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.