Location In-Pit Lowwall Dump Design and Stability at Mt Owen Mine Wayne Bartlett Senior Engineering Geologist Mt Owen Complex Overview In the past Mt Owen Mine lowwall dumps have failed. Lowwall dumps are integral to the mining process at Mt Owen. Important considerations in the management of lowwall dumps include; ~ dump design, ~ monitoring of slope movement and ~ management of the risk to personnel. Mining Method Mine Layout 1
Lowwall Orientation Lowwall Orientation Lowwall Orientation Lowwall Orientation Lowwall Orientation B7 Lowwall 2
Folding and Faulting Coal Seams Product Coal is thermal and semi-soft coking coal for supply to the export markets. Up to 11 coal seams are mined in Mt Owen. Some seams consist of up to 6 splits, splits greater than 0.4m and partings less than 0.4m are mined. The mined seams at Mt Owen include; ~ Lemington Seams, ~ Pikes Gully Seams, ~ Arties Seams, ~ Upper Liddell Seams, ~ Middle Liddell Seams, ~ Lower Liddell Seams, ~ Barrett Seams, ~ Upper Hebden Seams and ~ Lower Hebden Seams. 400mm Splits March 2008 June 2008 September 2008 3
December 2008 March 2009 June 2009 September 2009 December 2009 March 2010 4
June 2010 September 2010 December 2010 March 2011 June 2011 September 2011 5
December 2011 March 2012 June 2012 September 2012 Lowwall Stability In-Pit Lowwall Dump Design and Stability at Mt Owen Mine Lowwall dumping is necessary to minimise truck hauls and make use of available dump space. Due to the mine turning a corner there is limited dump space available. Dumping on a lowwall with a dip of up to 22 degrees. 6
Factor of Safety Factors Influencing Stability Stability analysis is an assessment of the factors driving failure and the factors that resist failure. Stability of a slope is referred to as a Factor of Safety (FoS). FoS = Factors Resisting Failure Factors Driving Failure Material Properties; ~ Cohesion, the strength required to shear rock and soil, ~ Angle of friction, the angle at which material will shear causing failure, and ~ Weight of material. Rock mass characteristics, discontinuities, faults Water and piezometric level Geometry of the slope Slope Stability Analysis Slope Stability Analysis Slope Stability Analysis Factors Decreasing Stability 400 300 200 100 0-100 -200 GALEA Version 5.01 Upper XW Rock & Soil Fr Rock normal Spoil Cat.2U Spoil Cat.2S Clay Band LHB Residual Fr Coal Analysis: 4 Single Stability Analysis Method: Sarma Surface: on-circular Over steepening of slope. ~ Removal of material at the toe. Mining deeper. ~ Loading at the crest. Dumping material at the crest of the slope. Change in water level. -300 Results Factor of Safety: 1.26 Critical Acceleration (Kc): 0.110-400 100 200 300 400 500 600 700 800 900 1000 1100 1200 Project: LOM - Revision 0308 - June 08 - Section 4, Mined to RL-120 and Fill B6 buttress to RL-30, dump RL185 and RL218 Dump Stability File: X:\14 Geotech\StabilityCelcs\LOM Analyses\Revised LOM 080313\LOM0803_0806d_dump04.gmf Licensed to: Thiess - Mt Owen Mine 7
Slope Monitoring Monitoring is installed so that we can monitor deformation to better understand the and predict failure. Automated Prism Monitoring Monitors prism movement. Measures prisms 2 or 3 times a day. Triggers alarms when thresholds are exceeded. Slope Stability Radar B6 Lowwall TARP Measures sub millimetre deformation of a slope. Scans a slope approximately every 2 to 8 minutes. Triggers alarms when thresholds are exceeded. Close to real-time alerts when movement exceeds thresholds. Low wall Conditions: L1 Green Existence of minor long term cracks. AD/OR Low wall monitoring is showing <10mm/day or 0.4mm/hr convergence. AD/OR Deceleration path in low wall monitoring over two weeks. L2 Yellow Opening of existing long term and development of new tensional cracks around low wall crest. AD/OR Development of isolated floor heave. AD/OR Low wall monitoring showing >10mm/day and < 180mm/day or 7.5mm/hour over a 12 hour period. AD/OR L3 Orange Rapid opening and slumping of cracks around low wall crest or toe. AD/OR Rapid onset of floor heave. AD/OR Isolated movement of low wall material. AD/OR Low wall monitoring showing > 7.5mm/hour and < 15mm/ hour over a 12 hour period. L4 Red Failure of low wall. AD/OR Imminent failure of low wall indicated by Rapid opening of cracks, floor heave, constant movement of spoil material. AD/OR Low wall monitoring showing > 15mm/hour increase in rate over a 12-hour period. Consistent acceleration path in low wall monitoring showing over 1 week. Rapid acceleration path in low wall monitoring showing over 1 week. 2005 B4 Lowwall Failure 2005 B4 Lowwall Failure 8
Monitoring Result of Failure B4 Failure 4500 4000 3500 3000 2500 2000 1500 1000 500 0 01-08-04 21-08-04 10-09-04 30-09-04 20-10-04 09-11-04 29-11-04 19-12-04 08-01-05 28-01-05 17-02-05 Date 9
Back Analysis of Failure GALEA Version 4.01 400 Failure along weak seam within the Lower Hebden. 300 Back analysis of failure allows calculation of material properties of the Lower Hebden; ~ Cohesion 0kPa and 200 ~ Phi of 12.5 degrees. 100 0 Analysis: 2 Single Stability Analysis Method: Sarma Surface: on-circular -100 Results Factor of Safety: 1.00 Critical Acceleration (Kc): 0.001-200 0 100 200 300 400 500 600 700 800 900 Project: 215KB B3 Switchback Failure Main Section Postfailure Backanalysis Check - Case 2 File: 25012-215kp_b3_analysis2.gmf Sherwood Geotechnical & Research Services Information Obtained Back analysis of the failure provided properties of failed materials. Current B6 Lowwall Movement Increased understandings of critical velocities and accelerations was derived from the analysis of slope movements. 10
Current B6 Lowwall Movement Current B6 Lowwall Movement Current B6 Lowwall Movement Current B6 Lowwall Movement B6 and B4 Comparison Radar Monitoring Monitoring Station Comparison with B4 Failure 4500 4000 B4 Total Displacement (mm) B6 Total Displacement (mm) B6 Total Displacement (mm) 3500 3000 2500 2000 1500 1000 500 0 0 50 100 150 200 250 300 350 Days 11
Radar Monitoring Lowwall Buttressing Lowwall Buttressing Lowwall Buttressing Lowwall Buttressing Buttress Construction RL-120 Barrett Box Cut 12
Buttress Construction RL-90 Buttress Buttress Construction RL-60 Buttress Buttress Construction Current Rate Movement RL-60 Ramp 3000 2500 B6 Displacement from SSR and Prisms B6 RL57 DP01 B5 RL65 DP01 B6 BH02 DP01 Crib P06 Lookout Displacement (mm) 2000 1500 1000 500 Crib P05 Crib Hut B6 Slope B6 Upper Slope B6 Mid Slope Buttress complete to RL- 50.5mm of rain in 37 48 hours Begin Construction of Buttress 61mm of rain in 24 hours Rainfall (mm) 0 12-01-08 12-02-08 14-03-08 14-04-08 15-05-08 Date Daily Rainfall 70 60 50 40 30 20 10 0 12-01-08 12-02-08 14-03-08 14-04-08 15-05-08 Date Conclusion Trigger Action Response Plan s (TARP s) are implemented to manage risk to personnel. Automated monitoring make it possible to analyse slope movement regularly and update the TARP level. Slope stability assessments, buttress and dump design are constantly updated to maintain stability as mining proceeds. 13