This study describes the importance on the evaluation of rock mass condition before mining and probability of failure during mining under slope surface in shallow depth. It is well known that several factors can affect the stability of underground openings such as the quality of rock mass, the in-situ stress, the depth below the surface and opening geometry. In addition to those parameters, if underground mining is conducted near the slope surface, the influence of slope surface should be taken into account during any mining activities. Consequently, stability analysis has been conducted for three different conditions including the evaluation on the strength of rock mass in the sloping surface, assessment on the stability of stope mining near the slope surface, and instability of stope in the nearground region in various mine condition. The preliminary results show the instability of rock mass near to slope surface is more severe than that of the rock mass far from the slope surface, therefore large instabilities of rock mass near the slope surface are experienced. In addition, the occurrence of failure zones of stope mining in the near-ground region under slope surface become more pronounced in weaker geological condition and higher stress ratio. All the investigations for these analyses are conducted by means of 3D finite difference software using FLAC3D.

1. Introduction

Stope mining is the most common mining method adopted in underground metal mines of Myanmar. However, the assessments on the stability of stope still remain quite limited in those mining industries. Currently, most of the underground metal mines are being mined-out or still mining at shallow ground part. Moreover, there are not so many recorded data regarding rock mass failures cases in underground mining due to cut and fill stoping methods in Myanmar. Hence, the study on the stability of underground mines in shallow part become one of the important issues to mitigate the unpredictable nature of rock failures. Two forms of instability are readily observed around underground openings: (1) structurally controlled gravity-driven processes (2) stress-induced failure or yielding (Martin et al. 2003). In many pieces of literatures, some instability indicators are usually defined in terms of failure zones, stress condition, displacement and extent of yield zones (Abdellah et al. 2018) (Karian 2016) (Purwanto et al. 2013). In this study, the evaluation on rock mass condition and mining under the sloping surface are described by the occurrence of natural and mining-induced differential stress and failure zones affected to mining activities. Considering the importance stability of underground mining under slope surface, some investigations on the characteristics of rock mass and mining condition are carried out at Modi Taung gold mine, one of the largest underground gold mines in Myanmar.

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