ABSTRACT:

In general, roughness profiles of rock joints consist of non-stationary and stationary components. A new strength criterion which includes one parameter to capture the non-stationary roughness and two parameters to capture the stationary roughness is suggested for modelling the anisotropic peak shear strength of rock joints at low normal effective stresses. In practice, to allow for modelling uncertainties, the new equation should be used along with a factor of safety of about 1.5.

INTRODUCTION

Strength and deformation behavior of joints and flow properties of joints depend very much on the surface roughness of joints. These effects arise from the fact that the surfaces composing a joint are rough and mismatched at some scale. The shape, size, number, and strength of contacts between the surfaces control the mechanical properties. The separation between the surfaces or the 'aperture' determines the hydraulic properties. A few methods have been suggested to characterize surface roughness of natural rock joints. The work done so far has been limited to characterization of surface roughness along linear profiles or in one dimension (1D). These investigations have led to controversial findings (McWilliams et al., 1993; Huang et al., 1992). In addition, since joint planes are two-dimensional, quantification of surface roughness on two dimensional planes is required. Usually, roughness on natural rock joint planes is anisotropic. However, anisotropic roughness quantification is not addressed in the literature. These clearly show our limited understanding on quantification of roughness of natural rock joints. A part of this paper presents findings of a research program initiated to characterize anisotropic roughness of natural rock joints.

The shear strength of rock joints should be studied under two major categories they are the filled joints and the unfilled joints. In this paper, the investigation is restricted to the shear strength behavior of unfilled joints. The shear behavior of unfilled joints depends on the following factors: (a) rock type (b) level of normal effective stress on the plane of sliding, (c) degree of roughness, (d) size of joint (scale effect), (e) degree of weathering, (f) presence of moisture, and (g) water pressure. Only the effect of factors (b) and (c) on peak shear strength is focused in this paper. The effect of roughness on the shear strength is more pronounced in the low normal effective stress range (0-0.4 times unconfined compressive strength). Therefore, in this paper, investigations are limited to the low normal effective stress range.

The main contributors for the development of peak shear strength criteria in the low normal effective stress range are Patton (1966), Ladanyi and Archambauit (1969) and Barton (1973). Movements along rock discontinuities in foundations, dams, tunnels and slopes can occur in various directions, depending on the external forces (such as external loads, water pressures, earthquake forces, etc.) acting on the structure and the kinematic constraints. Therefore, it is important to know the strength of rock discontinuities in different directions. The joint peak shear strength shows anisotropic properties due to roughness variation with the shearing direction in direct shear tests (Huang and Doong, 1990; Jing et al., 1992). Even along one particular direction, the shear strength of a natural joint can be different between the forward and the backward directions. To capture the above observations, either the existing shear strength criteria should be improved or a new peak shear strength criterion should be developed.

This paper presents findings of research initiated to develop an anisotropic peak shear streng

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