The disturbance factor (D) included in the Hoek-Brown criterion represents a strength reduction parameter associated to the damage induced in large open pit slopes as a result of the blasting process and also as a consequence of the stress relaxation and stress rotations induced by the excavations during the mining process. The spatial distribution of the D factor, as well as its value, remain unresolved issues in the current rock mechanics practice applied to slope stability analysis. While it is reasonable to expect that in the long term, numerical modeling based on lattice formulations or strain softening constitutive models will be able to address this issue properly, the current state-of-the-practice (using both limit equilibrium and numerical modeling approaches) will remain applicable for some time. Current methods would therefore benefit from more robust guidelines to define the extent of the disturbed zone, as well as the distribution of strength degradation of the rock masses within a mined slope. In this study, an empirically based guideline is proposed for the distribution of disturbance with depth in a slope. Furthermore, a first step is made at providing a mechanical interpretation of the disturbance of large open pit slopes in medium to hard rocks due to loss of confinement and is aimed at defining the extent of disturbance using lattice modeling. As the final purpose of the study is identifying the real effect of the disturbance in the strength properties and its location with respect to the slope surface, some initial attempts are also made to measuring the strength of undisturbed and disturbed rock mass samples using a Synthetic Rock Mass approach.

1. Introduction

The Hoek-Brown failure criterion is widely used in the current practice of slope stability analyses, as it provides an empirically solid and generally accepted means to assess the strength of rock masses. Out of all the parameters involved in the application of this failure criterion, the value and distribution of the so-called disturbance factor (D) behind the slope is the only one that lacks of an agreed procedure for its definition. This situation makes the analysis subjective and based on the experience of the modeler, although the influence of this parameter can be large and its application requires experience and judgment [1].

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