ABSTRACT:

Initiation of damage is defined as the onset of irreversible volumetric strains, and is determined by laboratory experiments. Axisymmetric triaxial tests following compression and extension stress paths have been performed, with an overall volumetric strain measurement. The experimental results are used to set up a non standard elastoplastic model dealing with instantaneous mechanical behaviour of rocksalt. Numerical implementation of the proposed model allows to demonstrate damage evolution for 2D simplified underground structures.

RÉSUMÉ:

L'initiation de l'endommagement est determinee à partir d'essais de laboratoire par l'apparition de deformations volumiques irreversibles. Pour ce faire, des essais de compression et d'extension triaxiale, avec mesure du volume global ont ete realises. Les resultats experimentaux permettent d'etayer un modèle rheologique du comportement instantane du sel gemme suivant Ie formalisme de l'elastoplasticite non-standard. L'lmplantation numerique du modèle permet d'evaluer I'evolution de l'endommagement du massif autour de cavites souterraines à geometrie simple. ZUSAMMENFASSUNG: Der Amfang der Beschadigung, als Erscheinung irreversibler Deformation, wurde anhand Laborversuche bestimmt. Kompressions - und Triaxial Extensions Versuche mit Messung des Totalvolumens wurden.unternommens. Die Versuche lassen ein rheologisches Model des augenblicklichen Verhaltens des Steinsaltzes, gemaß des Formalismus der nonstandard Elastoplastizitat, einfuehren. Numerische Berechnungen haben die Entwicklung der Beschadigung der Felsmasse in der Ungebung unterirdirscher Hohlraume erlaubtt.

1. INTRODUCTION

Underground storage cavities of hydrocarbons or wastes are performed in rocksalt for many years. The elastoviscoplastic behaviour and the natural low permeability (about 10-2°m2), of rocksalt are favourable regarding the storage safety. Nevertheless, the safety may decrease as a consequence of damage in the rocksalt mass, increasing permeability of salt. Rock salt damage has been studied during recent years, in laboratory [Hunsche, 1984, 1993, Hunsche et al., 1990], and from a theoretical point of view [Aubertin et al., 1991, 1993, 1995; Cristescu, 1989, 1993; Pouya, 1991; Thorel et Ghoreychi, 1993; Thorel, 1994]. Damage is defined as the coalescence of microvoids and microcavities of the material submitted to a mechanical loading [Lemaitre and Chaboche, 1978]. The development of microvoids in the polycrystalline rocksalt, is mainly localised at the grain boundaries, because of incompatible deformation between the lattice of sodium chloride monocristals [Stokes, 1966]. Moreover, the single crystal of sodium chloride displays a very small volumetric strain (about 10.7) [Davidge and Pratt, 1964]. The higher irreversible volumetric strains of the polycrystalline salt lead to damage. Damage onset of rocksalt is identified by performing laboratory experiments. A numerical model is backed up on those results, including damage and plasticity of rock salt. Application to underground structures is finally presented.

2. LABORATORY EXPERIMENTS
2.1. Material under study

Rocksalt samples were collected in a French Potash salt mine (MDPA) located in the Mulhouse basin, where in situ experiments have been performed [Ghoreychi, 1991, 1992; Cosenza and Ghoreychi, 1992] at a depth of about 520 m. Geological layer called "S1" in the local classification (85–95% of halite, with carbonate, anhydrite impurities), belongs to Stampien (30 millions years old). Samples have been cored in the purest part of this layer [Blanc- Valleron 1990; Ghoreychi, 1991; Streckdenfinger et al. 1993]. Acoustic measurements show isotropy of the samples used for the tests [Thorel and Ghoreychi, 1993].

2.2. Experimental set up and procedure

Axisymmetric triaxial tests are performed under different confining pressures p (0 to 60 MPa) and constant strain rate (about 10–5 s-1). Both compression and extension tests (figure 1) are performed with an overall or a local measurement of volumetric strain. After a hydrostatic path, compressive deviatoric test is performed by increasing the axial stress, while it decreases in extension tests. However, applied stress remains always compressive (with negative sign).

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