ABSTRACT

The understanding of the patterns, characteristics, and genesis of fractures in a rock mass is essential in coping with the solution of a number of problems in rock mechanics and rock engineering. This paper demonstrates that in multilayered systems, the development of fractures and their spacing vary according to the extension strain localization. The extension strain criterion of Stacey (1981) is indirectly validated and the concept of "strain-driven" fractures is introduced. FEM/DEM modeling has been used as available with the ELFEN code, which allows the transition of a rock mass from continuum to discontinuum to be simulated effectively assuming that, if the fracture criterion within the intact rock (represented by FEM) is met, a crack (represented by DEM) is initiated. Re-meshing allows the fracture process through the FEM mesh to be tracked and visualized, thus contact properties can be assigned to pre-existing fractures and newly generated fractures.

1 INTRODUCTION

In layered rock masses, such as in certain sedimentary rocks, fractures may develop preferentially within, and be relatively confined to, the less ductile layer. Gross (1993) defines a lithology-controlled mechanical layer as "a unit of rock that behaves homogeneously in response to an applied stress and whose boundaries are locatedwhere changes in lithology mark contrasts in mechanical properties". Field observations of layered rocks showthat opening-mode fractures often are confined by layer boundaries (Fig. 1). This remarkable behavior can influence the mode of failure and instability around underground excavations in layered rock masses, which is related to the magnitude of the in situ stress relative to the rock mass strength and to rock mass quality, a parameter which depends strongly on the degree of jointing and persistence. In layered rock masses fractures may be strain driven. The spacing of opening-mode fractures in layered media is shown to be proportional to the thickness of the fractured layer and is controlled by the geometry of the problem Figure 1. Example of opening-mode fractures in the layers of the Flysh Formation in Algeria (photo courtesy of Geodata, Torino). being considered. Experimental studies of this phenomenon (Garrett&Bailey, 1977; Narr&Suppe, 1991; Gross, 1993; Wu&Pollard, 1995) show that the spacing initially decreases as extension strain increases in the direction perpendicular to the fractures. At a certain ratio of spacing to layer thickness, no new fractures form and the additional strain is accommodated by further opening of the existing fractures. The spacing then simply scales with layer thickness, a phenomenon which is called "fracture saturation" (Wu&Pollard, 1995; Avenston et al. 1971). This is in marked contrast with existing theories of fracture, such as the stress-transfer theory (Cox, 1952; Hobbs, 1967), which predict that spacing should decrease with increasing strain at infinitum. According with Bai & Pollard (2000) it is possible to say that with increasing applied stress, the normal stress acting between such fractures undergoes a transition from tensile to compressive, suggesting a cause for fracture saturation.

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