Choudhury , P.B. and Raina, A.K. (2004) Crown pillar stability assessment in an underground copper mine using acoustic emission. International Journal of Rock Mechanics and Mining Sciences , 41 (3). pp. 399-400. ISSN 1365-1609

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Damage to a rock mass is associated with the growth of microcracks and brittle fracturing, so the analysis of acoustic emission (AE) signals is well suited to study this phenomenon. A fundamental difference between acoustic emission and other techniques involving wave propagation is that a wave is generated and transmitted in the rock itself in the case of acoustic emission—as opposed to being introduced into the rock by an external source. The most important parameters recorded and used for the analysis in the AE study are peak amplitude, threshold set by the user to filter out unwanted noise, number of times a signal crosses a preset threshold datum, event duration and counts that attains peak amplitude value. The transition from stable to unstable cracking is marked by a rapid rise in stronger events and a declining trend in weaker events during the course of rock deformation. The measurable characteristics of the acoustic event waveform are capable of revealing a number of insights into the progressive failure process of rock. This paper details an AE study at Khetri Copper underground mine of Hindustan Copper Limited to assess the crown pillar stability at the 300m level with the study involving analysis of the characteristics of the individual AE signals, and analysis of AE events sequence, including time sequences and frequency-count patterns. The crown pillar stability came into focus when the borehole extensometer (BX6) showed cumulative displacements, but a load cell installed beside it displayed no increase in load. The analysis of the acoustic observations monitored revealed (Fig. 1) the following. * Sensors located on the major and minor slip planes and on the floor of the 300m level did not record any acoustic emission. But, sensors mounted on BX6 did record low and high frequency emissions ranging from 1 to 1000 kHz. * The presence of high amplitude and continuous AE counts indicated the presence of continuous fracture/movements. The amplitude was feeble (in the range of 30–57 dB) and supplemented by discontinuous counts indicating no major change towards strata movement, failure or crack propagation. * The number of AE counts in a hit along with the Peaks to the Count for various data was scattered in a large rise time, indicating low energy stored in the emissions and hence low damage intensity. * More AE counts in a short duration indicated closure of cracks. Continuous monitoring revealed higher AE counts within a short time but with less Peak to counts, thereby revealing crack initiation or closure. * High frequency emissions recorded at the BX6 extensometer station attenuated very quickly. These high frequency signals denote the presence of defects at the grouted end of the extensometer. * One acoustic hit, of high frequency (1000 kHz), was recorded in a total monitoring span of approximately 10 h but had zero energy level, indicating that there was no continuous/drastic movement at the crown pillar. The real-time monitoring with acoustic emission provided essential information related to the changing stress condition in the rock mass. Some acoustic emission events did indicate localised crack initiation or closure. The acoustic emissions were of a feeble nature and did not indicate any major change in the stability of the 300m level crown pillar. The crown pillar stability was also evident from discontinuous recording of acoustic emission events and the low magnitude energy stored in any acoustic hit.

Item Type: Article
Uncontrolled Keywords: Acoustic emission; Frequency; Amplitude; Counts; Pillar stability; Microcracks
Subjects: Methane Emission and Degasification
Depositing User: Dr. Satyendra Kumar Singh
Date Deposited: 20 Nov 2011 05:12
Last Modified: 26 Nov 2011 09:28

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