All Excavations Over 20 Feet: OSHA Engineer Design Rules

OSHA’s requirements for excavations over 20 feet come down to one central rule: the protective system must be designed by a registered professional engineer, because the standard slope tables and pre-calculated options in Subpart P stop at 20 feet. The engineer’s written, site-specific design has to be on site while the work is being done. A narrow exception exists for manufactured shields and shoring specifically rated by the manufacturer for that depth. Everything else in Subpart P (soil classification, competent-person inspections, atmospheric testing, access and egress, spoil setback) still applies, and some of it applies more urgently the deeper you go.

Why 20 Feet Is the Cutoff

The appendices to 29 CFR 1926.652 repeatedly cap their allowable values at 20 feet. Appendix B’s Table B-1 limits its maximum allowable slope values to excavations “20 feet or less in depth” and states that sloping or benching for anything deeper must be designed by a registered professional engineer. Appendix F’s flowcharts carry the same note. OSHA concluded during the Subpart P rulemaking that deeper excavations create hazards the standardized tables were never built to address.¹

Past that depth, the design has to come from someone qualified to calculate it for the specific site, not from a chart.

What the Engineer’s Design Must Contain

Under 1926.652(b)(4) and (c)(4), the registered professional engineer must produce written plans that include the sizes, types, and configurations of materials used in the protective system, along with the engineer’s identity and approval. At least one copy of those plans must stay on site during construction so inspectors or the competent person can reference them at any time.²

The plans should also lay out how the protective elements interact and the sequence for installing and removing them as work progresses. Deep excavations often combine methods: sloping the upper portion, shielding the lower section, or using shoring with supplemental benching.

The Manufacturer’s Data Exception

OSHA does allow manufactured shield and shoring systems to be used past 20 feet without a separate engineering approval, but only if the manufacturer has specifically rated the system for that depth. In a 2003 letter of interpretation, OSHA stated: “Where a manufacturer’s system is used at a depth deeper than 20 feet, as long as its use at the depth in question is consistent with the manufacturer’s tabulated data, specifications, recommendations and limitations, the standard does not require the excavation contractor to obtain an approval from an RPE.”¹

This exception does not extend to sloping or benching. A second OSHA interpretation letter made clear that manufacturer’s tabulated data for sloping systems cannot simply reference “OSHA requirements” for depths over 20 feet. Because the appendices contain no slope values past that depth, a registered professional engineer must produce a specific sloping design.³

The practical line: a manufacturer’s trench shield rated to your working depth can stand on its own tabulated data. Sloping, benching, or anything the manufacturer hasn’t rated for that depth needs the engineer.

How This Applies to Each Type of Protective System

Protective systems fall into three categories, and at depths over 20 feet each has its own path to compliance.

Sloping: cutting back the trench walls at an angle steep enough to stay stable. Past 20 feet, the angle must be calculated by a registered professional engineer for the actual site conditions. The Type A, B, and C slope ratios in the appendices don’t apply.
Shoring: a bracing system, often hydraulic, that pushes against the trench walls to prevent inward movement. The engineer’s design specifies the grade of materials, placement of supports, and maximum loads. Manufactured shoring rated by its maker to the working depth can rely on the manufacturer’s data.
Shielding: a trench box placed inside the excavation to protect workers. Shields don’t prevent collapse; they create a protected zone inside the box. Either the manufacturer must have tabulated data for that depth, or an engineer must approve the shield’s use.

Soil Classification at Depth

Soil type still drives the design at any depth, and a competent person must classify the soil using both visual and manual tests before excavation begins. Manual tests include the thumb penetration test and instruments like a pocket penetrometer, which measures compressive strength directly.⁴

What changes past 20 feet is that the fixed slope ratios tied to Type A, B, and C soil no longer set the answer. The engineer takes the classification and site conditions and calculates the values, rather than reading them off a table.⁵

A Separate Engineer Trigger: Adjacent Structures

When an excavation runs close to an existing building, retaining wall, or other foundation, the dig itself can destabilize the structure. OSHA prohibits excavation below the base of any adjacent foundation or retaining wall that could reasonably pose a hazard to workers unless specific conditions are met. In most cases, a registered professional engineer must approve either the determination that the structure is far enough away to be unaffected or that the excavation work won’t create a danger to employees. This is separate from the protective system design, and it applies regardless of depth.⁶

The Rest of Subpart P Still Applies

The depth threshold changes who designs the protective system. It doesn’t lift any of the other duties in the standard, and several of them get harder to manage as excavations get deeper.

Competent Person Inspections

Every site needs a designated competent person who can identify existing and foreseeable hazards and who has the authority to take immediate corrective action, including shutting down work.⁷ OSHA has noted that trenching work’s “highly technical nature, as well as its inherent hazards, require a greater level of training and experience than a normal worker would possess.”⁸

The competent person must inspect the excavation, adjacent areas, and protective systems before the start of work each shift and as needed throughout the day. Additional inspections are required after every rainstorm or any other event that could change conditions, such as heavy equipment vibration or nearby blasting. If the inspection reveals signs of potential cave-in, protective system failure, or hazardous atmosphere, all exposed workers must be removed until the situation is corrected.⁹

Atmospheric Testing and Rescue Equipment

When an excavation is deeper than 4 feet and a hazardous atmosphere exists or could reasonably develop, such as in landfill areas or near stored chemicals, the air must be tested before workers enter. Testing checks for oxygen deficiency (below 19.5 percent oxygen), flammable gas concentrations, and toxic contaminants. If flammable gas is detected, ventilation must keep the concentration below 20 percent of the gas’s lower flammable limit.⁶

Where a hazardous atmosphere exists or could reasonably develop, emergency rescue equipment must be readily available: breathing apparatus, a safety harness and lifeline, and a basket stretcher. All rescue equipment must be attended while in use, meaning someone on the surface is monitoring it and ready to assist.¹⁰

Water

The competent person must monitor for water buildup continuously, and dewatering equipment should be in place before it becomes a problem. Uncontrolled water can rapidly erode trench walls and degrade the soil from a more stable classification to Type C, effectively undermining whatever protective system is in place.

Access, Egress, and the Trench Edge

Any trench 4 feet or deeper must have a stairway, ladder, ramp, or other safe exit route positioned so that no worker has to travel more than 25 feet laterally to reach it. In a deep, long trench, a single exit at the far end can turn a manageable emergency into a fatality.⁶

Excavated material and equipment must be kept at least 2 feet back from the excavation edge, or retained by devices that prevent anything from rolling in. Where workers or equipment need to cross over the excavation, walkways are required. Any walkway 6 feet or more above the trench floor must have guardrails meeting OSHA’s general fall protection standards.¹⁰

Underground Utilities and Surface Encumbrances

The estimated location of underground utility lines, including sewer, electric, gas, water, and telecommunications, must be determined before excavation starts. This means contacting the utility companies, giving them notice of the planned work, and asking them to mark their lines. If a utility company can’t respond within 24 hours (or the timeframe required by state or local law) or can’t pinpoint the exact location, work may proceed with caution using detection equipment or other acceptable means to locate the installations. As the excavation approaches the estimated utility location, the exact position must be confirmed through safe methods. While the trench remains open, underground utilities must be supported, protected, or removed.⁹

Trees, utility poles, boulders, and other surface features that could create a hazard must be removed or supported before excavation work begins.⁶

What Skipping the Engineer Costs

OSHA adjusts its civil penalty amounts annually for inflation. As of January 2025, the maximum penalty for a serious violation is $16,550 per violation, and the maximum for a willful or repeated violation is $165,514 per violation.¹¹ Failing to correct a cited violation can bring additional penalties for each day the hazard persists.

Missing the registered professional engineer requirement on an excavation over 20 feet is the kind of violation OSHA classifies as serious, since a cave-in at that depth carries a substantial probability of death or severe injury. If the employer knew the requirement and ignored it, the violation escalates to willful. When a willful violation causes a worker’s death, criminal prosecution becomes possible, with penalties including fines up to $10,000 and imprisonment up to six months for a first offense, doubling for subsequent convictions.¹²

1
Occupational Safety and Health Administration. Registered Professional Engineer Approval Requirements for Manufactured Trench Protection Systems Deeper Than 20 Feet
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Occupational Safety and Health Administration. 29 CFR 1926.652 – Requirements for Protective Systems
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Occupational Safety and Health Administration. Registered Professional Engineer Approval Requirements for Manufacturer Tabulated Data
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Occupational Safety and Health Administration. 29 CFR 1926 Subpart P Appendix A – Soil Classification
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Occupational Safety and Health Administration. Soil Classification Training Outline
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Occupational Safety and Health Administration. 29 CFR 1926.651 – Specific Excavation Requirements
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Occupational Safety and Health Administration. 29 CFR 1926.650 – Scope, Application, and Definitions Applicable to This Subpart
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Occupational Safety and Health Administration. Construction – Trenching and Excavations – Competent Person
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eCFR. 29 CFR 1926.651 – Specific Excavation Requirements
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GovInfo. 29 CFR 1926.651 – Specific Excavation Requirements
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Occupational Safety and Health Administration. US Department of Labor Announces Adjusted OSHA Civil Penalty Amounts
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Occupational Safety and Health Administration. Penalties – Section 17 of the OSH Act