int J Rock Mech Min Sci.
Material coefficients of multiphase thermoporoelasticity for anisotropic micro-heterogeneous porous media
Int J Solids Struct.
Poroelasticity of a micro-heterogeneous material saturated by two immiscible fluids
Int J Rock Mech Min Sci.
The excavation damaged zone in clay formations time-dependent behaviour and influence on performance assessment
J Phys Chem Earth.
Importance of anisotropy when estimating and measuring in situ stresses in rock
Int J Rock Mech Min Sci Geomech Abstr.
Poroelastic response of a borehole in a non-hydrostatic stress field
Int J Rock Mech Min Sci Geomech Abstr.
Poroelastic parameters of Meuse/Haute Marne argillites: effect of loading and saturation states
Appl Clay Sci.
Hydromechanical modelling of an excavation in an underground research laboratory with an elastoviscoplastic behaviour law and regularization by second gradient of dilation
Int J Rock Mech Min Sci.
On the validity of the “Brazilian” test for brittle materials. Int
J Rock Mech Min Sci Geomech Abstr.
Fracture initiation and propagation in intact rock – a review
J Rock Mech Geotech Eng.
FE modelling with strong discontinuities for 3D tensile and shear fractures: application to underground excavation
Comp Meth Appl Mech Eng.
Geometry and properties of the excavation induced fractures at the Meuse/Haute-Marne URL drifts
Rock Mech Rock Eng.
Analysis of long-term anisotropic convergence in drifts excavated in Callovo-Oxfordian Claystone
Rock Mech Rock Eng.
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2022, Computers and GeotechnicsCitation Excerpt :For direct coupling, which refers to the poroelastic interaction between the two physical fields, the stress/strain variation caused by the instantaneous excavation can lead to either pressurisation or depressurisation in surrounding low-permeability rocks around the tunnel, because subject to such an undrained (or nearly undrained) condition, the perturbed pore pressure field requires a sufficient period of time to reach an equilibrium. Many field observations point to the presence of such a phenomenon during underground excavation (Amann et al., 2017; Guayacán-Carrillo et al., 2017; Tsang et al., 2007), where low and high pressures are recorded in extensional and compressional areas, respectively, during the advance of the excavation face. The pressure fluctuation, in return, may modify the stiffness and brittleness of rock matrix (Dvorkin et al., 1995; D., 2007), causing fracture deformation (Huang et al., 2018; McClure and Horne, 2014; Wasantha et al., 2016) and brittle failure (Zhao et al., 2021).