ABSTRACT

ABSTRACT

Evaluation of ground stability and support requirements during block caving operations at Molycorp's Questa Mine was conducted with the help of a simple rock mechanics program consisting of underground mapping and classification of ground conditions, and stability and support monitoring by extensometer measurements. This paper describes how rock quality evaluations based on the Q system, convergence measurements, and computer aided analysis have proven useful in helping establish the support for a wide variety of rock conditions.

The type of supports used consisted basically of three systems, which were generally installed according to the location with respect to the cave, as follows: (1) cast concrete and steel beneath the cave; (2) shotcrete and fully grouted bolts in the peripheral areas; and (3) shotcrete and split sets outside the cave abutment areas. Exceptions to this general rule occur in some locations.

Support performance assessment was based on Q evaluations and convergence measurements, as well as roof fall experience. Results indicated that areas of very poor to poor ground need to be supported with steel and concrete or shotcrete and six meter long, untensioned, fully grouted bolts, depending upon the location in relation to the cave. Shotcrete and split sets performed well in areas of fair to good quality rock outside the cave abutments. Roof falls were structurally controlled, wedge-type failures, usually associated with water. No falls have occurred where the shotcrete is 50 mm or more in thickness.

1 INTRODUCTION

One of the major problems in mining and tunneling is the accurate prediction of ground stability and support requirements. No satisfactory quantitative solutions are currently available, but approximate solutions exist based on practical experience. The assessment of ground stability and support requirement is an evolutionary process that should begin in the early planning stages, and continue with development and mining. A practical rock mechanics program should be integrated into the operations as soon as development begins to monitor stability, verify the design, and assess support performance. This paper presents experience gained in this process during block caving at Molycorp's Questa Mine.

Figure 1. Mine location and general layout. (available in full paper)

Figure 2. General geological section. (available in full paper)

Figure 3. Mining system. (available in full paper)

Molybdenum has been mined at Questa, in Taos County of northern New Mexico, for over 60 years. In 1965, large-scale, open pit mining operations were initiated by Molycorp, a subsidiary of Union Oil Company of California, but by the mid 1970's, high stripping ratios forced the company to begin making plans to abandon surface mining and go underground. A large, deep, high-grade deposit was delineated by exploratory drilling southwest of the open pit. A gravity block caving method was selected for mining because of the size and shape of the orebody and the fractured nature of the rock mass. Figure 1 shows the mine location and general layout of the underground haulage level, production decline, shafts, open pit and mill site.

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