Introduction
Trenching and excavation rank among the most hazardous activities in the construction industry. A cubic yard of soil can weigh as much as a car, close to 3,000 pounds, according to OSHA. When a trench wall collapses, workers have seconds, not minutes, to react, and the outcome is often fatal.
OSHA’s excavation standards at 29 CFR 1926 Subpart P exist specifically to prevent these deaths, and yet trench fatalities remain a persistent problem. The 12 tips below translate those standards into practical site-level habits for supervisors, competent persons, and construction workers.
Tips
1. Assign a competent person before any excavation begins
OSHA requires that a competent person classify the soil, inspect the site daily, and authorise entry into any trench or excavation. This isn’t a paperwork role: the competent person must be physically present, capable of identifying hazardous conditions, and authorised to stop work immediately if something changes.
Many incidents occur because the competent person was assigned on paper but not actively performing the role on site. Put someone in that role who understands what they’re looking for and will act on what they find.
2. Know your protective system thresholds
Trenches 5 feet (1.5 metres) deep or greater require a protective system unless the excavation is made entirely in stable rock. Below 5 feet, a competent person may determine that no system is needed, but that determination must be made actively, not assumed. For trenches 20 feet (6.1 metres) or deeper, a registered professional engineer must design the protective system.
Three options are available: sloping, shoring, and shielding. Each has specific requirements that depend on soil type, depth, and site conditions.
3. Never skip soil classification
The type of protective system required depends directly on soil classification. Type A soil (the most stable, such as clay) allows steeper slopes than Type B or Type C. Type C soil, which includes granular soils and submerged soil, is the least stable and the most common cause of unexpected cave-ins.
Soil classification cannot be done visually alone. The competent person must perform field tests, including the thumb penetration test and the ribbon test, and account for fissures, water seepage, and previous disturbance. If in doubt, classify down, not up.
4. Keep spoils and materials at least two feet from the edge
Excavated soil (spoils), equipment, and materials must be kept at least 2 feet (0.6 metres) from the edge of any trench, as required by OSHA. The weight of material placed close to the edge increases the surcharge load on the trench wall and raises the risk of collapse.
This two-foot minimum applies to all materials, not just soil: pipes, equipment, vehicles, and any other surcharge load.
5. Inspect the trench at the start of every shift and after any event that could change conditions
Inspections are required before each shift begins and after any rainstorm, earthquake, or other event that could affect trench stability. Conditions that were safe yesterday may not be safe today if there has been overnight rain, freeze-thaw cycling, or nearby heavy equipment operation.
The inspection record should note the date, time, conditions observed, and any corrective actions taken. A verbal “it looked fine” does not constitute an inspection.
6. Provide safe entry and exit at intervals no greater than 25 feet
Ladders, steps, ramps, or other safe means of entry and exit must be provided in all trenches 4 feet (1.2 metres) or deeper, and positioned so that workers do not travel more than 25 feet (7.6 metres) laterally to reach one.
This matters most in an emergency. If a worker needs to exit quickly because conditions change, the time it takes to find a ladder or scramble out is time they don’t have.
7. Test the atmosphere before entry and continuously monitor it
Trenches can accumulate hazardous atmospheres including oxygen-deficient air, carbon monoxide from nearby equipment, and toxic gases from underground utilities or decomposing organic material. According to OSHA, workers must never enter a trench until a competent person has determined the atmosphere is safe.
Use a calibrated atmospheric monitor to test for oxygen content (19.5-23.5% acceptable range), combustible gases, and toxic contaminants before anyone goes in. In trenches near utility lines, traffic, or fuel-powered equipment nearby, continuous monitoring is the appropriate standard.
8. Locate and mark all underground utilities before breaking ground
Striking an underground utility line is a leading cause of excavation fatalities and injuries. Call 811 (the national utility notification service in the US) before any digging begins to have utilities marked. This is a legal requirement in all US states.
Even after utilities are marked, assume the marks are approximate and proceed carefully near any marked location. Hand digging within 18 inches of a marked utility line is the standard safe practice.
9. Keep water out and monitor for water intrusion
Water weakens soil, increases the weight of saturated material, and dramatically increases the risk of cave-in. Standing water in a trench is a hazard that requires immediate attention: dewatering equipment should be used and the trench re-inspected after water removal before workers re-enter.
Never allow workers to work in standing water. If water is entering faster than it can be controlled, stop work and reassess the protective system before continuing.
10. Apply the three Ss of trench safety: slope it, shore it, or shield it
OSHA’s summary of trench protection comes down to three options. Sloping involves cutting the trench wall back at an angle that matches the soil’s stability. Shoring means installing hydraulic or aluminium supports to hold the walls in place. Shielding uses a trench box or trench shield to protect workers within it, though it doesn’t prevent collapse, it prevents the collapsed material from reaching workers.
Each method has technical requirements and limitations. Benching, a variant of sloping, cannot be used in Type C soil. Shoring design must match the load conditions. Trench boxes must be properly sized and positioned for the depth and soil type.
11. Keep heavy equipment away from trench edges and manage vibration
Heavy equipment operating near a trench edge creates vibration and surcharge loading that can destabilise walls. OSHA requires that such equipment be kept away from trench edges and that all sources capable of affecting trench stability be identified.
Set physical barriers if needed to enforce equipment exclusion zones. Vibration from compactors, pile drivers, or heavy traffic on nearby roads should be factored into the trench hazard assessment, particularly in loose or granular soils.
12. Train every worker, and make sure they know they can stop work
Training requirements under OSHA’s excavation standards cover the hazards specific to trenching and excavation, the protective measures in use, and workers’ rights, including the right to refuse unsafe work. Training must be provided in a language and vocabulary that workers understand.
Beyond meeting the legal training requirement, make it clear on every site that any worker who believes a trench is unsafe can raise it without consequences. The incidents that happen are usually not surprises: something changed, someone noticed, and nobody stopped.
Common Mistakes
| Mistake | Why it’s dangerous |
|---|---|
| Assuming a shallow trench (under 5 feet) is automatically safe | A competent person must still evaluate conditions; 4-foot trenches have killed workers |
| Using the same protective system for all soil types | Type C soil requires different controls than Type A; soil classification must be done first |
| Treating the two-foot spoil rule as a starting point that can be relaxed | Spoil creep near the edge directly increases wall collapse risk |
| Conducting inspections only at the start of the week | Conditions change: rain, frost, vibration, and nearby activity all require re-inspection |
| Assuming utility marks are exact locations | Marks are approximate; hand digging near marked utilities is the safe practice |
Additional Recommendations
OSHA’s excavation standard (29 CFR 1926 Subpart P) is the primary reference for US sites. The OSHA publication “Trenching Safety: 5 Things You Should Know to Stay Safe” (Publication 3974) is a useful field reference for supervisors and workers. NUCA’s annual Trench Safety Stand Down, held every June, provides site-level resources and outreach materials.
Organisations looking to reduce trench incidents should also consider conducting mock trench inspections with competent persons, testing soil classification skills before assigning the role, and reviewing any near-miss or close-call incidents involving trenching, since these often reveal systemic gaps before a fatality occurs.
If your team handles excavation and trenching work regularly, structured safety training tailored to your specific project types and soil conditions can significantly reduce the risk of incidents and help supervisors build confidence in their competent person responsibilities.
Sources
- OSHA, “Trenching and Excavation Safety”
- OSHA, “Trenching Safety: 5 Things You Should Know to Stay Safe” (Publication 3974)
- NASP, “OSHA Guidelines for Trenching and Excavation”
- Safety By Design, “OSHA Trenching and Excavation Safety Tips and Requirements”
- Work Safety 24/7, “OSHA Trenching and Excavation Safety Fact Sheet”
- Maryland OSHA, “Trench Safety Stand Down 2024”
- ESC Steel, “An Overview of Trenching and Excavation Safety Guidelines by OSHA”