Françoise Homand

Modelling of induced anisotropic damage in granites

This paper deals with numerical modelling of induced damage in three granites. A continuous anisotropic damage model is proposed in the framework of thermodynamics and fracture mechanics. A second rank tensor is used to describe damage state which is directly related to orientation and density of microcracks. Both time independent and time dependent (or sub-critical) growth of microcracks are taken into account. A simple procedure for the determination of model parameters from standard laboratory tests is proposed. Comparisons between model simulation and experimental data are presented for some basic loading paths. Finally the application of the model to stability analysis of the Mine-by test tunnel of the URL in Manitoba (Canada) is presented. A comparison between numerical predictions and in situ observations makes it possible to evaluate the performance of the proposed model.

Geometric analysis of damaged microcracking in granites

Abstract Practically identical samples are tested at the same confining pressure and temperature but at different deviatoric stress levels. Thin sections are observed using an optical microscope and recorded as images in order to study the crack network evolution. The compound cracks are decomposed into elementary cracks (right segments) with constant orientation and then reassembled in order to determine crack length and cumulated crack length. The results of crack observations are discussed in the light of the mechanisms of crack evolution at microscopic level compared to the stress–strain curves. It results from our observations that mean crack length increases only moderately in comparison with maximal crack length and the number of cracks. Zhao reports similar results (cf. Zhao, Y., 1998. Crack pattern evolution and a fractal damage constitutive model for rock. Int. J. Rock. Mech. Min. Sci. 35 (3), 349–366). The evolution of microcracking can be attributed more to new crack nucleation rather than to growth of the pre-existing cracks.

Taking the soil–structure interaction into account in assessing the loading of a structure in a mining subsidence area

Underground mining of raw materials is often the cause of ground movements at the surface. Whether planned or accidental, such movements can cause considerable damage to structures located within the area of influence of underground mining works. Examples are the recent subsidences that took place at the end of the 1990s in the Lorraine iron mining field. A better understanding is necessary of how ground surface movements can be imparted to the supported structure and damage it. Indeed, it is too often considered that damage depends only on ground strain and no account has been taken of soil-structure interaction phenomena, which may affect considerably the structural behaviour. The stiffness of a structure is quantified in comparison to that of the ground as regards the various movements of the ground surface. This investigation highlights situations in which ground movements are integrally imparted to a structure. When this is not the case, the resulting complex soil-structure interaction phenomena is analysed. For this purpose, a finite-element software is used to generate models incorporating the ground material and a supported rigid structure. The ground movements are broken down into two basic movements in order to highlight the impact and the relative importance of one of these movements: ground curvature and horizontal strain. Structural stresses are quantified for different mechanical properties of the ground and the structure, as well as for different amplitudes of ground movement. This investigation made it possible to devise a methodology for analyzing structures in mining subsidence areas in order to determine cases in which soil-structure interaction phenomena must be considered. The investigation made it possible to hierarchically organize the ground and structure parameters, the variability of which has a significant effect on the behaviour of the structure affected by mining subsidence.

The effect of joint constitutive laws on the modelling of an underground excavation and comparison with in situ measurements

The paper discusses the results of a numerical study which examines the influence of joint constitutive models on the response of a specific jointed rock mass mane up of an anisotropic rock (slate). It presents a comparison between predicted convergences and displacements of rock mass surrounding a pilot gallery, and those measured during field investigations. To perform this analysis, the two-dimensional Distinct Element Method code, UDEC, is used and three joint laws are compared. Based on field investigations for site characterisation, two models are constructed according to fracture density (MODEL A and MODEL B). For each of them, the influence of joint constitutive law on the stability of the gallery is examined, and comparisons between the investigations and predictions are made. Examination of the results shows that there is no noticeable change in stress magnitudes between laws. The displacement magnitudes depend on (1) the constitutive law, (2) the input model parameters and (3) the fracture density. A parametric study in the case of MODEL A indicates that a relatively good march between predicted and measured displacements around the gallery can be observed in certain areas.

A short- and long-term rheological model to understand the collapses of iron mines in Lorraine, France

Abstract Mining collapses currently occur in Lorraine (France) above abandoned room-and-pillar iron mines. The mechanical behaviour of iron ore and marls was investigated using experimental studies (triaxial, hydrostatic and creep tests). Following this, an elasto-plastic constitutive law, with two strain hardening rules, is proposed to model the transition from hardening to softening. A creep constitutive law, which describes the delayed volumetric dilatancy of iron ore, is also proposed. Both laws have been implemented in a two-dimensional finite difference code (FLAC2D). The object of this modelling is to understand and predict the phenomena involved in the mining collapses. The results of numerical simulation show that the ageing of iron ore has to be considered when attempting to explain mining collapses.

Caractéristiques physiques et mécaniques du granite de la Vienne et de l'argilite de l'Est

ABSTRACT The physical and mechanical characteristics of a granite and a shale are presented. Some mechanical tests are carried out to characterize mechanical behaviour of these rocks: triaxial, proportionnal and lateral extensions tests. The evolution of microcracks network was also quantified by image analysis.

Comparaison des concepts et prédictions des modèles

ABSTRACT Different models presented in this volume are compared. This comparision concerns their concepts and procedures of parameters determination, as well as their predictions.

Modélisation microstructurale de l'endommagement du granite de la Vienne

ABSTRACT The microstructural approach in damage modeling, which is presented in this paper describes the evolution of micro-crack geometry as a function of history loading. If the crack geometry is known, the effectif properties could then be calculated foe any cracked roch by the mean of a micromechanical model. The PL evolution law which is necessary in the describing of crack geometry evolution is hardly based on the crack microscope observation as well as on the theory of fabric tensors. This approach is applied in the modeling of mechanical behaviour of Vienne granite. The result of model simulations are compared with laboratory tests.

Influence du vieillissement et de l'eau sur le comportement mécanique des mines de fer abandonnées de Lorraine

ABSTRACT In order to better explain mining collapses that occur in Lorraine, a hydro-mechanical constitutive law is proposed for the iron ore after performing laboratory tests. This law takes account of the active physical phenomena during a short-term loading and its simulation is compared with experimental results. The ageing and mine flooding influence on iron ore mechanical behaviour is discussed. The iron ore ageing is of anthropogenic origin and its characteristics are shown.

Modélisation de l'endommagement anisotrope du granite de la Vienne

ABSTRACT A modified elasto-damaged model counting for laboratory observed mechanisms is presented. Even if the general framework of the model is clearly phenomenological one, laws of state variable evolution are inspired from micromechanical mechanisms of crack propagation and they are constructed upon fracture mechanics bases. Basic hypothesis and the determination procedure of model parameters. The model is compared to the test laboratory results.