Quick answer. 29 CFR 1926 Subpart P is OSHA's excavation standard: 1926.650 (scope and definitions), 1926.651 (general requirements) and 1926.652 (protective systems), plus Appendices A to F. Every excavation 5 feet or deeper needs a protective system unless it is made entirely in stable rock; under 5 feet, only when the competent person sees signs of a possible cave-in (1926.652(a)(1)). When a trench box or shoring protects a vertical lower wall with a slope above it, it must extend at least 18 inches above the top of the vertical side (Appendix B), and OSHA has said the standard does not require a shield to extend above grade (interpretation of August 10, 2000).
The numbers
| Number | Rule | Where |
|---|---|---|
| 5 feet | At 5 feet deep, a protective system is required: slope it, bench it, shore it or shield it. Under 5 feet, only if the competent person sees a potential for cave-in. The one exception is an excavation entirely in stable rock. | 1926.652(a)(1) |
| 4 feet | At 4 feet deep, a way out: a ladder, stairway or ramp within 25 feet of lateral travel for every worker in the trench. Also at 4 feet, test the air where a hazardous atmosphere could exist. | 1926.651(c)(2), 1926.651(g)(1) |
| 25 feet | The farthest any worker can be from the ladder. | 1926.651(c)(2) |
| 2 feet | Spoils, materials and equipment stay at least 2 feet back from the edge, or behind a retaining device. | 1926.651(j)(2) |
| 3 feet | The ladder extends 3 feet above the edge and is secured. | 1926.1053(b)(1) |
| 20 feet | Deeper than 20 feet, the sloping or the support system has to be designed by a registered professional engineer. The tables in the appendices stop at 20. | Appendix F; 1926.652(b)(4), (c)(4) |
| 18 inches | When a trench box is combined with sloping above it, the box extends at least 18 inches above the bottom of the slope. | Appendix B, Figure B-1 |
| 6 feet | Walkways over an excavation need guardrails when the drop is 6 feet or more. | 1926.651(l) |
Before the first bucket
- Call 811 and wait the time your state requires. Two working days in Texas and California, three in several others. The ticket number goes on the JHA. The rule itself (1926.651(b)) says you locate the utilities before you open the ground, contact the owners, and expose them by safe means as you get close.
- Treat the marks as approximate. Inside the tolerance zone, 18 to 24 inches either side of the mark depending on the state, dig by hand or with vacuum excavation. Gas and electric lines are hit every day by crews that trusted the paint.
- Look at what is next to the hole. Buildings, footings, poles, sidewalks and pavement near the edge need support or a plan before you undermine them (1926.651(i)). Surface things that could fall in are removed or supported (1926.651(a)).
- Decide the protective system before the crew arrives, not after the trench is open. Depth, soil, water, the equipment you have, how long it stays open. If the answer is a trench box, the box and its tabulated data are on site before the first bucket.
- Name the competent person on the JHA. And make sure that person will actually be there every shift.
Soil types: A, B and C
Every protective system choice starts with the soil type, and the competent person decides it with at least one visual test and at least one manual test (Appendix A). The visual test is the cut face, the layering, the water, the spoil, whether the ground has been dug before. The manual test is the thumb penetration, the pocket penetrometer, the ribbon test, or a dry-strength test on a sample.
| Type | What it is | Maximum slope (Appendix B) |
|---|---|---|
| A | Hard, cohesive soil: clay, silty clay, sandy clay, hardpan, with an unconfined compressive strength of 1.5 tons per square foot or more. It stops being Type A if it is fissured, if it is near a source of vibration, if it has been disturbed before, if water is seeping, or if it sits in layers that dip into the trench. | ¾ horizontal to 1 vertical (53°) |
| B | Cohesive soil between 0.5 and 1.5 tons per square foot, angular gravel, silt, silt loam, sandy loam, dry unstable rock, and previously disturbed soil that is not Type C. | 1 to 1 (45°) |
| C | Cohesive soil under 0.5 tons per square foot, gravel, sand, loamy sand, any submerged soil or soil with water seeping in, submerged rock that is not stable, and layered systems that dip into the excavation at 4 to 1 or steeper. | 1½ to 1 (34°) |
Three things worth memorizing. Clay is not automatically Type A; most of what you dig has been disturbed before, and previously disturbed soil is at best Type B. Any water in the trench makes it Type C. And when the competent person is not sure, it is Type C. The slope tables, the shoring tables and the trench box's tabulated data all get more conservative as you go from A to C, so calling it C costs you a wider trench or a heavier box; calling it A when it is not can cost you a worker.
Sloping, shoring and boxes
- Sloping and benching (1926.652(b)). Cut the walls back to the angle for the soil type, or bench them in Type A and B soil (never in Type C). Needs room: a 10-foot-deep trench in Type C soil is 30 feet wider at the top than at the bottom. The simplest option when you have the space, and the one most often done wrong, because the slope flattens out as the day goes on and nobody re-measures.
- Shoring (1926.652(c), Appendices C and D). Timber or aluminum hydraulic shores holding the walls in place. Installed from the top down and removed from the bottom up, and in between, nobody is in the trench ahead of the shoring. Aluminum hydraulic shoring is fast and light, and comes with manufacturer's tabulated data that tells you spacing by depth and soil type.
- Shields, meaning trench boxes (1926.652(g)). The box does not hold the wall up; it protects the people inside it when the wall comes in. So the rules are about staying inside it: nobody in the box while it is being moved or set, nobody outside the box in the protected zone, the box rated for the depth and soil type (that is what the tabulated data on the plate is for), the box restricted from moving sideways, and a way in and out that doesn't mean climbing the wall. The excavation can go at most 2 feet below the bottom of the box, and only if the box is rated for the full depth and there is no sign of soil moving out from under it. Voids behind the box get backfilled.
- Engineered systems for anything deeper than 20 feet, anything the tables don't cover, and anything near a structure the tables weren't written for. A registered professional engineer designs it and the design is on site.
The box is a last line, not a reason to stop thinking. A box set in a trench with a 6-foot stack of spoil at the edge, a running excavator behind it and water in the bottom is a box in a trench that is actively trying to fail. Everything else in this guide still applies with the box in.
The competent person's day
1926.651(k) requires the competent person to inspect the excavation, the adjacent areas and the protective systems daily, before anyone enters, and as needed through the shift, and after every rainstorm or anything else that increases the hazard. When there is evidence of a possible cave-in, a failing protective system, a hazardous atmosphere or any other hazardous condition, exposed workers come out until it is fixed. In practice, every morning:
- Tension cracks along the top of the wall, sloughing or spalling on the face, bulging at the toe.
- Water: in the bottom, seeping from the face, pooling at the edge.
- Spoil pile set back 2 feet, equipment and material away from the edge, nothing surcharging the wall.
- The protective system: slope still at the angle, shores still tight and at the right spacing, box still at the right depth with no voids, 18 inches above the vertical cut.
- Ladders: within 25 feet, 3 feet above, tied off, in the box not outside it.
- Air test where required, and the meter calibrated.
- Utilities exposed and supported where they cross.
- The weather for the day and the plan if it changes.
Written down, with the date, time, soil type, protective system, findings and a signature, then again after rain, a vibration source starting, a utility strike, a change in depth, or anything else that changes the ground.
Water, air, loads and traffic
- Water (1926.651(h)). Nobody works in a trench with water accumulating in it unless there are precautions: a shield or support system designed for it, pumps watched by the competent person, a harness and lifeline. Runoff from the surface gets diverted. After rain, the competent person inspects before anyone goes back in.
- Air (1926.651(g)). In excavations deeper than 4 feet where a hazardous atmosphere could be present, near landfills, sewers, fuel tanks and gas lines, test for oxygen and flammable or toxic gas before entry and as often as needed. Below 19.5% oxygen or above 20% of the lower flammable limit, ventilate before entry. Where that kind of atmosphere could exist, rescue equipment is at the trench: breathing apparatus, a harness and line, a basket stretcher.
- Loads (1926.651(e)). Nobody under a suspended load, and nobody in the swing of the bucket. Workers stand clear of trucks being loaded and unloaded.
- Mobile equipment (1926.651(f)). When an operator cannot see the edge, a warning system: a spotter, barricades, stop logs, or the grade sloped away from the trench.
- Traffic (1926.651(d)). Workers exposed to traffic wear high-visibility garments, and the work zone has a traffic plan under 1926 Subpart G.
- Falls (1926.651(l)). Walkways over the trench where people have to cross, with guardrails if the fall is 6 feet or more. Barricades at remote excavations and wells.
What happens when OSHA drives by
Since October 2018, under OSHA's National Emphasis Program on trenching and excavation (CPL 02-00-161), any inspector who sees an open trench from the road can stop and open an inspection on the spot. No complaint, no referral needed. A crew in an unprotected trench is the single easiest willful citation in construction, and the 2026 maximum for a willful violation is $165,514 per violation; with a serious violation at $16,550 and the egress, spoil and inspection items stacking on top. Cal/OSHA requires a permit before you dig any trench 5 feet or deeper that a person will enter, on top of all of this.
The mistakes in every cave-in report
- "He was only going to be in there a minute." Hooking up the pipe, grabbing the laser, checking grade. The wall does not know how long you planned to stay.
- The box was on the trailer, or in the yard, or on the other job.
- The box was in the trench but 3 feet short of the bottom, or rated for Type B in Type C soil.
- The ladder was outside the box, or 60 feet away, or there wasn't one.
- The spoil pile was at the edge and the excavator was parked on top of it.
- It rained the night before and nobody looked again.
- The operator decided the soil was Type A because it was "hard clay." It had been dug for the old line twenty years ago.
- The competent person was at the other site.
- The 811 marks were a day old and the crew potholed with the bucket.
The trench JHA, the written program and the daily inspection, on the foreman's phone
29Safety's excavation and trenching program is written to Subpart P with the state additions, the JHA library has the trench steps and hazards already rated before and after controls, and the competent person's daily inspection is a form on the phone with photos, soil type and the protective system, timestamped and filed under the job.
See exactly what you'd get
The JHA plus a toolbox talk record, an inspection and more, in one PDF: five documents for a jobsite crew, or three for a plant. No watermark.
Plans from $199 a month. No per-person fees. 14 days free.
Rather have us write it? Two written programs ($295)
Questions we get
Do I need a trench box for a 4-foot trench?
Not by the depth alone. A protective system is required at 5 feet (29 CFR 1926.652(a)(1)), or shallower when the competent person finds a potential for cave-in: loose or previously disturbed soil, water, vibration, a surcharge load at the edge. At 4 feet you do need a ladder within 25 feet of every worker, and you need to test the air if a hazardous atmosphere could be present.
How often does the competent person have to inspect the trench?
Daily before any worker enters, as needed throughout the shift, and after every rainstorm or other event that could increase the hazard, such as vibration from nearby equipment, a change in depth, or a utility strike (29 CFR 1926.651(k)). If the inspection finds evidence of a possible cave-in, a failing protective system or a hazardous atmosphere, workers come out until it is corrected.
Can I slope the trench instead of using a box?
Yes, if you have the room. The maximum slopes are ¾:1 for Type A soil, 1:1 for Type B and 1½:1 for Type C (Appendix B to Subpart P), up to 20 feet deep. Benching is allowed in Type A and B soil only. The competent person classifies the soil first, and the slope has to be maintained all day, not just when it was cut.
How do I know what soil type I have?
The competent person classifies it under Appendix A with at least one visual and one manual test: how the cut face looks and layers, whether it was disturbed before, whether water is present, plus a thumb penetration, pocket penetrometer, ribbon or dry-strength test. Previously disturbed soil is at best Type B, any submerged or seeping soil is Type C, and when in doubt it is Type C.
Can workers ride in the bucket or be in the box while it is moved?
No to both. Nobody is permitted in a shield while it is being installed, removed or moved vertically (29 CFR 1926.652(g)(1)(iii)), nobody works under a suspended load (1926.651(e)), and the bucket is not a personnel platform.
Does the trench box have to go all the way to the bottom?
Within 2 feet of it. The excavation can extend up to 2 feet below the bottom of the shield only if the shield is designed to resist the forces for the full depth and there is no indication of soil loss from behind or below it (29 CFR 1926.652(g)(2)). If the box is combined with sloping above it, it also has to extend at least 18 inches above the bottom of the slope.
How far above the top of a trench does a shield have to extend?
Subpart P does not set a height above the ground. The 18-inch number is in Appendix B: when the lower part of the trench wall is vertical and shielded or supported, with the wall sloped back above it, the shield or support has to extend at least 18 inches above the top of the vertical side. In an August 10, 2000 interpretation letter, OSHA said the standard does not require trench shields to extend above grade. If the top of the box sits below grade, the wall above it has to be sloped back, and the box still sticks up 18 inches past the vertical cut. Spoil, materials and equipment stay at least 2 feet back from the edge or behind a retaining device (1926.651(j)(2)).
Unless it is in entirely stable rock, at what depth does a trench need a protective system?
5 feet. Every excavation 5 feet or deeper needs sloping, benching, shoring or a shield unless it is made entirely in stable rock, which Subpart P defines as natural solid mineral material that can be excavated with vertical sides and will remain intact while exposed. Under 5 feet, a protective system is required when the competent person's examination finds any indication of a potential cave-in (29 CFR 1926.652(a)(1)). Deeper than 20 feet, a registered professional engineer designs it (Appendix F).
What is 1926 Subpart P?
The part of OSHA's construction standards that covers all open excavations, trenches included: 1926.650 sets the scope and definitions, 1926.651 the general requirements (utilities, access, water, air, spoil, daily inspections), and 1926.652 the protective systems. Appendix A is soil classification, B sloping and benching, C timber shoring, D aluminum hydraulic shoring, E alternatives to timber shoring, and F the selection of protective systems. California, Washington and other State Plan states have their own versions with their own section numbers.
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