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Case Studies of Slope Stability Radar used in Coal Mines

David Noon

GroundProbe Pty Ltd 8 Hockings Street (PO Box 3934)

SOUTH BRISBANE, Qld 4101 AUSTRALIA Phone: 07 3010 8944 Fax: 07 3010 8988

Email: david.noon@groundprobe.com ABSTRACT This paper presents case studies about how the Slope Stability Radar (SSR) system provided adequate warning to safeguard people and equipment prior to a highwall and low wall failure at two Australian coal mines. At Drayton mine, the SSR was able to provide the mine with sufficient warning to move the shovel and trucks away from the highwall, while personnel safely watched 50,000 tonnes of bulk material coming down from the wall. At Mt Owen mine, the SSR alarm allowed the mine to evacuate equipment and personnel four hours prior to a 30,000,000 tonne low wall failure. These two case studies demonstrate how the SSR system was able to continuously monitor the stability of these critical slopes, enabling greater mine productivity whilst maintaining the highest quality of safety. INTRODCTION Ground instability in open cut mining operations is common, and mining can continue provided the wall does not collapse unexpectedly. Such risks can be virtually eliminated at the planning stage by reducing slope angles, but this carries a very high cost. Also, instabilities which develop whilst mining can lead to coal reserves being quarantined, representing a high cost. In spite of such safety measures, unexpected failures have occurred in the past. These issues motivated the development of the slope stability radar (SSR). The SSR system can detect and alert movements of a wall with sub-millimetre precision, with continuity and broad area coverage, and without the need for mounted reflectors or equipment on the wall. In addition, the radar waves adequately penetrate through rain, dust and smoke to give reliable measurements, 24 hours a day. SSR prototypes were tested under ACARP projects at Drayton, Moura, Callide, Tarong and Hunter Valley Operations coal mines from 1999-2002 (Reeves et. al, 2001). In 2003, GroundProbe® (http://www.groundprobe.com) was formed to provide SSR services to mine sites, and to date has provided services to numerous coal mines in Australia (Saraji, Goonyella Riverside, Burton, German Creek, Mt Owen, Liddell, Muswellbrook, Bengalla, Leigh Creek, and Bulga) and many of the large metalliferous mines in Australia and overseas. To date, SSR units have detected and provided timely warning of over 50 rock falls ranging from just a few tonnes to gross failures of many millions of tonnes. The reader is referred to an earlier paper by Noon, 2003 for more details about the SSR technology. This paper presents two case studies of the SSR providing sufficient warning for a highwall failure at Drayton and a low wall failure at Mt Owen. Case Study 1: Drayton Coal Mine Drayton mine used the SSR system to continuously monitor an unstable highwall while coal was being extracted. Figure 1 displays the displacement measured by the SSR over three days in wall areas 1 and 2 of the photograph. Two small rock falls were measured (0310hrs

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on 16/11/04 and 1755hrs on 17/11/04) prior to the main highwall failure at 0930 hrs on 19/11/04. Table 1 displays the five different alarm levels that were set by the mine geologist to warn the mine of the impending wall failure. Levels 1 to 4 sounded sequentially over the one hour period from 0730hrs to 0830hrs. Figure 2 is Figure 1 zoomed in to the last 4 hours prior to the main highwall failure. This figure shows the wall movement at the commencement of the shift (0730 hrs), the commencement of acceleration (0830hrs) and the start of the main highwall failure (0930hrs). Figure 3 shows the Level 5 alarm being triggered at 0830hrs. The mine immediately moved the shovel and trucks away from the highwall and personnel watched as the 50,000 tonnes of bulk material came down from the face. Figure 4 shows the photographs taken by the SSR on-board camera prior to (0800hrs) and after (1000hrs) the highwall failure. The mining equipment was moved away in sufficient time. Case Study 2: Mt Owen Coal Mine Mt Owen was monitoring an unstable low wall using traditional methods for over twelve months. When the movement rates became excessive, the mine utilised the SSR system to continuously monitor the spoil while coal was being extracted from the pit floor. Figure 5 displays the displacement measured by the SSR over 13 days in wall areas 1 and 2 of the photograph. One alarm level was set by the mine geologist at 70mm of movement over a 45 minute time period and over 1029 m^2 of wall area. Figure 6 displays the alarm sounding at 0343hrs on 29/1/05. The alarm sounded in the control room and mining equipment and personnel were evacuated in sufficient time prior to the failure occurring at 0740hrs on 29/1/05. Figure 7 shows the photograph taken after the low wall failure. The slump area was approximately 1km long and 200m high. The low wall slumped as a single entity causing a 30m slump at the top and a heave of 10-15 m on the pit floor. The mass of the slump was approximately 30,000,000 tonnes. CONCLUSIONS The SSR has intrinsically improved the safety management of coal mines by improving the available information on slope stability and thus allowing better decisions to be made. The technology overcomes the shortcomings of conventional geotechnical monitoring systems by providing extra warning time and greater coverage of the rock face. In conjunction with good management practices, the SSR system can dramatically reduce the risk of death, injury and equipment damage due to mine wall instability. Further, it gives confidence for mining to occur in areas that might otherwise be quarantined due to uncertainty over the extent of instability. These benefits translate to more assured productivity through quantifying and managing the risks associated with mining in pits which have potentially unstable walls. ACKNOWLEDGEMENTS We thank Stuart Argent from Drayton Coal and Darren Pisters from Mt Owen Coal for the effective utilisation of the SSR units, and in defining the alarm settings and response.

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REFERENCES Noon, D., 2003: Slope stability radar for monitoring mine walls, Mining Risk Management Conference, Sydney, NSW, 9 - 12 September 2003, 1-12. Reeves, B., Noon, D., Stickley, G. & Longstaff, D., 2001: Slope stability radar for monitoring mine walls, In Beeston, J.W. (Editor): Bowen Basin Symposium 2000 – The New Millennium – Geology. Geological Society of Australia Inc. Coal Geology Group and the Bowen Basin Geologists Group, Rockhampton, October 2000, 139-142

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0310 hrs 16/11 Small failure

1755 hrs 17/11 Small failure

0930 hrs 19/11 Major failure

Figure 1: SSR display showing wall movements over three days prior to the main highwall failure at 0930 hrs. Two small rock falls were measured prior to the main highwall failure.

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0730 hrs Start of shift

0830 hrs Increase acceleration

0930 hrs Start of material fall

Figure 2: Same as Figure 1 except timescale is zoomed in to the 4 hours prior to the main highwall failure.

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Alarm at 0830 hrs – 1 hour prior to major failure

Figure 3: Pixels that triggered the Level 5 alarm (20 mm over 30 minutes and over 60 m^2 area) at 0830hrs. Each square pixel is approximately 9 m^2 area on the wall.

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Figure 4: Photographs taken before (0800hrs) and after (1000hrs) the highwall failure on 19/11/04.

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Figure 5: SSR display of the displacement measured over 13 days prior to the low wall failure.

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Alarm at 0345 hrs – 4 hour prior to major failure

Figure 6: The triggered pixels at 0343hrs on 29/1/05.

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Figure 7: The slump area was approx 1km long and 200m high. The low wall slumped as a single entity causing a 30m slump at the top and a heave of 10-15 m on the pit floor. Approximate mass of the slump was 30 million tonnes.

Approx 30m slump in this area

Slump Area

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Table 1: Alarm Levels and Threshold Settings for Drayton Alarm Level Threshold Period Area on Wall 1 2 mm 30 min 15 m^2 2 4 mm 30 min 15 m^2 3 8 mm 30 min 15 m^2 4 15 mm 30 min 45 m^2 5 (Critical alarm) 20 mm 30 min 60 m^2