Trenching

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Trenching

• Competent Person

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What Is a Competent Person?

• Identifies trench hazards
• Is authorized to take prompt corrective measures
  to eliminate hazards
• Is responsible for daily trenching inspection
• Coordinates the emergency response plan
• Is knowledgeable about soils, protective systems,
  and OSHA rules
• Works with the professional engineer (PE), who
  designs protective systems

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Session Objectives

• As a competent person, you will be able to
• Recognize trenching hazards
• Understand protective systems for trenches
• Understand hazards and prevention measures for
  activities associated with excavation

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Trenching Fatalities and Injuries

• Over 30 workers die, 1,000 injured per year
• Common causes
• No protective system
• Failure to inspect
• Unsafe spoil placement
• Unsafe entry and exit

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What Is an Excavation?What Is a Trench?

• Excavationany hole or trench made by removing
  earth
• Trencha narrow excavation that is deeper than it
  is wide, no greater than 15 feet wide at bottom
• Trench walls will eventually fail

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What Is a Cave-in?

• Soil or rock falls into an excavation
• Occurs when soil gravitates downward
• Occurs when bottom third of a trench wall fails

Image Credit OSHA
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Consequences of a Cave-in

• Soil is dense and heavy
• Cave-ins can entrap, bury, injure, or immobilize
• Suffocationsoil prevents chest expansion
• The worker becomes immobilized

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Soil Classification

• Soil is classified into different types
• Each type requires a certain protective system
• Competent person determines soil type

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Soil Stability

• Soil stability affected by
• Grain sizesmall is better
• Water saturation
• Cohesivenesshow well soil sticks together
• Unconfined compressive strength (UCS)how easily
  the soil will shear

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Soil TypesType A

• Type A is
• The most stable soil type
• Cohesive soil with UCS of 1.5 tons per square
  foot (tsf) or more
• Cohesive soil that is predominantly clay
• Not fissured or subject to vibrations
• Not previously excavated or disturbed

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Soil TypesType B

• Type B is
• Cohesive soil with UCS gt 0.5 but lt 1.5 tsf
• Granular noncohesive soil
• Mixtures of clays, sands, and silts, where clay
  or sand are not dominant
• An A soil that is fissured or subject to
  vibration

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Soil TypesType C

• Type C is
• Cohesive soil with UCS less than 0.5 tsf
• Granular soil, including gravels
• Sandy soil where sand is dominant
• Submerged soil or seeping water
• A downgrade of Type B

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Soil Tests

• Plasticity test
• Dry strength test
• Thumb penetration test
• Pocket penetrometer

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Signs of Soil Distress

• Fissures or cracks in trench wall
• Wall of trench slumps or falls
• Soil bulges or heaves
• Excavations edge sinks
• Material ravels or trickles into excavation

Image Credit OSHA
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Soil DistressConditions

• Nearby vibrating machinery
• Nearby heavy, moving loads
• Seeping water or rain
• Hot, dry weather

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Trenching Hazards

• Any questions about specific trenching hazards?
• Any questions about cave-ins?
• Questions about hazards related to soil types?

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Trench Protective Systems

• Sloping and benching
• Shoring
• Shielding

Image Credit OSHA
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Sloping and Benching

• Slopingangling of walls at an incline
• Benchingseries of steps to angle walls
• Soil type determines angle of slope/bench
• Type A (slope ratio .751)
• Type B (ratio 11)
• Type C (benching not permitted) (ratio 1.51)

Image Credit OSHA
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Shoring

• Uses support walls to prevent a cave-in
• Usually built in place and designed by an
  engineer
• Components
• Uprights or sheeting
• Wales
• Cross braces

Image Credit OSHA
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Shielding

• Withstands forces of a cave-in and protects
  employees within
• Permanent or portable
• Trench boxes

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ShieldingTrench Boxes

• Often designed to stack
• Not for height extension
• Used with sloping and benching
• No one permitted inside when moving

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Whats Wrong Here?

• Identify the hazards
• Identify missing protective systems
• Identify missing protective measures

Osha.gov
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Trench Protective Systems

• Are there any questions about trenching hazards?
• The types of soil?
• The types of protective systems?

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Hazardous Atmospheres

• At excavations near sewers, landfills, and other
  potentially hazardous atmospheres
• Test atmosphere when deeper than 4 feet
• Ventilate or use appropriate PPE
• Maintain rescue and emergency equipment on site

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Falling Soil or Equipment

• Loose rock/soil that may fall from an excavation
  face
• Use scaling to remove loose soil
• Use shoring or shields
• Falling material or equipment
• Keep material/equipment 2 feet from edge
• Use retaining devices

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Adjacent Structures

• Excavations might endanger stability of
  buildings, walls, other structures
• Sidewalks and pavements must not be undermined
  unless supported to prevent collapse onto
  excavation workers
• Use shoring, bracing, or underpinning to ensure
  stability of nearby structures

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Water Accumulation

• Drowning hazard
• Take precautions
• Use special shoring or shield system
• Use a water removal system
• Use a safety harness and lifeline

Image Credit OSHA
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Entry and Exit

• Trenches 4 feet deep or more must have
• Entry and exit within 25 feet of worker
• Entries and exits include
• Ladder
• Stairway
• Sloped ramp

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Other Trenching Issues

• Mark all underground utilities
• Stand away from lifting/ digging equipment
• Use warning systemsor barricades
• Use hard hats

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Other Trenching Issues (cont.)

• Use fall protection systems
• No work above other workers
• Monitor excavation work

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Trench Inspections

• Inspect before work starts, throughout shift, and
  after rainstorm
• Inspect for
• Evidence of possible cave-ins
• Indications of failure of protective systems
• Potential hazardous atmosphere
• Remove workers if a hazardous condition exists

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Key Points to Remember

• Be aware of all the hazards associated with
  working around trenches
• Cave-ins occur suddenly and can entrap, bury, or
  injure
• Always use protection systems