The Dangers of Trenches
Trenches are among the most dangerous places on a construction site:
- One cubic yard of soil can weigh more than a car (3,000+ pounds)
- A trench collapse can occur in seconds with no warning
- Survival is rare once buried – suffocation occurs quickly
- Most fatalities occur in trenches less than 12 feet deep
- Most victims are not properly trained in trench safety
Basic Trench Safety Rules
The 4-Foot Rule
- Any trench 4 feet or deeper requires a safe way to enter and exit
- Ladders, steps, or ramps must be within 25 feet of all workers
The 5-Foot Rule
- Any trench 5 feet or deeper requires a protective system
- Only exception: trenches in stable rock (rare and must be verified by a competent person)
The 20-Foot Rule
- Trenches 20 feet or deeper require protective systems designed by a registered professional engineer
Types of Protective Systems
Sloping
- Cutting back trench walls at an angle away from the excavation
- Angle depends on soil type:
- Type A soil (most stable): ¾:1 slope (53°)
- Type B soil (medium stability): 1:1 slope (45°)
- Type C soil (least stable): 1½:1 slope (34°)
Shoring
- Installing supports to prevent soil movement
- Includes hydraulic or pneumatic shores, screw jacks, or timber shoring
- Must be installed from the top down
- Must be removed from the bottom up
Shielding
- Using trench boxes or shields to protect workers
- Does NOT prevent cave-ins, but protects workers inside the shield
- Must extend 18 inches above the trench if there's a potential for material to roll in
- Workers must stay inside the protected area
Benching
- Creating steps or benches in the trench wall
- Only for Type A and B soils (not Type C)
- Each step must be level
- Height cannot exceed width
Soil Types and Testing
Type A Soil (Most Stable)
- Clay, silty clay, sandy clay
- Has high unconfined compressive strength
- Not previously disturbed
Type B Soil (Medium Stability)
- Silt, sandy loam, unstable dry rock
- Previously disturbed Type A soil
- Medium unconfined compressive strength
Type C Soil (Least Stable)
- Gravel, sand, loamy sand
- Submerged soil or soil with water seeping in
- Unstable rock
- Low unconfined compressive strength
Soil Testing Methods
A competent person must test the soil using:
- Visual test (checking for cracks, water, layering)
- Thumb penetration test
- Pocket penetrometer or shearvane
- Ribbon test
Other Trench Hazards
Atmosphere
- Low oxygen levels
- Toxic gases (carbon monoxide, hydrogen sulfide)
- Flammable gases
- Test the air before entering trenches 4+ feet deep
Water Accumulation
- Never work in trenches with accumulated water
- Use pumps to remove water
- Inspect after rainstorms
- Divert water away from excavations
Adjacent Structures
- Vibration can cause trench failure
- Keep heavy equipment away from trench edges
- Support nearby structures if necessary
- Monitor for signs of movement
Falling Loads
- Keep spoil piles at least 2 feet from trench edges
- Never work under suspended loads
- Wear hard hats at all times
Daily Inspection Requirements
A competent person must inspect trenches:
- Before work begins
- Throughout the work shift
- After precipitation
- After any significant change in conditions
Look for:
- Signs of possible collapse (cracks, bulging)
- Proper protective systems
- Hazardous atmospheres
- Water accumulation
- Safe access and egress
Emergency Response
If a cave-in occurs:
- Do NOT enter the trench to attempt rescue
- Call 911 immediately
- Keep unnecessary personnel away
- Turn off all nearby equipment
- If safe to do so, try to stop any water flow into the trench
- Note the victim's last known position for rescuers