1. 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

2. 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.

3. Roof Falls with “Stack Rock”
Breaking like plates along laminations or beddings.
In the order of original lithology.

4. Thinly-laminated Siltyshale

5. Rock Properties

6. 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

7. High Horizontal Stress

8. Roof Fall History 40 roof falls over the last ten years.
Fall height: ft.
Primary bolt: 4-7 ft
Supplementary bolt: 8-16 ft

9. Roof Falls

10. Roof Fall I

11. Roof Fall II

12. Roof Fall III

13. Roof Fall Characteristics
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
Fall III
8 ft
6.5 ft, 7/8-in bolts with T-2 channel
8-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 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

16. Roof Monitoring
For four months
Observation
Roof scoping

17. Roof Initial Failure – Pressure Fracture or Buckling Failure

18. Roof Initial Failure

19. Roof Initial Failure

20. Pressure Fractures Developed within 2-3 blocks from the face
Can be at any location
Not necessarily along joint orientation

21. Roof Separations

22. Roof Horizontal Movement
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

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

25. 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

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. 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.

29. Conclusions
Primary 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.