ABSTRACT

The stability of underground caves in an abandoned gold mine was carried out through a series of geotechnical investigations. After the closure of the mine, both the natural caves and the man-made underground openings were converted to a tourist route since 1990's. Detailed field investigations including Rock Mass Rating (RMR), Geological Strength Index (GSI) and Q classification were performed to evaluate the stability of the caves and to design reasonable support systems of the unstable openings. Rock fall analysis has been carried out on the rock masses of the largest cave where discontinuities were investigated by the multistage convergent photographing method. Three dimensional numerical modeling has been conducted using MAP3D software package to examine the stability of the mine openings. Based on the results of rock mass classification and numerical analysis, suitable support design was recommended for mine openings.

INTRODUCTION

Hazardous abandoned mine problems include catastrophic land subsidence or sinkhole due to collapse of subsurface cavities, environmental impacts associated with harmful minewater drainage and muckpile of tailings. In the mean time, the abandoned underground mines used to be utilized as food storage, waste repository, mining museum or tourist attraction.

This study has been carried out to evaluate the stability of underground caves in an abandoned gold mine. Mine caves had been excavated until the entire mine was closed in 1940's. After the closure of the mine, both the natural caves and the man-made underground openings were converted to a tourist route since 1990's.

Rock mass classification and numerical analysis were carried out to evaluate the stability of caves. Detailed field investigations including Rock Mass Rating (RMR), Geological Strength Index (GSI) and Q classification were performed to evaluate the stability of the caves on the one hand and to design reasonable support systems of the unstable openings on the other.

Rock fall analysis has been carried out on the rock masses of the largest cave where discontinuities were investigated by the multistage convergent photographing method. Three dimensional numerical modeling has been conducted using MAP3D software package to examine the stability of the mine openings.

Based on the results of rock mass classification and numerical analysis, suitable support design was recommended for mine openings. In addition, special structural supports were design to protect the visitors from unexpected collapse of the large natural caves and the mine openings.

GEOLOGY AND UNDERGROUND STRUCTURES

Figure I shows the regional geological map of the studied area. The geology of this area consists of Joseon supergroup belonging to Paleozoic Era intruded by granitic rocks and dykes of Jurassic or Cretaceous Period. The predominant surrounding rock of the main roadway is Poongchon limestone intruded by acidic or basic dykes (Fig. 2). The formation of gold is supposed to be created due to intrusion of these igneous rocks.

Mining activities were carried out through the gold vein which develops along the fault zone inside the limestone bedrock. Rock raises and in-vein drifts were used as the main mining path way.

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