STSC Excavation and Trenching Safety 2 — Questions and Answers
Question 1: What is a 'competent person' required to do regarding excavation inspections under OSHA Subpart P?
- Inspect the excavation once per week
- Inspect before the start of work, as needed during the shift, and after rain or other events that could increase hazard (Correct answer)
- Inspect only when requested by workers or the project manager
- Inspect at the start and end of each day only
Correct answer: Inspect before the start of work, as needed during the shift, and after rain or other events that could increase hazard
OSHA 1926.651(k)(1) requires the competent person to inspect excavations, adjacent areas, and protective systems daily before work and as needed throughout the shift — especially after rain, freezing, or other hazard-increasing events.
Per OSHA 1926.651(k)(1), a competent person must inspect excavations and nearby areas for evidence of hazardous conditions (tension cracks, subsidence, water accumulation, frost, fissuring) before the start of work each day, as needed throughout the shift, and after every rain storm, seismic activity, or other occurrence that could increase hazard. If hazardous conditions are found, work must be stopped and workers must leave the hazard zone until the competent person has made the necessary safety corrections. These are ongoing responsibilities — a single morning inspection does not satisfy the requirement if conditions change.
Question 2: When is atmospheric testing required before workers enter an excavation?
- Only for excavations near industrial facilities
- When the excavation is 20 feet or more deep
- When the excavation may reasonably be expected to have an atmosphere containing harmful levels of gases, vapors, or oxygen deficiency (Correct answer)
- Only for excavations in confined space locations (indoors)
Correct answer: When the excavation may reasonably be expected to have an atmosphere containing harmful levels of gases, vapors, or oxygen deficiency
OSHA 1926.651(g)(1) requires atmospheric testing before workers enter any excavation that could reasonably be expected to have a hazardous atmosphere — including areas near landfills, fuel lines, sewer systems, or excavations where biological decomposition could occur.
Per OSHA 1926.651(g)(1), when any possibility of a hazardous atmosphere exists in an excavation, atmospheric testing must be performed before entry and continuously monitored during work. Sources of hazardous atmospheres in excavations include: methane from decaying organic matter or landfill migration; carbon monoxide from generator exhaust; hydrogen sulfide from sewers and organic decomposition; oxygen displacement by nitrogen or CO2; and vapor from nearby chemical spills. Excavations that become confined spaces (having limited entry/exit and meeting PRCS criteria) must follow the full confined space entry program under 1926 Subpart AA.
Question 3: A trench is being excavated in Type B soil. The trench is 10 feet deep. What is the required horizontal setback (slope) for each side of the trench?
- 3.75 feet from the base on each side (3/4:1 ratio)
- 5 feet from the base on each side (1:1 ratio) (Correct answer)
- 7.5 feet from the base on each side (3/2:1 ratio)
- No setback required with proper shoring
Correct answer: 5 feet from the base on each side (1:1 ratio)
Type B soil requires a slope of 1:1 (horizontal to vertical). For a 10-foot deep trench, each side must be set back 10 feet horizontally from the base — 5 feet measured from the trench centerline if full-width sloping is used.
OSHA Appendix B to Subpart P specifies that Type B soil must be sloped at 1:1 (45°) — for every 1 foot of vertical depth, the slope must extend 1 foot horizontally. For a 10-foot deep trench in Type B soil: (1) each wall must slope back 10 feet horizontally from the bottom; (2) the total trench width at the top would be the bottom width plus 20 feet (10 feet on each side). Alternative protection systems (shoring, trench box) may be used instead of sloping when slope width is impractical due to site constraints. Any combination of sloping, benching, and protective systems must be designed by a qualified person.
Question 4: What is the OSHA definition of a 'trench' as distinct from a general 'excavation'?
- Any underground opening more than 2 feet wide
- An excavation that is narrower than it is deep, with a maximum width of 15 feet at the bottom (Correct answer)
- Any excavation made by mechanical equipment
- An excavation specifically for utility installation
Correct answer: An excavation that is narrower than it is deep, with a maximum width of 15 feet at the bottom
OSHA 1926.650 defines a trench as a narrow excavation in which the depth is greater than the width, with a maximum width of 15 feet measured at the bottom.
Per OSHA 1926.650, an excavation is any man-made cut, cavity, trench, or depression made in the earth's surface. A trench is specifically defined as an excavation that is: (1) narrower than it is deep (depth greater than width), and (2) no wider than 15 feet at the bottom. Excavations wider than 15 feet at the bottom are classified as general excavations rather than trenches, though both are subject to Subpart P protective requirements when 5 feet or more deep. The distinction matters for regulatory application and for hazard assessment, as trenches have unique cave-in dynamics due to their relative dimensions.
Question 5: A trench protection system uses a hydraulic trench shield (trench box). Where must workers be positioned relative to the shield when working in the trench?
- Workers may work anywhere in the trench as long as they are supervised
- Workers must remain inside the protective shield at all times during excavation operations (Correct answer)
- Workers must stay within 2 feet of the shield at all times
- Workers may exit the shield to place bedding material at the trench bottom
Correct answer: Workers must remain inside the protective shield at all times during excavation operations
The entire purpose of a trench shield is to protect workers from cave-ins. Workers must remain inside the shield at all times during excavation — working outside the shield's protection defeats its purpose.
Trench shields (boxes) protect workers by transferring the load of a cave-in over the shield structure rather than onto the workers. However, protection only exists within the confines of the shield. Workers who exit the shield to work at the bottom of the trench outside the shielded area, or who work at the ends of the shield beyond its protection, are fully exposed to cave-in hazard. OSHA 1926.652(g)(1)(i) requires shields to be designed to resist the forces that may be imposed on them, and workers must remain within the protected zone. Shields must also be installed and extracted safely — never with workers in the trench outside the shield zone.
Question 6: After a significant rainfall, the competent person inspects a trench in Type A soil and finds that water has accumulated in the bottom of the trench and the walls show some cracking. What must happen?
- Workers may enter if they wear rubber boots
- The trench must be reclassified as at most Type C, water must be controlled, and the trench re-evaluated before workers re-enter (Correct answer)
- Workers may enter immediately if the water depth is less than 6 inches
- Type A classification can be maintained if additional shoring is added while workers are in the trench
Correct answer: The trench must be reclassified as at most Type C, water must be controlled, and the trench re-evaluated before workers re-enter
Water intrusion and cracking downgrade soil classification. Soil that was Type A can become Type C when subjected to water saturation. The trench must be reclassified, water controlled, and the protective system re-evaluated before worker re-entry.
Per OSHA Appendix B to Subpart P, soil exposed to moisture that alters its physical properties must be reclassified. Standing water in a trench indicates that pore water pressure has increased, which reduces effective stress and cohesion — key determinants of soil stability. Cracking is a visual indicator of soil distress. Soil that was Type A must be treated as Type C until re-tested or demonstrated to be stable after drying. OSHA 1926.651(h)(1) specifically prohibits workers from working in excavations where water has accumulated, unless water is being controlled and workers are protected from hazards posed by water accumulation.
STSC Excavation and Trenching Safety 2
What is a 'competent person' required to do regarding excavation inspections under OSHA Subpart P?