MassMin Chile 2004, Santiago, Chile, August 2004
Authors: L. J. Gilbride and J. F. T. Agapito (AAI) and R. Kehrman (Molycorp, Inc.)
The Molycorp, Inc., Questa Mine, located in New Mexico, currently mines using a gravity-draw panel cave to extract molybdenum sulfide ore from the 600-m-deep D Orebody. Prior to initial development, geotechnical studies were undertaken to predict ground response for the design of entry support on the Grizzly and Haulage levels and in transfer raise connections. Heavy abutment pressures were anticipated ahead of the undercut, followed by significant stress relief as a consequence of a post-undercutting mining sequence. Detailed three-dimensional continuum modeling was conducted to predict changing stress states during the undercutting sequence and to evaluate the performance of various concrete and steel liner designs. Lithologic variation across the orebody was simulated and proved meaningful for identifying different stress transfer mechanisms and liner pressures in different types of squeezing ground. Recommendations for concrete liner thickness, concrete strength, reinforcement, and steel liner thickness were developed from modeling and, ultimately, were implemented during construction. Since the cave was initiated in October 2000, ground support has performed reliably, with only occasional compression cracking and minor tensile separation of the Grizzly Level liner in response to passing abutment loads. Observations to date corroborate model predictions and validate initial support design for the new deep orebody.