1. Evaluating the Effectiveness of the 1750 Tonne Shields at Moranbah North Mine
Kelly Martin
Mehmet Kizil
Ismet Canbulat

2. Outline Project background Project aims and scope Methodology
Results of analysis
Summary of results
Conclusions

3. Project Background
Challenging geotechnical environment historically resulted in cavity formation on the longwall face with its associated reduction in productivity
Complex geology:
Depth
Sandstone channels
Faults
Ply split and rider seam split
Concerns raised about roof stability in future panels at greater depths
Determined that 1750t shields would be required to adequately control strata
Cavities have been THE primary cause of unplanned downtime at Moranbah and cavity management has been an ongoing battle. The intensity of cavity occurrences has been attributed to the complex geology found at the mine including the depth of cover, currently at about 300m, the presence of large strong sandstone bodies, numerous faults and rider seams. Due to the strata control issues encountered, concerns were raised about roof stability in future panels at greater depths upwards of 400m. An feasibility study was carried out and It was determined that 1750 tonne shields would be required to adequately control the strata.

4. Project Background Continued
1750t shields are highest capacity shields in the world
Replaced 980t shields due to:
Aging duty
Supports operating at yield for significant periods
Structural condition
Increasing depths, complex geology and associated geotechnical conditions
1750t shields installed at start of LW108 panel
The shield replaced the previous 980t shields due to their aging duty, the fact that they were constantly being operated on yield and the structural damage that they incurred. All these factors indicated that the shields would be incapable of controlling the strata at greater depths. The 1750t shields were installed at the start of LW108 and are the highest capacity shields in the world.

5. Aim: Scope: Project Aim and Scope
To determine the effectiveness of the new 1750 tonne shields
Scope:
Data analysis was confined to parallel sections of LW107 and LW108
Panels are adjacent to each other and are subject to similar conditions
Comparison of both panels using analysis results was used to determine effectiveness of 1750t shields
Only data related to cavity development and strata control were analysed

6. Methodology LVA data sorted and converted into pressure contour maps
Hazard map created using geological data and contour maps
Identified and analysed:
Number of cavities in each panel
Cavity occurrences in hazard zones
Lost time due to strata control issues
Lost time due to shield issues
Percentage of time spent at or above yield pressure
Percentage of time cavities were encountered in panel
Data analysis supplemented by:
Deputy delay reports
Fault maps
Geological data
Shield pressure and chainage data was downloaded from LVA and the data was then converted into pressure contour maps. The number of low pressure regions (indicative of cavities) was then analysed and compared for each panel. At the same time raw LVA data was used to determine percentage of time shields spent at or above yield pressure and the percentage of total panel time that cavities were encountered.
A hazard map was also created using mine geological data and the pressure contour maps to determine shield performance in geological hazard zones. Finally, the total hours lost in each panel due to strata control issues on the face and due to shield issues were analysed.

7. Leg Pressure Contours
LW107
LW108
Converted LVA pressure and chainage data into real-time coordinates using Surfer
Low pressure regions coloured red – indicate cavities (<250 bar)
To generate the pressure contours, the LVA data was converted into real-time coordinates with the shield number being assigned x-coordinates, the chainage values y-coordinates and averaged pressure values z-coordinates. Shield widths were added onto x-coordinate values and then all data rotated to align with the panels on the mine plan. The red areas represent low pressure regions and indicate the presence of cavities. It can be seen from these maps that there were significantly less cavities in LW108 than there were in LW107.
In order for the comparative assessment to be as accurate as possible, only the sections of the longwalls which were directly parallel to each other were used. As LW108 was significantly longer, this required starting the analysis of LW108 at the install point of LW107.

8. Number of Low Pressure Regions
LW107
LW108
Total Number of Cavities with lost time 38 6
TG Cavities with lost time 4
Face Cavities with lost time 34 2
LW108 had less than half of the low pressure regions that LW107 had with just 27 being identified while LW107 had 87. It should also be noted that the LVA data for the high hazard zone at the start of LW107 was missing and subsequently, the difference in numbers would actually, in reality, be much higher.
By cross-checking the dates corresponding to the locations of the low pressure regions with deputy delay reports, the number of low pressure regions on the contour maps which resulted in lost time due to cavities was able to be determined. It was found that for LW108 there were only six cavity occurrences which resulted in lost time with four of these cavities occurring in the tailgate. For LW107, however, it was found that there were a total of 38 cavities resulting in lost time with only four of these cavities occurring in the tailgate.
The remaining low pressure regions which were not found to have resulted in lost time due to cavity control can be attributed to cavities which did not result in lost time, shield mechanical issues or shield electrical issues

9. Geological Hazard Map LW108 LW107
The purple zones indicate fault zones, while the blue zone shows where the GM1-GM2 ply split is greater than 0.2m, the yellow zone shows where the GMR split is between 1-2m and the orange zone shows where there is potential for weighting. The GM1-GM2 ply split is only deemed hazardous when above 0.2m because this is when the ply has completely separated from the main seam and, due to the weak siltstone material existing between the main seam and top ply, as this distance increases, the roof strength deteriorates significantly which will also results in a reduced ability to accommodate any horizon control issues. The GMR split hazard zone was also determined for the same reasons and also since geotechnical issues have historically been experienced when the split is between 1-2m. The weighting potential zone indicates the presence of large sandstone bodies with a thickness of over 5m and a UCS of over 30Mpa. MNC has three of such channels.
LW107

10. Cavities in Hazard Zones
Number of Low Pressure Regions
LW107
LW108
Fault Zone
Multiple
Ply Split Zone 10
GMR Split Zone 9 1
GMR Split and Weighting Zone 6 3
Weighting Zone 41
For the missing zones and for the purpose of assessing shield performance in high hazard zones, deputy delay reports were used to identify any cavity occurrences.
The actual number of cavity occurrences in these circumstances could not be explicitly stated due to the deputy delay reports not stating explicitly whether the delays due to cavities recorded on consecutive days were due to a continuation of a single cavity or due to multiple separate cavities.
LW108 shields performed better in every high hazard zone showing that they were significantly more effective in terms of strata control in hazard zones. It is also should be noted that the majority of LW108 cavities occurred at the end of the panel which is at the greater depths and in the ply split zone.

11. Total Time Lost Due to Strata Control Issues
Not as large a difference as would be expected so further data filtering was undertaken to separate lost time into TG and face
17% less lost time in LW108 due to strata control issues

12. Time Lost Due to Face Cavities
LW107 had 148 hours of lost time due to cavities in the longwall face while LW108 lost just 20 hours. This is a significantly large difference which indicates that face support in LW108 was significantly more effective than that of LW107.
87% less lost time in LW108 due to face cavities

13. Time Lost Due to TG Cavities
Highlights the fact that most of the hours lost due to strata control in LW108 was because of issues in the tailgate and not actually due to face support issues
82% more lost time in LW108 due to TG cavities

14. Lost Time Due to TG Cavities Continued
Highlights again the support issues in LW108 TG. Only 14% of total time lost due to cavities occurred on the face for LW108 while 87% of the lost time in LW107 occurred due to face support issues. This result was not altogether suprising as LW108 was subject to numerous conditions and issues which likely led to this result.

15. LW108 TG Issues
and Delays
Double stress notch encountered at point corresponding to LW107 install road
Effects extended approx. 1 C/T (100m) into LW108 panel
Intense additional TG support required causing delays
Large unmapped faults encountered perpendicular to face
Led to major stoppages due to TG support issues
Additional delays due to gas levels unique to LW108
Prevented immediate entry to TG to install secondary support resulting in additional lost time
The final analysis of shield effectiveness should subsequently only focus on strata control issues which occurred at the FACE in order to provide an accurate comparative assessment
One of the main reasons for the LW108 TG issues was a double stress notch encountered at the point of the LW107 install road. The effects of this double stress notch extended approximately 100 m further into the panel and resulted in additional TG support including a high density of Link n Locks being required. Large unmapped faults situated perpendicular to the longwall face were also encountered which led to major stoppages as a result of TG support issues. Additionally, further delays were also encountered due to the unique gas levels in LW108 which prevented entry into the TG to install the required secondary support. This resulted in additional lost time. As a result of the TG support issues which were unique to LW108, and in order to provide an accurate comparative analysis, the final analysis of the shield effectiveness should focus largely on strata control issues which occurred at the FACE.

16. Lost Time Due to Shield Issues
LW107 significantly higher. Mechanical, electrical issues etc. likely due to ageing duty and also to overloading of supports as a consequence of shield capacity being too low for region
48% less lost time in LW108 due to shield issues

17. Shield Performance from LVA Data LW107: LW108:
Constant fluctuations in shield pressures
Regularly operated at or above yield pressure
Regularly operated at significantly low pressures
Shields adjacent to cavity zones consistently in yield
LW108:
Relatively consistent shield pressures
Rarely operated in yield
Even around cavity zones resulting in increased loading, the shields did not yield
Raw LVA data was used to determine the percentage of time that the shields were operating at or above the yield pressure as if a shield is constantly operated in yield it is feasible to say that the support is inadequate for the working conditions. Insufficient data was available to determine the actual amount of time that the shields spent in yield so, instead, the amount of time that the shields spent at or above the yield pressure was calculated. Additionally, the raw LVA data was also used to calculate the percentage of time that the shields operated at under 250 bar as pressures below this value is indicative of the presence of cavities.
The results of this particular analysis showed that the LW107 shield pressures were anything but constant with the shields regularly being either operated at or above the yield pressure or at significantly low pressures. Additionally, it was found that the shields adjacent to cavity zones consistently went into yield.
The results for the LW108 shields, however, showed a very different result. The shields displayed relatively consistent pressures, were rarely operated at or above the yield pressure and, even around cavity zones

18. Shield Performance Around Cavity Zones
Posi-set pressure (bar)
400
Yield pressure (bar)
450
In order to provide a graphical representation of the results, the shield leg pressures were plotted against the relevant shield numbers for a given date. This graph shows the typical behaviour of the LW107 shields around a cavity zone. The areas encircled red show were cavities occurred in the panel while the areas encircled green show the areas where the shields operated at or above the yield pressure. In this example, you can see that there are two cavities present and that there are numerous fluctuations in the shield pressures with multiple shields operating at or above the yield pressure, especially around the cavity zones.

19. Shield Performance Around Cavity Zones
Posi-set pressure (bar)
410
Yield pressure (bar)
465
This is the graph for the LW108 shields representing the typical behaviour of the shields around a cavity zone. It can be seen that even around a significant cavity zone and when increased loading was encountered, the shields did not go into yield. It also shows that the shield pressures remained relatively constant throughout the panel.

20. Shield Performance From LVA Data LW107 LW108
Percentage of time cavities were encountered in panel (%)
8.52
3.12
Percentage of time shields were at or above yield pressure (%)
6.47
0.61
The results of the analysis show that the shields in LW108 spent an insignificant amount of time operating at or above the yield pressure value while the LW107 shields had a significantly higher result. Similarly, the amount of time that the shields in LW107 spent operating under 250 bar (cavities) was significantly higher than that experienced by the LW108 shields. This result indicates that the shields in LW108 were more effective in terms of strata control and were more suited to the mining conditions.

21. Overall Performance Comparison
LW108
LW107
Number of Low Pressure regions 27 87
Time Lost (Face Cavities)(h) 20
148
Time Lost (Shields) (h)
314
603
Shield Rating in High Hazard Zones
Moderate - High
Low
Percentage of time cavities were encountered in panel (%)
3.12
8.52
Percentage of time shields were at or above yield pressure (%)
0.61
6.47
This table shows the overall shield performance comparison and it can be seen that the LW108 shields performed better in all categories.

22. Based on an acceptable yield percentage value of 5%:
Shield Suitability
Based on an acceptable yield percentage value of 5%:
LW107
shields not suited to the mining conditions
Shields spent 6.47% of time operating at or above yield pressure
Total time spent in yield would actually be significantly higher
LW108
More than adequate for mining conditions
Shields spent 0.61% operating at or above yield pressure
Total time spent in yield would be significantly higher
In future panels at greater depths and increased loading it is feasible to say that the 1750 tonne shields would be suitable to the conditions
Using the previously calculated values, and using an acceptable yield percentage value of 5%, the suitability of the shields to the mining conditions can be assessed.
It should be noted that the percentage values calculated give the percentage of time the shields operated at or above the yield pressure and not the actual total time the shields spent operating in yield. The actual time the shields spent in yield should include not only the time spent above the yield pressure, but also the time spent in yield as the leg pressure was released until the reset pressure was reached. Due to data being unavailable with the depth of detail required for the calculation, the actual total time the shields spent in yield could not be determined. However, as this value would be significantly higher than that calculated for the time the shields spent above the yield pressure, the previously calculated percentage values can still be used to determine if the shields were suitable to the mining conditions.
As the LW107 shields operated at or above the yield pressure for 6.47% of the time, this would indicate that the shields were working outside of their capacity and were unsuitable for the mining conditions. The LW108 shields, on the other hand, spent a significantly low amount of time operating at or above the yield pressure which indicates that they were more than adequate for the conditions. It is, however, feasible to say that in future panels at greater depths where increased loading will be encountered, the 1750 tonne shields would be suited to the mining conditions.

23. Conclusions
LW108 performed significantly better in geotechnical hazard zones
Only 2 face cavities in LW108 with lost time compared with 34 in LW107
Almost half the time lost due to shield issues in LW108
1750t shields found to be effective overall and more than adequate for mining conditions with greater potential for future panels

24. Anglo American Metallurgical Coal is acknowledged with gratitude for the permission to publish this paper. Steve Winter and Andrew Laws are thanked for their willingness to share their knowledge and for providing the necessary data for the project.

25. Thank you