Managing Uncertainty in Geological Engineering Models for Open-Pit Feasibility

Open-pit mines rely on geology for economic feasibility based on mineral value and geotechnical parameters. Pit slope angles may control economics; therefore, open-pit mine design utilizes a reliability approach that specifies confidence interval, precision index, and practical strength values. The geological model for economic evaluation is developed before pit slope stability analyses are undertaken and is based on lithology and mineralization, both of which are essential for the geological engineering model. The preliminary mine plan includes a shell for the ultimate pit based on assumed pit slope configuration that includes haul-road benches. Geological engineering characterization utilizes the geological model to identify locations for geotechnical bore holes and detailed mapping of rock structure to document aspects critical for slope stability. Uncertainty is common; rock structure variability is represented from direct observation of selected outcrops and from detailed logging of rock core. Lithologies that form the ultimate pit walls are sampled for unconfined compression testing. Samples of a single lithology from different bore holes reflect formation variability. Typically, a few (3-15) samples of each lithology are tested. The desired precision index and reliability may not be met for a specified confidence interval with a limited program; these parameters cannot be determined until test-result variability is known. Geological uncertainty may be managed for open-pit mines by using practical strength values, drilling a few additional bore holes, and performing additional unconfined compression tests.