Highwall Safety for Metal and Nonmetal Surface Mines

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Highwall Safety for Metal and Nonmetal Surface
Mines

• Mine Waste and Geotechnical Engineering Division
• Pittsburgh Safety Health Technology Center

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Ground Control

• Maintaining mass stability by controlling the
  movement of excavations in the ground, which can
  be either rock or soil

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Ground Control Problems

• Scale
• Mass Stability/Instability - Failure or
  movement of massive sections of a highwall or
  spoil bank
• Falling Rocks - movement of relatively small
  loose rocks that are not firmly attached to a
  highwall or spoil bank

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RISK

• CONDITION (HAZARD)
• EXPOSURE

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Preface

• While ground control failures are often easy to
  explain after-the-fact, they are often very
  difficult to predict before-the-fact.
• Minimizing exposure to potential hazards is
  always the best policy.

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Risk Avoidance

• Use common sense
• expect the unexpected
• stay alert
• Never place yourself or others in a vulnerable
  position, no matter how good the wall or bank
  looks

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Outline

• Highwall Stability
• Spoil Bank Stability
• Stockpile/Surge Pile Stability
• Impoundments / Refuse Piles

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Highwall Stability
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Factors Influencing Rock Cut Stability

• Properties of Intact Rock
• Properties of Rock Mass- joints, faults,
  fissures, bedding planes
• Water Conditions- rainfall, snow melt, springs or
  seeps, ice on highwall face, etc.
• Environmental Factors - freeze/thaw, soil/rock
  decomposition, etc.
• Mining-induced Factors - stress changes, blasting
  effects, equipment vibration, etc.

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Types of Highwall Failures

• Planar Failure
• Sliding Wedge Failure
• Toppling Failure
• Circular Failure
• Rock Falls

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Planar Failure
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Mohr-Coulomb Shear Strength Equation

• ? Shear Strength (psf)
• C Cohesion (psf)
• ? Normal Stress (psf)
• u Pore water Pressure (psf)
• ? Angle of internal friction (degrees)

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Wedge Failure
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Wedge Failure Conditions

• Two discontinuities meet
• Intersection point daylights on highwall
• Line of intersection dips toward the pit
• Weight of sliding wedge exceeds frictional
  resistance along the bottom

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Toppling Failure
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Localized Failure -Loose Rocks
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How Do We Handle Loose Rocks?

• The answer depends on whether it is in an area
  that requires temporary or permanent stability?
  
• Longer stability is warranted for - highwall
  above portal, road cut, rock-cut spillway, etc.

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Rock-removal methods for slope stabilization.
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Rock slope reinforcement methods
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Slope stabilization with rock bolts.
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Rock BoltUsed to tie multiple pieces of rock
together.
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Benching

• - Reduces weight (Driving Force) on existing
  discontinuities!
• - Eliminates some discontinuities!
• - Acts as an effective Rock Catcher!

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Angle Drilling

• In some cases, angle drilling can significantly
  reduce the potential for rock falls from a
  highwall

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Slope Stability and Water

• Groundwater in slopes is often a contributory
  cause of instability.
• Water adds weight to the sliding mass, lubricates
  the failure planes, and acts as a driving force.

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Effect of Water in Joints
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Effects of Ice
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Remedies

• Flatten Slopes
• Better Scaling
• Wider Benches
• Monitoring
• Visual
• Instrumentation

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Circular Failure
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Circular Slope Failure
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Working Above and Near the Edge of a Slope or
Highwall
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Causes

• Dragline was not on a firm level bench in full
  contact with the ground
• elevating the edge aggravated the situation by
  causing uneven load distribution with greatest
  load right at the edge.
• bubble levels were not checked
• Dragline was very close to the edge
• no allowance for change in ground conditions
• Sidewall was not examined for overhangs, etc.

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Causes

• Leveling Jack Set on Soft Material
• Contributing
• Position of the drill
• Long Axis Parallel to Highwall
• Close to the highwall
• Operators cab on lower highwall side
• Lack of Inspection or Recognition of Ground Hazard

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Wedge Failure in Open Pit Highwall
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Highwall Safety

• Program Information Bulletin
• P00-7
• Metal and Nonmetal Mine Safety and Health

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Highwall-Related Accidents 1990-2000

• Highwall Failures
• 6 Fatal Accidents
• Falling Rocks
• 4 Fatal Accidents
• 60 Non-fatal Accidents
• Near Misses ?

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Alerts Mine Operators to

• Hazards Associated with unstable highwalls
• overhangs
• unstable walls, banks, and slopes
• Hazards associated with loose rock accumulating
  on or near highwall faces

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• 30 CFR 56/57.3200 any unsafe ground condition
  existing where persons work or travel must be
  taken down or supported prior to starting work or
  traveling in the area.

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• 30CFR56.3130 Mining methods shall be used that
  will maintain wall, bank, and slope stability in
  places where persons work or travel in performing
  their assigned tasks. When benching is
  necessary, the width and height shall be based on
  the type of equipment used for cleaning of
  benches or for scaling of walls, banks, and
  slopes.

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• 30 CFR 56.3131 In places where persons work or
  travel in performing their assigned tasks, loose
  or unconsolidated material shall be sloped to the
  angle of repose or stripped back at least 10 feet
  from the top of the pit or quarry wall. Other
  conditions at or near the perimeter of the pit or
  quarry wall which create a fall-of-material
  hazard to persons shall be corrected.

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Examination

• 30CFR56/57.3401 Persons experienced in examining
  and testing for loose ground shall be designated
  by the mine operator. Appropriate supervisors or
  other designated persons shall examine and, where
  applicable, test ground conditions in areas where
  work is to be performed prior to work commencing,
  after blasting, and as ground conditions warrant
  during the work shift.

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Examination

• 30CFR56/57.3401 (continued) Highwalls and banks
  adjoining travelways shall be examined weekly or
  more often if changing ground conditions warrant.

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Conclusion

• Preventing ground control problems is one of the
  greatest challenges facing surface coal mine
  inspectors.
• Clues to impending ground instability can often
  be found in unlikely places - on top of highwalls
  and on the slope of spoil banks
• Focus should be on minimizing exposure to
  potential ground hazards