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The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone. Although the main objective has been the simulation of the COx claystone behaviour, the model can be readily used for other argillaceous materials. The constitutive model developed is then applied, via a suitable coupled hydromechanical formulation, to the analysis of the excavation of a drift in the Meuse/Haute-Marne 2 Underground Research Laboratory. The pattern of observed pore water pressures and displacements, as well as the shape and extent of the damaged zone, are generally satisfactorily reproduced. The relevance and importance of rock anisotropy and of the development of a damaged zone around the excavations are clearly demonstrated.

The paper presents the numerical simulation of an underground excavation in the Callovo-Oxfordian argillaceous formation, performed in the Meuse/Haute-Marne underground research laboratory, within the context of deep geological nuclear waste disposal. The constitutive model adopted incorporates a number of characteristic features of the hydromechanical behaviour of the Callovo-Oxfordian claystone, and other indurated clays, including anisotropy and time-dependent deformations. Model parameters have been largely determined from available experimental results. Particular attention has been given to the incorporation of a nonlocal formulation in order to simulate localised deformations objectively. It is shown that the numerical analysis is able to provide a very satisfactory reproduction of the extent and configuration of the excavation-induced fractured zone, the development of rock displacements, and the evolution of water pressures in the rock. The analysis is also able to provide a deeper insight into the mechanisms underlying the generation of water pressures inside the rock mass.

A cross-anisotropic formulation for elasto-plastic constitutive models based on a non-uniform scaling of the stress tensor is described. Taking advantage of the material symmetries characterising cross-anisotropy, only two scaling factors, one for the normal stress components and one for the shear stress components, are required. It is shown that the formulation can be easily introduced in already implemented models with minor modifications. The performance of this formulation is investigated by reproducing the strength variation of anisotropic rocks in triaxial tests. The numerical simulation of an unsupported excavation is also presented to show the effect of different scaling factors and bedding plane orientations.Peer ReviewedPostprint (author's final draft

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A constitutive framework for the simulation of the time-dependent behaviour of soft structured clays has been developed from the combination of a number of existing modelling approaches. The formulation accounts, in a natural way, for timedependent phenomena such as loading rate dependency, stress-relaxation, ageing, and creep deformations under constant load. In addition, the effect of structure is accounted for. To demonstrate the potential of the proposed approach, a specific model based on this framework is described and applied to the simulation of a number of relevant laboratory tests from the literature.

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