Presentation on theme: "Peter Zhang, Senior Geotechnical Engineer Scott Wade, Senior Geologist"— Presentation transcript:
1 Roof Bolting in Low Seam Mining With “Stack Rock” Roof Under High Horizontal Stress Peter Zhang, Senior Geotechnical EngineerScott Wade, Senior GeologistEd Zeglen, Chief Mining EngineerScott Peterson, Director GeologyRod Lawrence, Director Technical ServicesMike Mishra, VP EngineeringTechnical Services, An Affiliate of Alpha Natural Resources, Inc.Rick Smith, Mine SuperintendentGary Deemer, General ManagerRobert Bottegal , Chief EngineerAmfire Mining Company, LLC
2 Stack RockThin sheets of sandstone or sandyshale interbedded with thin layers of shale, coal or mica flakes, or very frequently thin films of carbonaceous materials.Stack rock is weak because of poor cohesion between mica or shale rick laminations.
3 Roof Falls with “Stack Rock” Breaking like plates along laminations or beddings.In the order of original lithology.
13 Roof Fall Characteristics Fall HeightFall ShapePrimary SupportSupplementary SupportFall I5 ftFlat top, steep breaking angle at corners4.5 ft, 7/8-in bolts with T-2 channelnoneFall II6-7 ftFall III8 ft6.5 ft, 7/8-in bolts with T-2 channel8-ft cable bolts
14 New Bolting Plan 6.5 ft combination bolts with straps 12 ft cable bolts on 6 ft spacing
15 Variations of the Basic Bolting Plan Primary boltsAdditional two 4.5’ 7/8-in resin bolts on 8 ft spacing3 ft spacingSupplementary bolts14 ft cable bolts16 ft cable bolts through parallel straps16 ft post-tension cable bolts
16 Roof MonitoringFor four monthsObservationRoof scoping
17 Roof Initial Failure – Pressure Fracture or Buckling Failure
22 Roof Horizontal Movement Tend to be parallel to major horizontal stressAlong diagonal of an intersection towards the centerWithin 5 ft of the immediate roof.Shifting 0.02 – 0.5 in
23 Effect of Fully-grouting and Pre-tensioning Fully grouting the bolt cannot prevent roof lateral shifting, but may reduce the amount of shifting.Pre-tension cannot prevent or close separations in the immediate roof.
24 Causes of Roof FallsWeak thin-laminations and low cohesion between laminationsHigh horizontal stressJoints when they are dense and deep.
25 Support RequirementsBeam building - to maintain the immediate roof as an effective beam.Suspension – to use cable bolts to suspend the roof in case primary bolted roof fails.Use straps to reduce buckling failure
26 Distribution of the Highest Separations in the Inby Area
27 Support Requirements – Primary vs. Secondary 6-ft primary bolts can cover the separated roof in 90% of the inby area.10 ft cable bolts can cover the separated roof in 95% of the outby area (95% of the area, separations are less than 8 ft high).Scoping at each block for needs of 14 ft or 16 ft cable bolts.
28 ConclusionsThinly-laminated silty shale is much weaker under horizontal stress than under vertical loading.Initial failure of the thinly-laminated silty shale is buckling failure of laminations.Roof falls occur in the order of original laminations and with flat top and steep breaking angle at corners.
29 ConclusionsPrimary bolts should be enough in length to cover most of the separations (>90%) in the inby area.Supplementary bolts should be enough in length to cover the most of the separations (>95%) in the outby area and capacity to suspend the dead weight of the separated roof in the outby area.