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.