The west wall of Chuquicamata Mine experiences large on-going displacements of the order of two to four meters per year. Slope behavior is affected by the presence of a pervasive fault and an adjacent zone of sheared rock near the toe of the slope. Deformation of these materials is expressed in toppling behavior farther upslope. Previous analyses attempted to estimate safety factors for the slope using UDEC. Difficulties in identifying both a clear point of failure and a failure mode suggested that other criteria should be considered in assessing the acceptability of west wall slope designs. Slope displacement and displacement rates were considered as other criteria. However, these criteria require use of material models that can represent timedependent behavior. Such models have not been previously available in programs used to study discontinuum slope behavior. The modeling described in the paper demonstrates that the timedependent behavior of the west wall can be reasonably simulated when the sheared zone is represented by a two-component power-law creep model combined with a Mohr-Coulomb elasto-plastic model. Measured slope movements and changes in displacement rates are compared to model predictions over a period of six years.
Chuquicamata Mine is one of the largest open-pit mines in the world. Currently, the mine is 4.5 km in length (north-south), 2.7 km wide (east-west) and nearly 800 meters deep. The current Production rate is about 500,000 tpd, with about one-third of that (170,000 tpd) being ore. Expansions (or pushbacks) that start at the pit crest take about seven years to reach the pit bottom. Therefore, it is imperative that slope behavior is well understood and slope angles be carefully selected. The west wall has overall slope angles close to 30 degrees and experiences large on-going displacements on the order of two to four meters per year. Slope behavior is affected by the presence of the west fault and an adjacent zone of sheared rock near the toe of the slope. Deformation of these materials is expressed in toppling behavior farther upslope. Analyses to date have attempted to estimate safety factors for the west wall using UDEC(Universal Distinct Element Code) [Itasca 2000]. Difficulties in identifying both a clear point of failure and a failure mode have suggested that safety factors are probably not appropriate means to assess the acceptability of west wall slope designs. A more appropriate means may be slope displacement or displacement rates. However, previous analyses have largely ignored the time-dependent behavior of the slope. Consequently, there is some uncertainty as to how the west wall slopes will perform in the future and whether the future behavior is acceptable. This paper presents an initial attempt to calibrate and model the time-dependent behavior of profiles P3 and P5 of the west wall (see Fig. 1) using UDEC. The time period considered in this paper is January 1996 to February 2002. Both profiles have good historical information about movement through prism records, as described in the next section.
