Shouyi Xie

Influences of temperature and water content on mechanical property of argillite

High-level radioactive waste is heat-emitting; the temperature can increase up to 100 °C in the gallery, and thus increase the density of microcracks in the host rock of argillite. In the meantime, the host rock undergoes swelling or shrinking induced by wetting or drying during ventilation or sealing process. Therefore, it is necessary to study the influence of temperature and water content, as well as their coupling effects, on the mechanical behaviour of argillite for the safety of underground disposal of high-level radioactive waste. For this purpose, we performed classical triaxial compression tests, micro-indentation tests and mini-compression tests on Callovo-Oxfordian (Cox) argillite under both temperature-controlled and relative humidity-controlled condition. Five levels of relative humidity (from 33 to 99%) and five levels of temperature (from 20 to 95 °C) were studied. The evolutions of elastic modulus, strength and deformability of Cox argillite were related to the temperature and relative humidity. It is found that the mechanical properties of Cox argillite were significantly deteriorated by the temperature and relative humidity, as well as their coupling effects.

An Experimental Study and Constitutive Modeling of Saturated Porous Rocks

Abstract This paper is devoted to the experimental characterization and constitutive modeling of saturated porous rocks. A typical porous chalk is investigated. Drained hydrostatic and triaxial compression tests are first performed to characterize the basic mechanical behavior of chalk. Drained triaxial tests with constant interstitial pressure are then carried out to study the effects of interstitial pressure on the plastic deformation and failure criterion. Finally, undrained triaxial compression tests are performed to investigate poromechanical coupling in saturated conditions. Based on the experimental data and some relevant micromechanical considerations, a micromechanics-based plastic model is proposed and extended to poroplastic coupling using the effective stress concept. The proposed model is verified through comparisons between the numerical results and experimental data for both drained and undrained tests.

An experimental study of crack growth in claystones

An experimental study is presented for the characterisation of crack propagation in claystones. Three point bending (TPB) and double torsion (DT) tests are conducted in order to determine the material toughness and crack propagation velocity, respectively. The experimental data obtained from both TPB and DT tests show that the value of critical stress intensity factor is a function of loading rate and it is affected by the material structural anisotropy. The crack propagation velocity is determined as a function of applied stress intensity factor. The data obtained from the present work will provide an experimental background for the modelling of time-dependent deformation in claystones.

Experimental Investigation on Mechanical Behavior and Permeability Evolution of a Porous Limestone Under Compression

Abstract This paper presents an experimental investigation on the mechanical behavior and permeability evolution of a typical porous limestone, the Anstrude limestone. Hydrostatic and triaxial compression tests are first performed under drained condition to study the basic mechanical behavior of the porous rock. Permeability measurement under both hydrostatic and triaxial compression is carried out for investigating effects of stress state on the permeability evolution along the axial direction of sample. The obtained results allow to identifying two basic plastic deformation mechanisms, the plastic shearing and pore collapse, and their effects on the permeability evolution. Under low confining pressures, the permeability diminution in the elastic phase is controlled by deviatoric stress. After the onset of plastic shearing, the deviatoric stress induces a plastic volumetric dilatation and a permeability increase. When the deviatoric stress reaches the peak strength or after the onset of shear bands, the permeability slightly decreases. Under high confining pressures, the deviatoric stress also induces a permeability diminution before the onset of plastic pore collapse. After the onset of pore collapse, the deviatoric stress leads to a plastic volumetric compaction and permeability decrease. When the deviatoric stress reaches the onset of plastic shearing, the two plastic mechanisms are in competition, the permeability continuously decreases but with a reduced rate. Finally, after the compaction–dilatation transition, the plastic shearing dominates the deformation process while the pore collapse still controls the permeability evolution.

An Experimental and Numerical Investigation of the Mechanical Behaviour of a Concrete and of its Permeability Under Deviatoric Loading

This paper investigates the mechanical behaviour of a concrete under deviatoric loading. Special attention is placed on understanding the evolution of concrete permeability at different stress levels. The studied cement-based material, called CEMI, is the potential candidate for the engineering barrier of underground repositories for nuclear wastes. The description of an original experimental method, based on applying a transient flow, is firstly presented. Two groups of experimental tests are realized. The first group is conventional triaxial tests performed under different confining pressures. The other group is triaxial compression tests accompanied with simultaneous pressure evolution phases. In the pressure evolution phases, the evolutions of water pressures are measured on the upper and lower boards of samples which are mechanically loaded. The experimental investigation shows that the mechanical behaviour of concrete is characterized by: important residual strains developed in both axial and lateral directions and a progressive decrease of elastic stiffness during loading/unloading cycles. In view of this, a coupled elastoplastic damage model is proposed for studied material. In the proposed model, two damage variables are introduced in order to take into account the dissymmetric behaviour of concrete observed under tensile and compression conditions. On the other hand, the experimental results obtained during the pressure evolution phases show that the evolution of measured pressures dependents strongly the associated mechanical behaviour induced by external loading. As the movement of fluid through concrete is essentially controlled by the permeability of concrete, a phenomenological relationship for the permeability of studied material is then proposed to study numerically the evolution of pressures. Afterwards, the previous relationship is introduced in generalized hydraulic diffusion equation. The coupled hydromechanical tests (i.e. the triaxial compression tests accompanied with simultaneous pressure evolution phases) are simulated. A good agreement is obtained between experimental data and numerical simulations. Finally, the principal numerical results of a coupled hydromechanical test are analysed and discussed in order to give a good understanding the hydromechanical coupling process developed in CEMI.

Experimental study and numerical modelling of thermo-mechanical behaviour of Tournemire argillite

In this paper, we present an experimental investigation and a numerical modelling of thermo-mechanical behaviour of Tournemire argillite. In the first part, the results of triaxial compression tests under two different temperatures are presented in order to show the thermal effect on the mechanical behaviour. In the second part, taking into account the induced thermal effects on shear strength at critical state, a new thermo-elasto-plastic model is proposed for Tournemire argillite. An internal variable is introduced in this model for the purpose of describing thermal effect on elasto-plastic properties. In addition, the post-peak behaviour is considered as a plastic strain softening in the model.

Couplage comportement mécanique et perméabilité : Cas d'une pâte de ciment pétrolier dégradée chimiquement à 90 °C

ABSTRACT This experimental study is interested in the chemical degradation effect on the hydraulic behaviour of a cement paste, according to the mechanical loading under temperature. The tests carried out show that the permeability decreases with increase in confining pressure, in a more important way for chemically degraded material. Under creep with weak confining, the permeability of degraded material decreases, contrary to that of sound material, because of the preponderance of the compaction effect with regard to the induced microcracking. Under simultaneous creep and chemical degradation, the permeability increases due to the dissolved chemical products.

Lateral Decompression Behaviors of a Hard Claystone in Excavation-Damaged Zone of Galleries

We propose a new type of mechanical tests, named the lateral decompression tests, to evaluate the mechanical properties of the surrounding rocks in the shifts based on elastic stress solutions of circular openings. We carried out lateral decompression test with a developed autonomous device to evaluate the mechanical properties of the Callovo-Oxfordian (COx) claystone, the hosted rock and geological barriers of the repositories for radioactive waste disposal in France. In the tests, the samples were firstly subjected to a hydrostatic stress that corresponds to the representative in situ mean stress (p = 12 MPa). Then, the samples were loaded to a possible failure at the given constant mean stress (∆p = 0). The results show that the COx claystone failed in the lateral decompression tests at the in situ mean stress of 12 MPa. A shear band controls the failure pattern in lateral decompression tests, which is similar to that in conventional triaxial tests. However, the failure prior to rupture is more brittle than the failure in conventional triaxial tests. The failure of the COx claystone under the tested mean stress suggests that the excavated galleries need external supports to keep its stability after tunneling in the COx formation.