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
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.
Roof Falls with “Stack Rock”


Breaking like plates along laminations or
beddings.
In the order of original lithology.
Thinly-laminated Siltyshale
Rock Properties
Mining Condition
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
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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
High Horizontal Stress
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
Roof Falls
Roof Fall I
Roof Fall II
Roof Fall III
Roof Fall Characteristics
Roof Fall
Fall
Height
Fall Shape
Primary Support
Supplem
entary
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
New Bolting Plan


6.5 ft combination bolts
with straps
12 ft cable bolts on 6 ft
spacing
Variations of the Basic Bolting Plan

Primary bolts


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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
Roof Monitoring
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
For four months
Observation
Roof scoping
Roof Initial Failure – Pressure
Fracture or Buckling Failure
Roof Initial Failure
Roof Initial Failure
Pressure
Fractures
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Developed within 2-3
blocks from the face
Can be at any location
Not necessarily along
joint orientation
Roof Separations
Roof Horizontal
Movement
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
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
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
Effect of Fully-grouting and Pretensioning
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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.
Causes of Roof Falls
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Weak thin-laminations and low cohesion
between laminations
High horizontal stress
Joints when they are dense and deep.
Support Requirements
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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
Distribution of the Highest Separations in the
Inby Area
Support Requirements
– Primary vs. Secondary
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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.
Conclusions
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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.
Conclusions
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
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.