Managing uncertainty in underground excavation design associated with rock mass characterization using core drilled for mineral exploration

Design of underground mining operations, in their preliminary stages, often requires obtaining geomechanical data from re-logged rock core initially drilled for mineral exploration. The degree of alteration of this core due to handling and storage time increases uncertainty in the geotechnical parameters assessed and in the estimated required excavation support. Uncertainty is primarily related to changes in observed joint infill characteristics during storage. This paper presents a simple approach to assess this uncertainty quantitatively, illustrated with a case study in the Peruvian Cordillera. The Q-System was adopted for such assessment and to estimate the required excavation support. From results of the case study presented in this paper, uncertainty associated with the use of stored core was not considered significant when compared to the natural variability of the rock mass. Numerical simulations of excavation geometries for different rock mass qualities were performed to validate estimated support requirements through the Q-System. These simulations required estimations of strength characteristics of the rock mass and discontinuities. These were estimated from measurements in all core available (old and fresh) and considered the uncertainty in the adopted strength envelopes quantitatively. The range of model results and sensitivities to effects of modeling validated the recommended support requirements. During excavation, support requirements would be based on an assessment of Q as the excavation proceeds (an observational approach).