Trenching and Shoring

1
Trenching and Shoring

• 29 CFR 1926.650

2
A Trenching Tragedy

• Had a false sense of security
• Knew they were out of compliance
• Thought the soil was stable
• Conditions changed overnight
• A worker died

3
Trenching Statistics

• About 400 U.S. workers die in trench-related
  accidents each year
• About 6,400 are seriously injured

4
Trenching and Shoring Goals

• Hazards, soil, protective systems
• Safe work practices and hazard awareness

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What Is a Trench?

• A narrow excavationthat is deeper than it is
  wide
• No more than 15 feet wide at bottom
• Walls will eventually fail

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What Is a Cave-in?

• Soil or rock that suddenly falls or slides into
  an excavation
• Sufficient quantity to entrap, bury, injure, or
  immobilize
• Soil gravitates downward, pressure pushes soil
  inward toward the trench
• Bottom third of wall typically fails first
• Soil above the collapsed lower wall follows

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Cave-in Injuries

• Soil weighs 125 lbs. per cubic foot
• A worker can be crushed by soil, rock, or an
  object
• Suffocationeven if workers head is not
  buried, soil prevents chest expansion
• Worker becomes immobilized by soils suction
  effect

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Soil Classification

• Grain size
• Saturation
• Cohesiveness
• Unconfined compressive strength

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Soil Types

• Type A (most stable)dense and heavy clay
• Type Bsilt, sandy loam, medium clay
• Type C (least stable)gravel, loamy sand, soft
  clay

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Sloping and Benching

• Sloping angling of walls at an incline
• Benching series of steps to angle walls
• Soil type determines angle of slope/bench
• Type A 3 feet horizontal to 4 feet vertical
  (3/41)
• Type B 4 feet horizontal to 4 feet vertical
  (11)
• Type C 6 feet horizontal to 4 feet vertical
  (1-1/21)
• Benching not permitted for Type C soil

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Shoring

• Support walls designed to prevent cave-in
• Usually built in place and designed by an
  engineer
• Components include uprights (sheeting), wales,
  and cross braces

12
Shielding

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

13
Trench Boxes

• Often designed to stack
• Never use sheeting to extend the height
• Can be used in conjunction with sloping and
  benching
• No one permitted inside when being raised or
  lowered

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Trenching and Shoring Goals

• Hazards, soil, protective systems
• Safe work practices and hazard awareness
• Quiz

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Excavation Inspections

• Inspections conducted before work starts,
  throughout shift, after rainstorm
• Excavations inspected for
• Evidence of possible cave-ins
• Indications of failure of protective systems
• Potential hazardous atmosphere
• If hazardous condition found, workers are
  removed

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Signs of Soil Distress

• Fissures or cracks on excavation face
• Slumping of material from excavation face
• Bulging or heaving of material at the bottom of
  excavation wall
• Sinking of excavations edge
• Ravelling, or small amounts of material (e.g.,
  pebbles) trickling into excavation

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Conditions Causing Soil Distress

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

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Hazardous Atmospheres

• Excavations near sewers, landfills, hazardous
  substances storage area
• Test atmosphere when deeper than 4 feet
• Ventilation or appropriate PPE
• Rescue and emergency equipment

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Falling Soil or Equipment

• Protect workers from loose rock/soil that may
  fall from an excavation face
• Use scaling to remove loose soil
• Use protective barricades, such as shoring or
  shields
• Protect workers from material or equipment that
  could fall into the excavation
• 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, pavement not undermined unless
  supported to prevent collapse on excavation
  workers
• Shoring, bracing, or underpinning used to ensure
  stability for employee protection

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Water Accumulation

• Never work in an excavation where water is
  accumulating without proper precautions
• Special shoring or shield system
• Water removal system
• Use of safety harness and lifeline

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Other Trenching Issues

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

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Other Trenching Issues (cont.)

• Trenches 4 feet deep or more must have exit means
  within 25 feet of every worker
• Use fall protection
• Do not work on sides of sloped or benched
  excavation above other workers
• For parallel construction have a worker on top to
  watch excavation walls to warn trench workers of
  potential hazards

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Trenching and Shoring Goals

• Hazards, soil, protective systems
• Safe work practices and hazard awareness

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Summary

• Cave-ins occur suddenly and can entrap, bury, or
  injure
• Soils have varying stability that determines the
  appropriate protection
• Always use protection systems
• Be aware of signs of soil distress
• Be aware of all the hazards associated with
  working around excavations