Investigation of Shield Convergence in Underground Longwall Coal Mining, A Case Study Luke Clarkson Graduate Geotechnical Engineer, BMA Thesis Presentation (goodyoda13 2014)
Limitations Limitation #1. Limitation #2. Chart Showing Loading Rates against Convergence Rates for Broadmeadow LW8 Weighting Event 25/04/2013 to 04/05/2013 Time until Stability Restored: 67 hours Distance Travelled in this time: 5.6m Zero Convergence recorded 138 hours and 22.15m past event initiation. 1-5 Minute Segment vs 5-10 Minute Segment Limitation #3. Tilt sensors geometrically calculate - Evaluation of the earlier and shorter term loading and convergence the typical h1, h2 and h3 values. rates during the set to yield period may eliminate any compromise of the data caused by early yield in dedicated high set longwalls. Caterpillar Tilt Sensor Resolution: Approx. 8mm, 0.1 ° CSIRO Tilt Sensor Resolution 0.01 ° (PDR Engineers, 2013)
Events Analysed LW8 Weighting Event Cavity #1 Cavities #2 & #3 Detrimental Influence of Dyke Intrusion (SCT, 2013) Load Cycle Map Showing Recorded Pressures over LW8 Panel Parameter Longwall 8 Longwall 9 Weighting Event Weighting Event Seam Thickness (m) 6.88 6.90 Depth of Cover (m) 235 270 Floor Strength (MPa) 32.5 29 Roof Strength (MPa) 16 22 MP42 (overburden 27.5 – 30 sandstone unit) Thickness 16.5 (m) Microseismic Events 150 4 Preceding Weighting Comparison between Geological parameters in LW8 and LW9 Events
Conclusions Made on Prediction of a Weighting Event • There is no correlation between number of yields and convergence rate. • Loading in excess of 2 bar/ minute during set-to-yield over 5-6 cutting cycles is indicative of an oncoming weighting • Previously estimated correlations between loading rate (5-10 min) and convergence rate are deemed to be subject to geological and operating conditions: 1.5 bar/ min pressure increase rate correlates to… Event Periodic yet Stable Conditions (up to) 8.12mm/ hour convergence rate Broadmeadow LW8 Weighting Event 22.04mm/ hour convergence rate Cavity #1 19.7mm/ hour convergence rate Broadmeadow LW9 Weighting Event 9.76mm/ hour convergence rate • Increased thickness of a competent, overburden unit will increase the intensity of a weighting after the unit breaks. • (Related to the previous point) A longer set-to-yield time can be indicative of normal conditions, or indicative of strata unit competence and extended cantilevering. • 6.6% (or less) time spent in yield is reflective of stable although still periodic weighting influenced longwall mining conditions. • ≥2 yields/ cycle for 3 consecutive cycles as an indicator of oncoming weighting.
Conclusions Made on Prediction of Cavities • Loading rates are not recommended as a parameter to monitor in anticipation of cavities. • Number of yields on the shield directly under the influence of a cavity are not recommended to monitor in anticipation of cavities. • A predictive tool for cavities is observation of the number of yields of adjacent shields. • An additional indicator would be total convergence in a single cycle or for 3-5 cycles. • Deterioration of conditions as a result of structural geology should be an ongoing consideration.
Convergence in Underground Longwall Mining Analysis of Shield Closure vs Difference in Shield Heights
Analysis of Shield Closure vs Difference in Shield Heights Charts Showing Cumulative Difference in Tip Height compared to Average Shield Closure in Cycle N.B. A positive displacement reflects downward movement Overburden Sandstone 2000 1400 fracturing point, reflected by Instability Duration, 5.6m of retreat until clear 1200 increased convergence/ 1500 Shield Closure (mm) Displacement (mm) convergence rates 1000 21.44mm/ hr 13.33mm/ hr 800 10.15mm/ hr 1000 600 Cumulative 6.31mm/ hr 500 400 Diff. in Tip 13.95mm/ hr Height (mm) 200 0 0 Shield -200 -500 Closure (mm) Cumulative convergence allows -400 overlying sandstone to break -1000 -600 12:05 16:20 20:35 1:00 5:15 9:30 13:45 18:00 22:15 2:40 6:55 11:10 15:25 19:40 0:05 4:20 8:35 12:50 17:05 21:20 1:45 6:00 10:15 14:30 18:50 23:05 3:30 7:45 12:00 16:15 20:30 0:55 5:10 9:25 13:40 17:55 22:10 2:35 6:50 11:05 15:20 19:35 27/04 28/04 29/04 30/04 1/05 2/05 3/05 4/05 Time • Shield closure is represented as an average ‘per cycle’. • Time until Stability Restored: 67 hours • Distance Travelled in this time: 5.6m • Zero Convergence recorded 138 hours and 22.15m past event initiation
Analysis of Shield Closure vs Difference in Shield Heights Charts Showing Cumulative Difference in Tip Height against Cumulative Shield Closure over time N.B. A positive displacement reflects downward movement Cumulative Shield Closure LW8 5500 5500 Displacement (mm) Overburden Sandstone fracture Instability Duration, 5.6m of retreat until clear 4500 4500 3500 3500 Linear increase between tip (mm) Cumulative Diff. height and shield closure 2500 2500 readings in Tip Height (mm) 1500 1500 500 500 Cumulative Shield Closure -500 -500 (mm) 12:05 16:05 20:05 0:15 4:15 8:15 12:15 16:15 20:15 0:25 4:25 8:25 12:25 16:25 20:25 0:35 4:35 8:35 12:35 16:35 20:35 0:45 4:45 8:45 12:45 16:50 20:50 1:00 5:00 9:00 13:00 17:00 21:00 1:10 5:10 9:10 13:10 17:10 21:10 1:20 5:20 9:20 13:20 17:20 21:20 27/04 28/04 29/04 30/04 1/05 2/05 3/05 4/05 04:25 Time Shield tip height Reduction LW9 > Average Shield Closure 1200 1200 Cumulative Shield Closure Overburden sandstone fracture 1000 1000 Displacement (mm) 800 800 Cumulative Initial condition reflected height less than cut height. 600 600 Negative fluctuations throughout reflect an attempt to Difference in 400 400 (mm) return to appropriate operating height Shield Height 200 200 (mm) 0 0 -200 -200 Cumulative -400 -400 Shield Closure in Cycle (mm) -600 -600 4:10 6:20 8:30 10:40 12:50 15:00 17:10 19:20 21:30 23:40 2:00 4:10 6:20 8:30 10:40 12:50 15:00 17:10 19:20 21:30 23:40 2:00 4:10 6:20 8:30 10:40 12:50 15:00 17:10 19:20 21:30 23:40 2:00 4:10 6:20 8:30 10:40 12:50 15:00 17:10 19:20 21:30 23:40 2:00 4:10 19/07/2014 20/07/2014 21/07/2014 22/07/2014 23/07/2014 Time
Canopy Attitude ( ° ) Canopy Attitude ( ° ) -10 10 10 12 14 16 -8 -6 -4 -2 0 2 4 6 8 0 2 4 6 8 Analysis of Shield Closure vs Difference in Shield Heights 12:05 4:10 27/04 15:45 6:10 19:25 8:10 23:05 10:10 19/07/2014 2:55 12:10 6:35 14:10 28/04 10:15 16:10 13:55 18:10 17:35 20:10 21:15 22:10 1:05 0:20 4:45 2:20 8:25 29/04 4:20 12:05 6:20 15:45 20/07/2014 8:20 19:25 10:20 23:05 12:20 2:55 14:20 6:35 16:20 30/04 10:15 (PDR Engineers 2013) 18:20 13:55 20:20 17:35 22:20 21:15 0:30 1:05 Time 2:30 Time 4:45 4:30 8:25 1/05 6:30 12:05 21/07/2014 8:30 15:50 10:30 Charts Showing Canopy Angle against H1:H3 Ratio over time 19:30 12:30 23:10 14:30 3:00 16:30 6:40 18:30 2/05 10:20 20:30 14:00 22:30 17:40 0:40 21:20 1:10 2:40 4:40 4:50 8:30 6:40 3/05 22/07/2014 12:10 8:40 15:50 10:40 19:30 12:40 23:10 14:40 3:00 16:40 6:40 18:40 4/05 10:20 20:40 23/07/2014 14:00 22:40 17:40 0:50 21:20 2:50 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 H1:H3 Ratio LW8 LW9 H1:H3 Ratio Ratio H1:H3 Angle (°) Canopy Ratio H1:H3 Angle (°) Canopy
Analysis of Shield Closure vs Difference in Shield Heights Rocscience – Phase2D Finite Element Model - Pillars exhibit influence 60m from each gate end - Shields in the centre of the panel are expected to experience the worst conditions - Strength Factor = Factor of Safety - Given excavation modelled with no structural support aside from coal pillars White area represents excavated Heading region in front of coal face 0 100 200 300 Distance along Face (m) 27/04 28/04 29/04 30/04 01/05 02/05 03/05 04/05 Time 12:05 17:40 23:15 05:00 10:35 16:10 21:45 03:30 09:05 14:40 20:15 02:00 07:35 13:10 18:45 00:30 06:05 11:40 17:20 22:55 04:40 10:15 15:50 21:25 03:10 08:45 14:20 19:55 01:40 07:15 12:50 18:25 Microseismic monitoring will allow for 0 Displacement (mm) a better understanding of anticipated 500 geological influences, and the Shield #115 associated magnitudes of effects, on Shield #110 1000 different shields across the face. Shield #100 1500 Shield #90 2000
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