Overview: 29 CFR 1926 Subpart P
OSHA's excavation standard (29 CFR 1926 Subpart P) applies to all open excavations made in the earth's surface, including trenches. It is one of the most frequently cited OSHA standards because trench collapses are among the most deadly construction hazards. A cubic yard of soil weighs approximately 2,700-3,000 pounds, and workers buried by even a partial collapse face serious injury or death.
The standard requires that all employees in excavations 5 feet or deeper be protected from cave-ins by an adequate protective system, unless the excavation is made entirely in stable rock. For trenches less than 5 feet, a competent person must determine whether a protective system is needed based on soil conditions.
Soil Classification
OSHA classifies soil into three types based on unconfined compressive strength and other characteristics. Soil classification determines the required slope angle for open-cut trenches.
Type A Soil
The most stable soil type, with unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Examples include clay, silty clay, and hardpan. Type A soil is rare in practice because it must not be fissured, subject to vibration, or previously disturbed. Required slope: 0.75H:1V (53 degrees from horizontal).
Type B Soil
Moderately stable soil with unconfined compressive strength between 0.5 and 1.5 tsf. Examples include angular gravel, silt, silt loam, and previously disturbed soils that would otherwise be Type A. Also includes dry unstable rock. Required slope: 1H:1V (45 degrees from horizontal).
Type C Soil
The least stable soil type, with unconfined compressive strength of 0.5 tsf or less. Examples include granular soils (gravel, sand, loamy sand), submerged soil, soil from which water is freely seeping, and any submerged rock that is not stable. Type C is the default for most practical purposes because it is the most conservative classification. Required slope: 1.5H:1V (34 degrees from horizontal).
Protective Systems
OSHA recognizes four types of protective systems for trenches 5 feet or deeper:
1. Sloping
Cutting back the trench walls at an angle inclined away from the excavation. The required angle depends on soil type. This is the most common method for open trenches in residential and commercial work. Sloping requires more excavation (wider trenches) but no special equipment.
For a 6-foot-deep trench in Type C soil, the slope setback is 6 × 1.5 = 9 feet on each side, making the total top width = bottom width + 18 feet. Use the OSHA trench sloping calculator to calculate the required dimensions for your trench.
2. Benching
Excavating the sides of the trench in a series of horizontal steps (benches) rather than a continuous slope. Benching is not allowed in Type C soil. For Type A soil, bench height cannot exceed 4 feet. For Type B, bench height cannot exceed 4 feet and the vertical side of each bench must be no more than the bench height.
3. Shoring
Installing support structures (hydraulic, mechanical, or timber) to prevent cave-ins. Shoring allows narrower trenches than sloping, which is important in urban areas where space is limited. Shoring systems must be designed for the specific soil type and trench depth.
4. Shielding (Trench Boxes)
Using prefabricated steel or aluminum trench boxes that protect workers inside the shield. The trench can still collapse, but workers inside the box are protected. Trench boxes are common for utility installation because they can be moved along the trench as work progresses.
Key Depth Thresholds
| Depth | Requirement |
|---|---|
| Under 5 ft | Competent person evaluates; protective system may not be required if examination shows no cave-in potential |
| 5 ft to 20 ft | Protective system required (sloping, benching, shoring, or shielding). Competent person selects the system. |
| Over 20 ft | Protective system must be designed by a registered professional engineer. Tabulated data and simplified methods no longer apply. |
Competent Person Requirements
OSHA requires a “competent person” on site for all excavation work. The competent person must be able to identify existing and predictable hazards, classify soil, and has authority to take corrective measures. This person inspects the excavation daily (and after any rain or other event that could affect stability) before workers enter.
Additional Safety Requirements
- Spoil piles must be kept at least 2 feet from the edge of the excavation.
- A means of egress (ladder, ramp, or stairway) must be provided within 25 feet of workers in trenches 4 feet or deeper.
- Underground utilities must be located before digging. Contact 811 (Call Before You Dig) at least 48 hours in advance.
- Surface water and ground water must be managed to prevent accumulation in the trench.
- Workers must not work beneath loads handled by digging or lifting equipment.
Calculate Your Trench Requirements
Use our OSHA trench sloping calculator to determine the required slope setback and total top width for your trench based on depth and soil classification. The calculator defaults to Type C (most conservative) and includes safety warnings for deep excavations.
For trench volume estimation with OSHA-compliant dimensions, use the trench volume calculator in trapezoidal mode.