In Japan, a bench cut method is mainly applied for limestone quarries. As most of the mines are located in the mountain regions and steep slope mountains, the shaft and the underground opening for installation of facilities such as crushing equipment are applied in order to provide workability and economic efficiency for transportation and crushing ores. However, as the mining operation proceeds, the level of the pit will be down and the distance between the mining area which is the bottom of the pit and the opening for underground facilities such as crushing equipment become to be short. In order to maintain the safe mining operation and the stability of the underground opening, the appropriate distance between them has to be maintained. Therefore, in order to develop the guidelines for safety distance between the mining area and the underground opening for installation of facilities such as crushing equipment under different rock mass conditions in a limestone quarry, a series of numerical simulations are conducted by means of finite differential code “FLAC3D”.

1. Introduction

Currently, more than 200 limestone mines are in operation in Japan. A bench cut method is mainly applied for limestone quarries of the open-pit mine. In general, the limestone ore excavated from a limestone quarry at a high elevation is introduced into the shaft to achieve efficient transportation by utilizing the gravity. After that, the ore is crushed by the underground crusher and transported by the belt conveyor to the facility of processing at the foot. As most of the mines are located in the mountain regions with a steep slope, the shaft and the underground opening for installation of facilities such as crushing equipment are applied in order to provide workability and economic efficiency for the transportation and crushing ores. However, the stability of the shaft and the underground opening is suspected when the rock mass conditions are poor due to the existence of a fractured zone. In addition to that, as the mining operation proceeds, the distance between the surface and the underground opening becomes to be short with the down of the level of the pit. In order to maintain the stability of the underground opening, the appropriate distance between them has to be maintained. However, there are few studies about the influence of the rock mass conditions especially fractured zone and the mining operation on the stability of the underground opening. Therefore, in order to discuss the safety distance between the surface and the underground opening for installation of facilities such as crushing equipment under different rock mass conditions in a limestone quarry, a series of numerical simulations are conducted by means of finite differential code “FLAC3D”.

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