Roof Bolting in Low Seam Mining With Stack Rock Roof Under High Horizontal Stress Peter Zhang, Senior Geotechnical Engineer Scott Wade, Senior Geologist.
Post on 31-Mar-2015
216 Views
Preview:
Transcript
Roof Bolting in Low Seam Mining With “Stack Rock” Roof Under
High Horizontal Stress
Peter Zhang, Senior Geotechnical EngineerScott Wade, Senior Geologist
Ed Zeglen, Chief Mining EngineerScott Peterson, Director Geology
Rod Lawrence, Director Technical ServicesMike Mishra, VP Engineering
Technical Services, An Affiliate of Alpha Natural Resources, Inc.
Rick Smith, Mine SuperintendentGary Deemer, General ManagerRobert 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
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
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
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 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
For four months Observation Roof scoping
Roof Initial Failure – Pressure Fracture or Buckling Failure
Roof Initial Failure
Roof Initial Failure
Developed within 2-3 blocks from the face
Can be at any location Not necessarily along
joint orientation
Pressure Fractures
Roof Separations
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
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.
Causes of Roof Falls
Weak thin-laminations and low cohesion between laminations
High horizontal stress Joints when they are dense and
deep.
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
Distribution of the Highest Separations in the Inby Area
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.
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.
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.
top related