Abstract

Kinematic analyses of wedge failures in rock slopes are usually carried out based on stereographic techniques. Nevertheless, this methodology presents several limitations that could lead to poorly accurate conclusions. In line with this idea, the main objective of this work is to evaluate the effect of joint spacing in the estimation of the factor of safety of rock slopes affected by potential wedge failures. For this purpose, a 3D numerical distinct-element code (3DEC) was selected to carry out a good number of simulations in which the factor of safety a slope, affected by different discontinuity sets, was studied by using the Shear Strength Reduction (SSR) method. Different values of joint spacing, cohesion and friction angles were considered, combined with two angles of the slope face under study. The joint spacing has been found to relevantly affect the values of the factor of safety, which showed variations of up to 40% in comparison with those obtained from limit-equilibrium methods for rock slopes with similar structural features. This work provides an insight into a more realistic interpretation of rock slope analyses against wedge failures, and particularly to more accurate estimations of the factor of safety.

Introduction

Excavations in rock masses are common operations in the fields of mining and civil engineering, whether for the development of foundations, tunnels or slopes. All these activities involve an interaction with the natural environment, which presents characteristics that are conditioned by its origin and geological history, which are modified at the time of excavation. A well-known feature of any rock mass is that its mechanical behavior is greatly determined by the presence of discontinuities. According to [1], the discontinuity spacing can be considered as one of the most relevant indicators when determining the so-called quality’ of a rock mass. It seems therefore reasonable that the magnitude of this parameter may influence the general stability of slopes. This feature is illustrated by the photographs shown in Figure 1, in which two rock masses are affected by several discontinuity sets causing wedge failures, but with very different spacings.

A preliminary stability analysis of the slopes shown in Figure 1 would often require plotting the main joint sets by means of a stereographic projection, in such a way that potential instability mechanisms can be identified. This can be carried out by resorting to commercial software like Dips [2] even though other freeware options like Stereographic Projection [3], InnStereo [4] or Stereonet [5] are currently available. After that, the Limit Equilibrium Method (LEM) is often used to estimate the factor of safety of the slope.

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