ABSTRACT:

This paper presents the results of an assessment of the hydrologic stability of the crown pillar that will remain after extraction of zinc and copper ores at the Crandon deposit in Forest County, Wisconsin. The crown pillar, defined as the rock remaining between the uppermost extent of mining and the base of surficial glacial deposits, is to be left in place to protecthe surface and the glacial aquifer system. Hydrologic stability refers to potential changes in vertical hydraulic conductivity resulting from mining. Changes in hydraulic conductivity could result from mechanisms which include collapse of the crown pillar, joint movement, or strain changes in the crown pillar. The issue of hydrologic stability of the crown pillar had been raised by the Department of Naturai Resources (DNR), State of Wisconsin, as a potential concern because of solute release to the groundwater regime after refiooding the backfilled mine. To address this issue, a series of twoand threedimensional numerical analyses were conducted to predicthe impact of mining to the crown pillar for the worst possible scenario. A three-dimensional simulation of the entire mine area was first conducted to evaluate the stress and strain changes in the crown pillar. A detailed analysis for selected locations within the crown pillar was then used to investigate the potential of joint shear and joint interaction. Maximum allowable strain, safety factors against joint slip, and change of hydraulic conductivity in the crown pillar were evaluated. A minimum crown pillar thickness of 100 ft was established based on the analysis results. The numerical analyses concluded that there will be no measurable impact to the hydraulic regime due to mining.

INTRODUCTION

This paper presents the results of an assessment of the hydrologic stability of the crown pillar that will remain after extraction of zinc and copper ores at the Crandon deposit in Forest County, Wisconsin. The zinc and copper ores at the Crandon deposit are oriented in the east-west direction, with ore extending approximately 4900 fi along the strike length of the orebody. The ore dips from 70 o to 85 o and is situated approximately 150 to 2300 ft below the ground surface. The crown pillar is the rock remaining between the uppermost extent of mining and the base of surficial glacial deposits, and is left to protect the aquifer system.

In this study, the hydrologic stability refers to the integrity of the crown pillar with regard to fracture conductivity that could be impacted by mining. Earlier mining plans for the Crandon orebody proposed leaving a crown pillar with thickness (vertical height) varying between 70 and 310 ft along the orebody.

The Crandon orebody has an average width of 100 ft. Mining extraction will produce increasing stress and deformation in the crown pillar as the excavation progresses upward and the thickness of the crown pillar is reduced. Deformation of the crown pillar will be induced by the mining of the ore and concentration of the horizontal stress. A strain criterion developed from coal mining experience, for the preservation of aquifers and aquitards, has been adopted as the limit criterion for this study. This quantitative criterion is that induced horizontal strain should be less than e0.005 inch/inch (Singh 1992) for no significant impacts to the aquifers or aquitards from mining.

This paper summarizes the analytical approach and results of the analyses used as part of the pre-mining and permitting stages of this project. The outcome of these studies included establishing the recommended minimum dimension of the crown pillar and assessing the impacts of mining on the hydrologic integrity of the crown pillar. Hydrologic mo

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