Trenching and Shoring 29 CFR 1926.650

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

Cave-in Injuries • 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

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

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

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 False 5. 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.

Trenching and Shoring 29 CFR 1926.650