Roof Bolting in Low Seam Mining With

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Roof Bolting in Low Seam Mining With Stack Rock
Roof Under High Horizontal Stress

• Peter Zhang, Senior Geotechnical Engineer
• Scott Wade, Senior Geologist
• Ed Zeglen, Chief Mining Engineer
• Scott Peterson, Director Geology
• Rod Lawrence, Director Technical Services
• Mike Mishra, VP Engineering
• Technical Services, An Affiliate of Alpha Natural
  Resources, Inc.
• Rick Smith, Mine Superintendent
• Gary Deemer, General Manager
• Robert Bottegal , Chief Engineer
• Amfire Mining Company, LLC

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Stack Rock

• Thin 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.

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Roof Falls with Stack Rock

• Breaking like plates along laminations or
  beddings.
• In the order of original lithology.

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Thinly-laminated Siltyshale
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Rock Properties
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Mining Condition

• Mining height 48 in
• Overburden depth 450-470 ft
• Entry width 19 ft
• Immediate roof laminated silty shale or shale,
  or sandstone
• Roof joints N30W in shale or siltyshale
• High horizontal stress

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High Horizontal Stress
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Roof Fall History

• 40 roof falls over the last ten years.
• Fall height 5-12 ft.
• Primary bolt 4-7 ft
• Supplementary bolt 8-16 ft

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Roof Falls
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Roof Fall I
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Roof Fall II
12
Roof Fall III
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Roof Fall Characteristics
Roof Fall Fall Height Fall Shape Primary Support Supplementary Support
Fall I 5 ft Flat top, steep breaking angle at corners 4.5 ft, 7/8-in bolts with T-2 channel none
Fall II 6-7 ft Flat top, steep breaking angle at corners 4.5 ft, 7/8-in bolts with T-2 channel none
Fall III 8 ft Flat top, steep breaking angle at corners 6.5 ft, 7/8-in bolts with T-2 channel 8-ft cable bolts
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New Bolting Plan

• 6.5 ft combination bolts with straps
• 12 ft cable bolts on 6 ft spacing

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Variations of the Basic Bolting Plan

• Primary bolts
• Additional two 4.5 7/8-in resin bolts on 8 ft
  spacing
• 3 ft spacing
• Supplementary bolts
• 14 ft cable bolts
• 16 ft cable bolts through parallel straps
• 16 ft post-tension cable bolts

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Roof Monitoring

• For four months
• Observation
• Roof scoping

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Roof Initial Failure Pressure Fracture or
Buckling Failure
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Roof Initial Failure
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Roof Initial Failure
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Pressure Fractures

• Developed within 2-3 blocks from the face
• Can be at any location
• Not necessarily along joint orientation

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Roof Separations
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Roof HorizontalMovement

• Tend to be parallel to major horizontal stress
• Along diagonal of an intersection towards the
  center
• Within 5 ft of the immediate roof.
• Shifting 0.02 0.5 in

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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.

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Causes of Roof Falls

• Weak thin-laminations and low cohesion between
  laminations
• High horizontal stress
• Joints when they are dense and deep.

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Support Requirements

• Beam 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

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Distribution of the Highest Separations in the
Inby Area
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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.

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Conclusions

• Thinly-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.

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Conclusions

• Primary bolts should be enough in length to cover
  most of the separations (gt90) in the inby area.
• Supplementary bolts should be enough in length to
  cover the most of the separations (gt95) in the
  outby area and capacity to suspend the dead
  weight of the separated roof in the outby area.