ABSTRACT

ABSTRACT: High-grade ore found in a shaft pillar at the Homestake Mine prompted a request for a Bureau of Mines study to determine if selected areas of the shaft pillar could be mined without jeopardizing the shaft. The design approach was to perform a finite-element study of the shaft pillar. Plan view and vertical sections were developed using two-dimensional, elastic, plane strain assumptions. Input into the model consisted of in situ stress measurements, geologic mapping, rock mass properties, and previous mining history. Results from the study indicated that ore-bearing sections of the shaft pillar could be removed within the displacement tolerances acceptable for the shaft. Homestake engineers, using the results of the finite-element study and considering other factors, have designed and are beginning to mine the shaft pillar.

1 INTRODUCTION

Located in the northern Black Hills at Lead, South Dakota, the Homestake gold mine has been in operation since 1876. Current annual production is 1.8 million tons and development extends to more than 8,000 feet below the surface. Vertical crater retreat (VCR) mining accounts for approximately 60% of total underground production (Muir, 1988), with the remainder coming from mechanized cut-and-fill methods (Smith, 1987).

Concerned with stope and pillar sizing for VCR mining in a large ore block at increasing depth, Homestake entered into a cooperative rock mechanics research program with the Spokane Research Center of the Bureau of Mines and the Department of Mining Engineering of the University of Utah. The extensive field investigation became part of the Bureau's research program for developing advanced concepts for mining deep ore bodies. The success of this study, which was conducted on a panel between the 6950 and 7100 levels, led to a second cooperative agreement between Homestake and the Bureau (see Pariseau, 1985).

The Ross Shaft is one of two main shafts servicing the Homestake Mine. Approximately 50% of all ore is transported through the shaft, which also contains the main pump column and half of all the other service lines for the mine. During sinking operations in the early 1930's, a large ore body was intersected between the 3200 and 3800 levels. Subsequent development proved the existence of a large ore zone, and a shaft pillar was defined. The pillar, which extends for 61.9 m along the strike of the ore zone and is bounded by geologic contacts between the ore body and the Poorman and Ellison Formations, contains 8.8% of the mine's proven gold reserve. Production goals dictated mining the pillar while maintaining the integrity of the Ross Shaft (Corso, 1988).

The goals of the cooperative program were to: (1) provide an instrumentation plan to monitor shaft and pillar loads and deformations, (2) provide input for calibrating computer models, (3) develop a reliable computer model to be used for evaluating shaft stability, and (4) provide guidelines for stope sizes and sequencing.

2 APPROACH

Although a precise measurement of the amount of shaft deformation that can be tolerated has not been determined, it appears reasonable to suppose that as long as deformation is within the elastic range, displacements will be tolerable.

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