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Trenching and Shoring29 CFR 1926.650
A Trenching Tragedy
False sense of security Knew they were out of compliance Thought the soil was stable Conditions changed overnight A worker died
Trenching Statistics
About 400 U.S. workers die in trench-related accidents each year
About 6,400 are seriously injured
Trenching and Shoring Goals
Hazards, soil, protective systems Safe work practices and hazard awareness Quiz
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
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
Soil weighs 125 lbs. per cubic foot
A worker can be crushed by soil, rock, or an object
Suffocation—even if worker’s head is not buried, soil prevents chest expansion
Immobilized by soil’s suction effect
Cave-in Injuries
Soil Classification
Grain size Saturation Cohesiveness Unconfined compressive strength
Soil Types
Type A (most stable)—dense and heavy clay Type B-silt, sandy loam, medium clay Type C (least stable) —gravel, loamy sand,
soft clay
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 (3/4:1)–Type B: 4 feet horizontal to 4 feet vertical (1:1)–Type C: 6 feet horizontal to 4 feet vertical (1-1/2:1)–Benching not permitted for Type C soil
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
Shielding
Withstands forces of a cave-in and protects employees within
Permanent or portable Trench boxes
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
Trenching and Shoring Goals
Hazards, soil, protective systems Safe work practices and hazard awareness Quiz
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
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 The sinking of excavation’s edge Ravelling, or small amounts of material
(i.e., pebbles) trickling into excavation
Conditions Causing Soil Distress
Nearby vibrating machinery Nearby heavy, moving loads Seeping water or rain Hot, dry weather
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
Falling Soil or Equipment
Protect workers from loose rock/soil that may fall from an excavation face
–Scaling to remove loose soil–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
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
Water Accumulation
Workers have drowned in the water at the bottom of a trench or excavation
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
Other Trenching Issues
Mark underground utilities Stand away from lifting/digging equipment Use of warning systems or barricades Use hard hats
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 Worker on top watches excavation walls to warn
trench workers of potential hazards
Trenching and Shoring Goals
Hazards, soil, protective systems Safe work practices and hazard awareness Quiz
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
Quiz
1. Describe two signs of soil distress:__________________________________________,__________________________________________.
2. Describe why a worker buried up to the neck would not be able to breathe:__________________________________________.
3. When working in a 4-foot trench, there must be an exit within 25 feet.
True or False
4. Shielding is designed to prevent an excavated wall from caving in.
True or False5. Name a portable device used for shielding:
__________________________________________.
Quiz (cont.)
6. How does water or rain impact the classification of soil?
___________________________________
7. Trenches near landfills may not contain enough oxygen to support life.
True or False
8. Describe a way to protect trench workers from falling soil or objects:_______________________________
9. Excavations need to be inspected only right after they are first dug.
True or False
10. If you don’t know the soil type, what slope angle should you use to be safe?_____________________________
Quiz Answers
1. Signs of soil distress include cracks, slumping, bulging, sinking edge, or trickling pebbles.
2. The soil exerts about 800 lbs. of pressure on the chest, which prevents expansion.
3. True.
4. False. Shoring prevent cave-ins and shielding protects workers from a cave-in.
5. A trench box is a portable device that is usedfor shielding.
Quiz Answers (cont.)
6. Rain or water decreases the stability of soil. Saturated soil can be very unstable.
7. True. The trench might be filled with a heavy gas (from the landfill) that displaces oxygen.
8. Remove loose soil by scaling, provide protective barriers, keep material 2 feet from trench’s edge.
9. False. Inspect excavations daily, throughout the shift, and after conditions change.
10. 1-1/2 feet horizontal for every vertical foot. So a 10-foot-deep-trench would slope out 15 feet.