Trenching

1. Trenching Competent Person

2. 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

3. Session Objectives • As a “competent person,” you will be able to: • Recognize trenching hazards • Understand protective systems for trenches • Understand hazards and preventive measures for activities associated with excavation

4. 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

5. What Is an Excavation?What Is a Trench? • Excavation—any hole or trench made by removing earth • Trench—a narrow excavation that is deeper than it is wide, no greater than 15 feet wide at bottom • Trench walls will eventually fail

6. 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

7. Consequences of a Cave-in • Soil is dense and heavy • Cave-ins can entrap, bury, injure, or immobilize • Suffocation—soil prevents chest expansion • The worker becomes immobilized

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

9. Soil Stability • Soil stability affected by: • Grain size—small is better • Water saturation • Cohesiveness—how well soil sticks together • Unconfined compressive strength (UCS)—how easily the soil will shear

10. Soil Types—Type 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

11. Soil Types—Type B • Type B is: • Cohesive soil with UCS >0.5 but <1.5 tsf • Granular, cohesionless soil • Previously disturbed soils except those which would otherwise be classified asType C soil • An “A” soil that is fissured or subject to vibration • Dry rock that is not stable

12. Soil Types—Type 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

13. Soil Tests • Plasticity test • Dry strength test • Thumb penetration test • Pocket penetrometer

14. Signs of Soil Distress • Fissures or cracks in trench wall • Wall of trench slumps or falls • Soil bulges or heaves • Excavation’s edge sinks • Material ravels or trickles into excavation Image Credit: OSHA

15. Soil Distress—Conditions • Nearby vibrating machinery • Nearby moving, heavy loads • Seeping water or rain • Hot, dry weather

16. Trenching Hazards • Do you understand specific trenching hazards? • About cave-ins? • About hazards related to soil types?

17. Trench Protective Systems • Sloping and benching • Shoring • Shielding Image Credit: OSHA

18. 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 (slope ratio .75:1) • Type B (ratio 1:1) • Type C (benching not permitted) (ratio 1.5:1) Image Credit: OSHA

19. 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

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

21. Shielding—Trench Boxes • Often designed to stack • Not for height extension • Used with sloping and benching • No one permitted inside when moving

22. What’s Wrong Here? • Identify the hazards • Identify missing protective systemsand measures Osha.gov

23. Trench Protective Systems • Do you understand trenching hazards? • The types of soil? • The types of protective systems?

24. 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

25. 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

26. 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

27. Water Accumulation • Drowning hazard • Take precautions • Use special shoring or shielding system • Use a water removal system • Use a safety harness and lifeline Image Credit: OSHA

28. Entry and Exit • Trenches 4 feet deep or more must have: • Entry and exit within 25 feet of worker • Entries and exits include: • Ladders • Stairways • Sloped ramps

29. Other Trenching Issues • Mark all underground utilities • Stand away from lifting/ digging equipment • Use warning systemsor barricades • Use hard hats

30. Other Trenching Issues (cont.) • Use fall protection systems • No work above other workers • Monitor excavation work

31. 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

32. 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