An Integrated Real-Time Roof Monitoring System for Underground Coal Mines

CSIRO has been conducting a five-year research project under the sponsorship of JCOAL and Ulan Mine to develop a real-time roof monitoring and roof fall warning system for underground coal mines. A preliminary system has been developed and successfully trialled twice in the gateroads at Ulan Mine during 2004 and 2005. The system integrates the displacement monitoring, stress monitoring and seismic monitoring in one package. It includes: • GEL multi-anchor extensometers • Vibrating wire uniaxial stress meters • ESG seismic monitoring system with microseismic sensors and high frequency AE sensors. The monitoring system has been automated and the data are automatically collected by a central computer located in an underground non-hazardous area. The data are then transferred to surface via an optical fibre cable. The realtime data can be accessed at any location with internet connections. The trials of the system in two tailgates at Ulan Mine have demonstrated that the system is effective for monitoring the behaviour and stability of roadways during longwall mining. The continuous roof displacement/stress data have showed clear precursors of roof falls. The seismic data (event count and locations) have provided insights into the roof failure process during roof fall. The sensor type and real-time communication system are flexible and can be tailored to meet site-specific monitoring needs. INTRODUCTION Roof fall is a major hazard in underground coal mines in Australia. It can cause fatalities, injuries and significant economic losses. Fatalities and injuries due to roof falls in Australian underground coal mines have been significantly reduced in recent years, thanks to the efforts of mine operators and industry regulators. Production loss due to roof falls continues to be a major industry concern for underground coal mines. Roof falls have caused the stoppage of longwall mining and/or roadway development for days or weeks. Real-time monitoring and early detection of imminent roof fall allowing preventative action to be taken, will increase safety margins and bring significant economic benefit to the mining industry. A research project was established in 2002 under the sponsorship of JCOAL and CSIRO to develop a roof fall monitoring and warning system for underground coal mines. The project is of 5 years duration total. During the past 4 years, an integrated real-time roof monitoring system has been developed. The system has been successfully trialed twice at the tailgate of longwall panels 20B and 21 at Ulan Mine. This paper presents the key feature of the system and the results from the first field trial at LW20B. More details can be found in Guo et al. (2004). INTEGRATED REAL-TIME MONITORING SYSTEM The integrated CSIRO roof monitoring system consists of: (1) Extensometers (measuring roof displacement); (2) Stress meters (measuring stress change in roof and rib); (3) Seismic sensors (detecting seismic events and mapping roof damage); (4) Real-time data acquisition and communication system. 1 CSIRO Exploration and Mining, PO Box 883, Kenmore, QLD 4069, Australia 2 Ulan Coal Mines Limited, Ulan, NSW 2850, Australia 2006 Coal Operators’ Conference The AusIMM Illawarra Branch 6 – 7 July 2006 65 Extensometer GEL extensometers manufactured by GEL Instrumentations Pty Ltd have been used (see Figure 1). The extensometers used have 6 anchors which were specifically made for CSIRO. The technical specifications of the 6 anchor GEL extensometers are given below: • Measurement range: 0 – 170 mm • Accuracy: 0.5 mm • Borehole diameters: 27 mm – 55 mm Stress sensors The stress monitoring device used in the monitoring system is the 4300 EX vibrating wire stress meter (Figure 2) supplied by Geokon Geotechnical Instrumentation. It measures the stress change (not the absolute stress). The features of this type of stress meter include: • High sensitivity (sensitivity = 0.014-0.07 MPa); • High range (compression = 0-70 MPa, tension = 0-3 MPa); • Simplified installation. Borehole grouting is not required; • Corrosion resistant; • Waterproof; • Long-term stability; • Suitable for remote reading and automatic logging. The required borehole diameter is 37-39 mm. Seismic monitoring system The seismic system includes seismic sensors (both low and high frequency) and a data acquisition system. The new acquisition system is the ESG HMSi integrated seismic monitoring system (http://www.esg.ca/home/products/hmsi1.htm) modified to enable low-frequency microseismic (MS) and highfrequency acoustic emission (AE) data acquisition cards to be combined on one chassis. This system can run on a standard PC under the MS Windows operating system. The system is capable of acquiring eight channels of MS data, at a sampling rate of up to 40 kHz, as well as four channels of AE data, at a sampling rate up to 40MHz. Extra cards can be installed to increase the number of MS and AE channels available. There are two different types of sensors used in the monitoring system: the low-frequency MS sensors, which includes uniaxial and triaxial accelerometers, and high-frequency AE sensors with built-in preamplifiers for the AE signal detection. The MS sensors are used to detect larger events with fracture radii of tens of centimeters or greater, whereas the AE sensors are aimed at the small, grain-to-centimetre sized fracturing events. The MS accelerometers used are the A1030 and A3005 accelerometers from ESG with a frequency range of 50Hz to 5 kHz, and 3 Hz to 8 kHz, respectively. The R6 sensors used as ultrasonic sources, and the R6i sensors, used as AE receivers, are made by Physical Acoustics Corporation. They have an operating frequency ranging from 40 to 100 kHz. Potentiometer