OSHA excavations (Subpart P) for the CHST
A trench can fail in seconds and workers rarely get a second chance, which is why excavations get their own line in the CHST blueprint. Subpart P (29 CFR 1926.650 to 1926.652) is short, and its numbers are specific. This guide maps them in our own words, without reproducing the standard.
Check anything you rely on against your own licensed code book and the edition your AHJ enforces.
Where Subpart P sits in the blueprint
Excavations are item 3 in the Domain 1 knowledge list, and Domain 1 is 36.6% of the CHST. Expect questions on protective systems, the competent person and the conditions that change the answer, such as water or traffic.
The sections to know
- 1926.650 gives scope and definitions: trench, excavation, competent person, protective system, shield.
- 1926.651 holds the general requirements: locating underground utilities before digging, keeping spoil and equipment back from the edge (the figure is 2 feet), a way out of any trench 4 feet or deeper within 25 feet of lateral travel, testing the air where a hazardous atmosphere could exist, controls for accumulated water, and a competent person's inspection daily and after anything that changes conditions, such as rain.
- 1926.652 requires protection from cave-ins. Protection is needed unless the excavation is in stable rock, or is under 5 feet deep and a competent person finds no sign of cave-in. It then offers the choices: sloping and benching, support systems, or shield systems. The soil classification and slope tables are in Appendices A and B.
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Soil classification
The competent person classifies soil as stable rock, Type A, Type B or Type C, with Type C the weakest. Visual and manual tests decide it, and Appendix A describes them. Soil that has been disturbed before, or that sits in a trench with water, usually falls toward the weaker types. The slope you may use depends on the type, so a wrong classification is a wrong slope.
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What the competent person does
Inspects before work and as conditions change, classifies the soil, chooses the protective system, and removes workers when there are signs of a hazard. Under 1926.651(k) the inspection is a duty, not a courtesy.
Common mix-ups
- 4 feet and 5 feet are different triggers. Four feet drives egress and atmosphere testing. Five feet drives the protective system rule.
- A spotter is not a protective system. Neither is a short work time.
- Spoil belongs back from the edge. Piles at the lip add load and fall onto workers.
- Shields protect workers inside them. They do not prevent a cave-in outside the shield.
How to practice
Draw a trench and label depth, soil type, water, spoil and egress. Then ask which rule each feature triggers. The trench sloping calculator gives the maximum slope and the top width for any soil type and depth. Work questions that describe a changed condition and ask what the competent person does next.
Slope run and top width are worked with examples in the CHST formulas guide.
Related guides: fall protection (Subpart M) for edges and confined spaces (Subpart AA), since some excavations and trenches become confined spaces.
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Take the free 10-question check. It covers the four CHST domains, needs no signup and explains every answer, so you see whether this subpart is a weak spot. The CHST study guide shows where it fits in a six-week plan.
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Sources
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- 29 CFR 1926 Subpart P, Excavations, OSHA.
- CHST Examination Blueprint (CHST5, V.2022.04.12), Board of Certified Safety Professionals.
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