Molycorp, Inc.?s Questa Mine plans to switch from gravity draw in Block 1 to a load-haul-dump (LHD) draw system in the East and West blocks of the D Orebody. Changes in mining method require modifying the support design. This paper summarizes the numerical modeling effort and findings in evaluating abutment stress conditions and ground support requirements for LHD mining. A three-dimensional, elastic-plastic, FLAC3D model was constructed comprising a detailed mesh representing the excavation geometry of the LHD drawlines encapsulated within a coarse, global mesh representing the entire D Orebody. The mining sequence was simulated to output stress changes at the LHD Level during advance of the caving front. The drifts at the LHD Level were mined before or after several key caving steps to represent pre-undercut and post-undercut conditions. A fictitious support pressure was then applied to the walls of the drifts and incrementally relaxed to measure convergence as a function of support pressure. LHD Level support requirements were determined from these ground-support interaction analyses. Conclusions were that a thick shotcrete liner would be required for the drifts in weak andesite, while light shotcrete and rockbolts would be sufficient in areas of strong aplite (granite porphyry).


Molybdenum has been mined at Molycorp, Inc.?s Questa Mine, New Mexico for over 80 years. A gravity block-caving method was planned by the end of 1976 because of the well-fractured nature of the rock mass and the size and shape of the deposit [1]. Underground mine development began in 1979, followed by initial production in 1983 from the Goathill Orebody. In its block caving operations, both a manual gravity draw system and a highly-mechanized load-haul-dump (LHD) draw system were selected to remove broken ore from the production level.

By the end of 2000, production began in the D Orebody the east of the original Goathill Orebody [2]. The traditional gravity draw system was adopted to mine Block 1 of the D Orebody (Figure 1). Presently, Molycorp plans to convert to an LHD draw system for the East and West blocks. The East Block mining sequence was simulated using FLAC3D to estimate stress levels imposed on the LHD lines. Computed convergence within the LHD lines was used to develop ground-support interaction curves for representative locations within the East Block at different stages of mining. The curves, which describe the relationship between entry convergence and passive pressures imparted on the surrounding rock mass by the support system, were ultimately applied to estimate support requirements in the LHD lines.

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