Abstract.

The West Wall of the Chuquicamata open pit has been historically referenced as an example of block toppling behavior, particularly in the upper part of the slope, located specifically in the material known as Fortuna Granodiorite and containing structural systems of significant persistence which dip into the slope at a somewhat steep angle. This rock mass is, in turn, supported on a strip of weak material associated to a shear zone located to the west of the main regional feature that defines the orebody, named the West Fault. Through the years, this failure mechanism has led to metric displacements which have accumulated and which continue to play a relevant role in the geotechnical design of the open pit. The mechanism, however, has been properly managed by the Geotechnical Direction staff at the mine, allowing the development successive pushbacks. Of relevance to the study presented in this article is the observation that whenever the mining plan of the pit involves excavations at the toe of the west wall slope (particularly in the referred shear zone), a correlation is observed with the activation of the slope, in terms of increased deformation rates for the wall. The observed behavior suggests that the interpretation of the west wall stability by means of the traditional factor of safety analysis is likely inadequate, as the slopes would be considered marginally stable and mining could have not been developed over the centennial life that the mine has had so far. Instead, a time dependent deformational approach seems to be a more appropriate option for dealing with slopes showing toppling behavior. A viscoelastic constitutive model is normally used to assess phenomena such as creep. This study summarizes the results obtained from a UDEC model developed for calibrating the observed behavior of prisms installed in the West Wall with readings ranging between years 1995 through 2013.

1. Introduction

The time-dependent behavior of the Chuquicamata west wall slope was already studied by Lorig and Calderón [1]. At that time, the shear zone was described with a viscoelastic constitutive model (creep), which was calibrated considering prism readings from 1995 through 2002. Two dimensional analyses were carried over for a selected sector of the pit using the distinct element code UDEC [2]; and applying a built-in creep constitutive model for the shear zone. The analysis presented here represents a continuation of that study; by including updated monitoring information and also geomechanical data for the slope.

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