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.
Geometry planning alone does not establish compliance or approve a protective system. A competent person must classify the deposit, assess site conditions, and select or verify the protective system that fits the actual excavation.
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.
Appendix A does not permit a shortcut default classification. Each soil or rock deposit must be classified by a competent person based on at least one visual analysis and at least one manual analysis, and the deposit must be reclassified if site conditions change.
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. 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 plan trench geometry from Appendix B maximum allowable slope tables after the competent person has classified the deposit and evaluated site conditions.
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, and Appendix B benching configurations apply only to excavations 20 feet or less in depth.
On this site, the calculator's benching mode uses a 4-foot lowest bench and 5-foot subsequent benches for Type A, and a 4-foot lowest bench and 4-foot subsequent benches for cohesive Type B soil.
Type B benching should be used only for cohesive Type B soil. If the deposit is granular, layered unfavorably, saturated, or otherwise outside the Appendix A and Appendix B assumptions, the benching geometry alone does not establish compliance and the competent person must select or verify another protective system.
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 | Appendix B sloping and benching configurations no longer apply. Deeper sloping or benching needs a registered professional engineer design under 1926.652(b), while support and shield systems must stay within applicable manufacturer limits, approved tabulated data, or a registered professional engineer design under 1926.652(c). |
Competent Person Requirements
OSHA defines a competent person as someone who can identify existing and predictable excavation hazards and who has authority to take prompt corrective measures.
For trench work, the competent person classifies the soil or rock deposit under Appendix A, reevaluates that classification when conditions change, and selects or verifies the protective system that matches those conditions.
OSHA's trenching eTool also summarizes the competent person's recurring duties: inspect the excavation, adjacent areas, and protective systems before each shift, after rainstorms, and after any other hazard-increasing event; monitor for cave-in, water-accumulation, and atmospheric hazards; maintain safe access and egress; and remove or correct hazardous conditions before employees are exposed.
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. One-call notice periods vary by state, so follow the applicable state or local one-call requirements for your project.
- 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 plan slope setback and total top width from OSHA Appendix B maximum allowable slope tables after a competent person classifies the deposit and evaluates site conditions.
The benching mode also shows the site's Type A and cohesive Type B bench geometry assumptions, but those outputs do not by themselves approve benching or select the protective system for the actual trench.
For trench volume estimation using the planned top width from that geometry, use the trench volume calculator in trapezoidal mode.
Sources
Review the primary OSHA sources directly before relying on any planning output or site decision.
OSHA 29 CFR 1926.652 - Requirements for protective systems
OSHA 29 CFR 1926 Subpart P Appendix A - Soil Classification
OSHA 29 CFR 1926 Subpart P Appendix B - Sloping and Benching