Modelling Fractured Rock Heterogeneity in an Open Pit: Groundwater Model Calibration using Pilot Points

Claudia Martínez
Monday, May 16, 2016
First presented: 
Water in Mining, May 18-20, 2016 Santiago de Chile, Chile
Published paper
Water Management

Water content in rock units present in open pit mines has significant effects on pore pressure and in slope stability. For this reason, pore pressure distribution throughout the pit represents one of the most important factors in slope stability analysis, obtainable through groundwater numerical modelling. Generally, the hydrogeological units are defined using discrete geological-geotechnical domains related to its hydraulics parameters. However, once implemented in the numerical model, this approach may lead to an oversimplified zonation of piecewise parameter uniformity. As a result, the model has a limited capacity to express hydraulic property complexity, leading to diminished potential for using site data and evaluating predictive uncertainties. This paper describes the application of “pilot points” methodology as a means of spatial hydraulic property characterization in an open pit mine. It uses MODFLOW-USG with Quadtree Refinement and PEST. A multilayer model was divided into zones based on a geological model, and pilot points were used to evaluate intrazonal hydraulic property variations. The calibrated transmissivity fields show substantial heterogeneity with regions of high hydraulic conductivity, which correlates well with the location of main faults. Null Space Monte-Carlo uncertainty analysis shows important predictive error for groundwater inflows to the pit as well as for pore pressure distributions. These results are valuable for identifying data needs and optimizing future hydrogeological works.

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