ABSTRACT:

Rooffalls are the most common problems encountered during various stages of longwall mine operation resulting in loss of production and machinery. A real time microseismic monitoring system was installed at Rajendra underground coal mine and acquired rock fracturing information from different roof layers. These data were analysed and obtained stress concentration zones, which ultimately resulted in rooffalls. Based on the microseismic event release rate advanced strata instability information was provided to mine management through daily reports. This information has helped in carrying out effective and optimized blasting operations to distress the high stress zone. The microseismic data also helped in mapping real time fracture process to get the extent of fractures in roof strata. The significant and reliable precursor found was microseismic event release rate (ERR) for rooffalls. Daily reports were submitted to mine management regarding stress concentration zones and rockfall information, which were confirmed from underground observations. The details of the microseismic technique and its application to longwall mine problems are discussed in this paper.

1. INTRODUCTION

India is the world?s third largest coal producing country in the world after China and USA. As the most abundant fossil fuel in India, it is heavily used in India?s energy sector, approximately 55% of energy needs are catered by Coal. The International Longwall census report says that early efforts at Longwall mining in India were poorly implemented but with experience, better equipment specification and improved infrastructure have increased the chances of success. Today in India, there are examples of moderately successful mechanized Longwall faces, which produce on an average 1500 Mt per day. Powered Support Longwall technology contributes nearly 50% of global production of coal from underground mines and is the most prevalent methodology in the leading coal producing countries. According to International longwall census done in 1997, on an average the production of coal from longwall mining continues to grow [1] "Mehta , 2003". Longwall technique was introduced in India during 1960?s and powered support longwall mining was introduced in 1970?s. In most of the longwall faces, the main problem was roof strata caving. It was mainly due to the presence of massive sandstone bed which in general is difficult to cave and cave-in dynamically and violently. In order to understand the behaviour of strata the microseismic technique has been experimented in one of the longwall face at Rajendra underground coalmine of South Eastern Coal Fields (SECL) in Madhya Pradesh [2] "Sivakumar et.al., 2002". Today it is widely accepted that seismic data, in the form of event-triggered seismograms in real time yield much useful information about state of the rockmass [3] "Mendecki, A. J,1999". If the dynamic ranges of the seismic system are good enough then the time, location, radiated energy and scalar seismic moment of a seismic event can be routinely estimated from several associated seismograms. Having recorded and processed a number of microseismic events within a given volume of interest over time, then one can quantify the changes in the strain and stress regimes and in the rheological properties of the rock mass deformation associated with the seismic radiation.

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