Title: Roof Bolting in Low Seam Mining With
1Roof 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
2Stack 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.
3Roof Falls with Stack Rock
- Breaking like plates along laminations or
beddings. - In the order of original lithology.
4Thinly-laminated Siltyshale
5Rock Properties
6Mining 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
7High Horizontal Stress
8Roof 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
9Roof Falls
10Roof Fall I
11Roof Fall II
12Roof Fall III
13Roof 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
14New Bolting Plan
- 6.5 ft combination bolts with straps
- 12 ft cable bolts on 6 ft spacing
15Variations 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
16Roof Monitoring
- For four months
- Observation
- Roof scoping
17Roof Initial Failure Pressure Fracture or
Buckling Failure
18Roof Initial Failure
19Roof Initial Failure
20Pressure Fractures
- Developed within 2-3 blocks from the face
- Can be at any location
- Not necessarily along joint orientation
21Roof Separations
22Roof 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
23Effect 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.
24Causes of Roof Falls
- Weak thin-laminations and low cohesion between
laminations - High horizontal stress
- Joints when they are dense and deep.
25Support 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
26Distribution of the Highest Separations in the
Inby Area
27Support 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.
28Conclusions
- 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.
29Conclusions
- 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.