Evaluation of roughness parameters of rock fractures created by different loading modes

A. Khosravi, R. Simon, N. Falaknaz

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Engineering Geology and Rock Mechanics

ABSTRACT: In this paper, the effect of loading conditions (tension and compression) on the joint roughness is investigated. Three types of fractures are studied based on a morphological point of view. The first fracture was created by a modified splitting method in order to create a tensile joint, while the second and third fractures were created from uniaxial and triaxial compression tests. In the modified splitting method, a standard sized cylindrical specimen is grooved longitudinally on the top and bottom and is split using a round bar on the upper groove. The roughnesses of fracture surfaces were quantified. The results show that the roughnesses of the rock joint created by a modified splitting method are higher than those of a joint created by uniaxial and triaxial compression tests. Tensile fractures would have greater shear strength than the fractures created by compression.

RÉSUMÉ: Dans cet article, l'effet des conditions de chargement (traction et compression) sur la rugosité du joint est étudié. Trois types de fractures sont étudiés selon un point de vue morphologique. La première fracture a été créée par une méthode de fractionnement modifiée (splitting), tandis que les deuxièmes et troisièmes fractures ont été créées à partir dessais de compression uniaxiale et triaxiale. Dans la méthode fractionnement modifiée, un spécimen cylindrique de taille standard est rainuré longitudinalement sur le haut et le bas et il est divisé en utilisant une barre ronde sur la rainure du haut. Les rugosités des surfaces de rupture ont été quantifiées. Les résultats montrent que les paramètres de rugosité du joint créé par une méthode de fractionnement modifiée sont plus élevés que ceux d'un joint créé à partir d'essais de compression triaxiale ou uniaxiale. Les fractures de tension auraient une plus grande résistance au cisaillement que les fractures créées par compression. 1. INTRODUCTION Fractures, as a principal form of discontinuities existing in the rock mass can control the rock mass mechanical and thermo-hydrological behavior. The joint shear strength depends on two main components, the matrix strength and geometrical parameters of the surface. Hence, in order to estimate the shear strength of a joint, the surface roughness and spatial situation of two corresponding joint walls should be taken into consideration. Since joint surface roughness can influence both the mechanical and hydrological behaviors of a rock mass, several studies have been undertaken to characterize the joint surface roughness and correlate the joint mechanical behaviors. 2D and 3D profilommetry methods are used to characterize the fracture topography. Most of the roughness characterization methods so far have been proposed based on 2D linear profiles (Barton and Choubey, 1977; ISRM, 1978; Tse and Cruden, 1979; Maerz et al., 1990; Poon et al, 1992; Kim and Lee., 2009; Jang et al., 2014). These methods are usually used in laboratory for a joint surface < 1 m2. Some of these 2D roughness quantification methods are still used even after the emerging of modern 3D topography scanning methods with proper resolutions. New 3D surface roughness measuring tools and methods are also used to obtain a complete and comprehensive roughness surface characterization of the joint. An accurate quantification of roughness can improve prediction of the mechanical characterizations of joints. Having 3D surface information allowed researchers to provide new generation of the 3D roughness quantification parameters (e.g. Belem et al., 2000; Tatone and Grasselli, 2009). However, lack of availability and cost of new optical and laser technologies forces the use of primary profiling methods (2D) as easy and inexpensive methods to provide the roughness profiles. Having the rock joint roughness profiles and their Joint Roughness Coefficient (JRC)s could help the immediate and in-situ rock mass characterization and rock mechanical design. There is a paucity of information about the roughness of the fractures produced in different modes (Brady and Brown, 1985). The roughness of fractures created using uniaxial compression, tension and triaxial stress state has been evaluated by Seredin et al. (2013). These fractures were created on rocklike materials. The highest roughness parameter was obtained with uniaxial tensile test. They showed that the roughness decreases when the confining stress applied in triaxial compression tests increases (i.e. 3 =2-30 MPa). This investigation, although capable of characterizing different modes of fractures, has been conducted on rocklike materials which may not reflect the real response of rock fractures. The effect of confinement has also been investigated by Amitrano and Schmittbuhl (2002) on Granite samples for 3=20 to 80 MPa. The results of this study indicated that the roughness parameters decreased when the confining stress is between 20 MPa to 60 MPa, while the roughness parameter increased when 3 = 60 - 80 MPa. This study

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