A 2D investigation of the fracture roughness created by a modified splitting method

A. Khosravi, R. Simon

Dans les comptes rendus d’articles de la conférence: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Engineering Geology and Rock Mechanics

ABSTRACT: In this study, the objective is the investigation of the loading effect on the roughness homogeneity of the fracture created by a modified splitting technique on Gabbro. First, each lateral profile is divided into three segments: upper, middle and lower. The results show that the JRC value of the middle segment is less than the other side segments. Secondly, the JRC values of both lateral and longitudinal profiles are calculated separately. The JRC values of the longitudinal profiles are close to each other and no scattering was observed between the profiles from one side to another. However, the observation on the lateral profiles shows a chaotic behavior in the JRC values. A microscopic view of the diametral cut of the split sample shows that the fracture passes through the matrix (clinopyroxene) and deviates from the main fracture trajectory when it encounters plagioclase crystals.

RÉSUMÉ: Dans cette étude, l'objectif est l'étude de l'effet du chargement sur l'homogénéité de la rugosité de la fracture créée par une technique modifiée de de rupture en tension de (Gabbro). Dans la première étape, chaque profil latéral est divisé en trois segments, soit supérieur, central et inférieur. Les résultats montrent que la valeur du JRC du segment central est moindre que les autres segments latéraux. Dans la deuxième étape, les valeurs de JRC des deux profils latéraux et longitudinaux sont calculées séparément. Les valeurs du JRC des profils longitudinaux sont proches lun de autre et on n'observe pas de dispersion entre les profils. Mais, l'observation sur les profils latéraux montre un comportement chaotique dans les valeurs du JRC. Une analyse microscopique de la coupe diamétrale de l'échantillon fractionné montre que la fracture passe à travers la matrice (clinopyroxene) et s'écarte de la trajectoire de la fracture principale lorsqu'il rencontre des cristaux de plagioclase. 1. INTRODUCTION In order to understand the mechanical behavior of a rock mass, it is essential to have a better understanding of the mechanical behaviour of fractures. One of the effective factors in the mechanical behavior of fractures is the roughness, which is defined as the fracture surface topography. There are several geological properties which influence the physical shape of a fracture roughness such as: the mineral composition, the type of cement and degree of cementation, grain size and the secondary minerals. From a morphological point of view, the three main effective aspects are the material type, the grain size and the matrix material (Kabeya, 1999). In this regard, the effect of loading modes as the cause and source of the roughness surface are not explicitly well known and most of the time they have been ignored. In nature, there are three ways of applying a force to initiate and propagate a crack on rock material. They are commonly called fracturing modes: Mode I or opening as a tensile fracture; mode II or in-plane shear and mode III or out-plane shear (Brady and Brown, 1985). In mode I, the crack is created in a pure tensile form; however, in the other two modes both tensile and shear cracks play a role. In modes II and III, in terms of applying the external forces, the ratio of shear to tensile cracks differs. Analyses of the fracturing modes while taking into consideration all relevant parameters (material and loading conditions) are very complex. As to crack initiation and propagation, several researchers have performed numerical and experimental studies. To follow the fracturing in different loading modes on rock material, tensile tests, uniaxial and triaxial compression tests were performed. Crack propagation in a sample with pre-created flaws was studied by employing the code Rock Failure Process Analysis (RFPA2D) (Tang et al, 1998). The flaw network was used as a representation of irregular inhomogeneity of rock materials. Their results showed that the system of 2D cracks coalescence in different loading modes (mixture of modes I and II). Bobet and Einstein (1998) experimentally studied crack coalescence in the pre-cracked samples under uniaxial and biaxial compression. In the case of non-overlapped flaws, it has been observed that the cracking mode varies with the confining stress applied on the samples. In both numerical and experimental studies, the fracturing in mode I has been neglected. They showed the influence of confining stress on the crack coalescence which implicitly indicated the roughness profile of the created fracture in 2D. As a result, increase of confining stress decreases the roughness of the created fracture surface.

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