1.4 4, 5, 6 and 20 feet: every threshold in Subpart P
Subpart P has four numbers that change what you owe: 4 feet, 5 feet, 6 feet and 20 feet. They come from different paragraphs, they do different things, and mixing them up is how a crew ends up with a compliant slope and no ladder. This chapter puts every depth threshold in the standard in one table, then explains the three that get confused most.
1 Every threshold in one place
| Depth | What it triggers | Paragraph |
|---|---|---|
| Any depth | Protective system for each employee, unless one of the two exceptions applies | 1926.652(a)(1) |
| Any depth | Underground installations located before opening the excavation; daily inspection by a competent person where exposure is anticipated; spoil and equipment 2 ft back | 651(b)(1), 651(k)(1), 651(j)(2) |
| 4 ft or more (trenches) | Stairway, ladder, ramp or other safe means of egress, no more than 25 ft of lateral travel away | 651(c)(2) |
| Greater than 4 ft | Atmosphere tested before entry where oxygen deficiency (<19.5%) or another hazardous atmosphere exists or could reasonably be expected | 651(g)(1)(i) |
| Less than 5 ft | The only depth-based exception to the protective system, and only with a competent person's examination | 652(a)(1)(ii) |
| 6 ft or more | Guardrails per 1926.502(b) where a walkway crosses over the excavation | 651(l) |
| 6 ft or more (depth) | Fall protection at excavation edges not readily seen, and at wells, pits and shafts (Subpart M) | 1926.501(b)(7) |
| Up to 8 ft | Type A: unsupported vertically sided lower portion allowed, maximum 3½ ft vertical | App. B Fig. B-1.1 |
| 12 ft or less | Type A short-term slope of ½:1 (63°) for excavations open 24 hours or less; 8-12 ft with an unsupported vertical side needs 1:1 and a 3½ ft maximum vertical | App. B Table B-1 note 2, Fig. B-1.1 |
| Less than 20 ft | The range Table B-1's maximum allowable slopes cover | App. B Table B-1 |
| Greater than 20 ft | Sloping or benching designed by a registered professional engineer | App. B Table B-1 note 3 |
| 2 ft below the bottom of a shield or support member | Maximum extra excavation allowed, and only under conditions | 652(g)(2), 652(e)(2)(i) |
| 18 in above the top of a vertical side | How far a shield or support must extend when the lower portion is vertical | App. B Fig. B-1 |
Two patterns are worth noticing. Several duties have no depth at all — locating utilities, the daily inspection, the spoil setback. And the numbers that do exist are not a ladder of increasing severity; they answer different questions. 4 feet is about getting out and about air. 5 feet is about cave-ins. 6 feet is about falling. 20 feet is about who signs the design.
2 Why 4 feet appears twice, and means two things
1926.651(c)(2) is about escape: in trench excavations 4 feet or more in depth, a stairway, ladder, ramp or other safe means of egress must be located so as to require no more than 25 feet of lateral travel. It applies to trenches, and it is measured in walking distance, not straight-line distance.
1926.651(g)(1)(i) is about air: where oxygen deficiency or a hazardous atmosphere exists or could reasonably be expected — the standard names landfill areas and excavations near stored hazardous substances — the atmosphere must be tested before employees enter excavations greater than 4 feet in depth. It applies to excavations generally, and it is conditional on the hazard being foreseeable.
So the same measurement triggers a ladder in every trench, and a gas meter only where bad air is plausible. Crews that hear "four feet" and think only of the ladder miss the second one on exactly the jobs where it matters: a dig in old fill, next to a fuel tank, downhill from a landfill, or into a sewer.
3 The 20-foot line, and what it really costs
Note 3 to Table B-1 is one sentence: "Sloping or benching for excavations greater than 20 feet deep shall be designed by a registered professional engineer." Table B-1 itself is titled for excavations less than 20 feet deep, which is why the tables stop being useful right where the geometry gets expensive.
Do the arithmetic once and the 20-foot line explains itself. In Type C soil at 1½:1, a 20-foot excavation is cut back 30 feet on each side. A 3-foot-wide trench becomes 63 feet of disturbed ground. Past that depth the standard stops offering a table because there is no generic answer: the loads, the groundwater and the surcharges have to be looked at by an engineer.
| Depth | Type A (¾:1) | Type B (1:1) | Type C (1½:1) |
|---|---|---|---|
| 5 ft | 3 ft 9 in | 5 ft | 7 ft 6 in |
| 8 ft | 6 ft | 8 ft | 12 ft |
| 12 ft | 9 ft | 12 ft | 18 ft |
| 16 ft | 12 ft | 16 ft | 24 ft |
| 20 ft | 15 ft | 20 ft | 30 ft |
| Over 20 ft | Registered professional engineer designs the slope or bench | ||
Total top width is the trench width plus twice the cut-back. That column is why shoring and trench boxes exist, and it is the number to put in front of anyone who says sloping is always cheaper.
4 Measuring depth honestly
Thresholds are only as good as the measurement. Four rules of thumb keep the number defensible:
- Measure at the deepest point where someone will be, not the average. Trench bottoms fall toward the main; bell holes are deeper than the run.
- Measure from the working surface people stand on at the top, and remember that a spoil pile does not raise the ground level — but it does add a surcharge load.
- Re-measure when the grade changes. The trench that started at 4 ft 6 in is over 5 feet somewhere down the run, and nobody announces it.
- Write it down. The depth belongs on the daily inspection record with the soil type and the system, so the next shift inherits a number rather than an impression.
5 A worked example
Water service replacement in a residential street. The trench runs 70 feet, starts at 4 feet at the main and finishes at 7 feet at the house. Dry sand with gravel. There is an asphalt driveway 4 feet off one side and a parked backhoe near the other. Which thresholds fire?
- 1Protective system: yes, at least where it exceeds 5 feet — and under 5 feet only if a competent person examines the ground and finds no indication of a cave-in. Sand and gravel is granular, which is Type C.
- 2Egress: yes from the 4-foot end onward — a ladder within 25 feet of lateral travel of every person in the trench, moved as the work advances.
- 3Atmosphere: not obviously foreseeable on a clean water line in native soil, but ask what else is buried there; old fill or a nearby fuel tank changes the answer.
- 4Sloping route: Type C at 1½:1 needs 10 ft 6 in of cut-back per side at 7 feet. With a driveway 4 feet away, the slope does not fit — this trench wants shoring or a box.
- 5Surcharge: the driveway and the backhoe both load the wall. Appendix B (c)(3)(iii) makes the competent person reduce the actual slope below the maximum allowable, which only makes the geometry worse.
- 6Spoil: 2 feet minimum from the edge, and further back if the pile is large — 651(j)(2).
That is the same fact pattern as MIFACE 17MI127, where the wall against the driveway came in on two men. The thresholds are not paperwork; they are the difference between that trench and this one.
6 Where Part 2 picks up
Every number in this chapter that involves a slope depends on one input: the soil type. Table B-1 is unusable until somebody classifies the ground, and the person who classifies it has to be a competent person doing at least two kinds of analysis. That is Part 2, and it is the single most skipped requirement in Subpart P.
- Locating utilities, the daily inspection and the 2-foot spoil setback have no depth threshold at all.
- 4 feet triggers two different rules: egress within 25 feet of lateral travel in trenches, and atmospheric testing where bad air is foreseeable.
- 5 feet appears only inside the exception to the protective system requirement, never as the trigger.
- 6 feet brings guardrails on a walkway over the excavation, and fall protection duties from Subpart M.
- Past 20 feet, sloping or benching must be designed by a registered professional engineer — and at 20 feet Type C already means 30 feet of cut-back per side.
- Measure depth where people work, re-measure as the grade falls, and never measure from the top of the spoil pile.
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