As part of a study to provide a complete set of guidelines for the development of re-entry protocols, a survey of re-entry practices at 18 seismically active mines, mostly in Ontario, was analyzed and evaluated. Some results from the survey are: the majority of re-entry incidents are triggered by production blasts, resulting in classical Omori-style decay in seismic event frequency over a number of hours. The distance from active mining that seismic events typically trigger re-entry restrictions is between 50-100 m. Seismic parameters typically evaluated are event frequency and magnitude, energy, and moment. Due to the possibility of large aftershocks in the event decay sequence, re-entry is generally delayed for a minimum time window once normal levels of seismic activity have been re-established. Based on this compilation, a complete set of ?best practice? guidelines are proposed. These guidelines provide standardization for protocol development, and guidance for mines with little history of seismic activity.
1. INTRODUCTION
analysis and evaluation of current re-entry practices after large events/blasts and rockbursts,
preliminary guidelines for the development of re-entry protocols at a mine, based on a synthesis and analysis of currently used protocols, including advice on instrumentation, analysis of data, and final development of the protocol, and
identification of needs for a second of stage more focussed and in-depth research.
A common characteristic of deep mines in hard rock is induced seismicity. Following large seismic events/blasts, there is an increased but short term risk that additional seismicity can cause a rockburst and therefore the policy adopted by mines is to restrict access to the affected areas for a specific time period. This is the re-entry protocol. As part of a study to provide a complete set of guidelines for the development of re-entry protocols, a short-term project has been undertaken to collect information on current re-entry practices and to produce preliminary ?best practice? guidelines for the development of a re-entry protocol in seismically active mines. These results are briefly summarized in this paper. The main results include: 2. APPROACH
To evaluate current re-entry practices and identify the key parameters that control seismicity following blasts and large seismic events, a questionnaire was developed and distributed to 60 operating mines in seven different countries, with a total of 18 seismically active mines replying to the survey. The main mining methods of the surveyed mines are; entry mining (7), open stope (9) and caving (2). 75% of these mines were from Ontario, representing a solid basis of information from Ontario?s operating mines. The principal aspects assessed for each site surveyed included:
description of current re-entry protocol being used,
decision indicators used in the re-entry protocol,
site conditions (mining method, geology, depth, rock properties),
descriptions of seismic monitoring systems, and
main concerns, needs for improvement.
