Slope It, Shore It, Shield It: A Field Guide to Trench Protective Systems and the Competent Person

By James Trench, Trenchsafety

A trench wall can look solid right up until it fails, and when it does, it gives almost no warning. That is why OSHA’s excavation standard, 29 CFR 1926 Subpart P, is built around protective systems chosen before anyone enters the cut, plus a person on site with the knowledge and the authority to stop the work. This article walks through the three basic protective approaches (sloping, shoring and shielding), the depth thresholds that drive the decision, and what a competent person actually has to do.

When a protective system is required

The rule starts with a clear trigger. Under 29 CFR 1926.652(a)(1), every employee in an excavation must be protected from cave-ins by an adequate protective system unless the excavation is made entirely in stable rock, or it is less than 5 feet deep and a competent person has examined the ground and found no indication of a potential cave-in.

Two points follow. First, 5 feet is not a safe zone; a shallower trench still needs a competent person’s examination, and that person can require protection. Second, the system must be able to resist “all loads that are intended or could reasonably be expected,” per 1926.652(a)(2). That includes spoil piles, equipment and traffic near the edge.

At the other end of the scale, OSHA’s Trenching and Excavation Safety fact sheet summarizes the rule this way: trenches 20 feet deep or greater require a protective system designed by a registered professional engineer or based on tabulated data prepared and/or approved by one, in accordance with 1926.652(b) and (c). Appendix B makes the same point for sloping: sloping or benching for excavations greater than 20 feet deep shall be designed by a registered professional engineer.

Step one: classify the soil

Most protective-system choices depend on soil type. Appendix A requires a competent person to classify each deposit as Stable Rock, Type A, Type B or Type C, from most to least stable, based on at least one visual and at least one manual test.

The definitions are stricter than many crews assume. No soil is Type A if it is fissured, subject to vibration from heavy traffic or pile driving, or previously disturbed. Granular soils such as gravel and sand, submerged soil, and soil with freely seeping water are Type C. In a layered system, the deposit is classified by its weakest layer. If conditions change, for example after a rain, the competent person must re-evaluate and reclassify as needed.

Sloping and benching

Sloping cuts the trench walls back at an angle away from the excavation; benching steps them back in horizontal levels. Under 1926.652(b), the employer can use one of four design options. The simplest, Option 1, needs no soil classification at all: slope no steeper than 1½ horizontal to 1 vertical (34 degrees), the Type C configuration.

If the soil has been classified, Appendix B Table B-1 sets maximum allowable slopes for excavations less than 20 feet deep:

| Soil or rock type | Maximum allowable slope (H:V) | |—|—| | Stable rock | Vertical (90°) | | Type A | ¾:1 (53°) | | Type B | 1:1 (45°) | | Type C | 1½:1 (34°) |

Appendix B also allows a short-term slope of ½:1 in Type A soil for excavations 12 feet deep or less that stay open 24 hours or less. When the competent person sees signs of distress, such as cracking, slumping, bulging or ravelling, the slope must be cut back further. OSHA’s fact sheet notes that benching cannot be done in Type C soil.

Sloping is often the simplest system on open sites, but it needs room. A deep cut in Type C soil gets wide quickly, which is why sloping is often impractical near roads, buildings and existing utilities.

Shoring

Shoring is a support system that holds the trench walls in place, using hydraulic, pneumatic, timber or screw-jack members. Under 1926.652(c), designs can come from Appendices A and C (timber shoring), Appendix D or manufacturer’s data (aluminum hydraulic shoring), other tabulated data approved by a registered professional engineer, or a design by a registered professional engineer.

The practical requirements matter as much as the design. When manufacturer’s tabulated data is used, it must be followed, any deviation needs the manufacturer’s written approval, and that data must be kept in writing at the jobsite during construction. Members must be connected to prevent sliding, falling or kickouts. Installation and removal must protect workers, and removal starts at the bottom of the excavation, with backfilling progressing together with removal (1926.652(e)).

Shielding

Shields, commonly called trench boxes, do not stop the walls from moving. They protect the workers inside if the walls do fail. The rules in 1926.652(g) are short but frequently overlooked:

• Shields must not be loaded beyond what they were designed to withstand.
• They must be installed to restrict lateral or other hazardous movement under sudden loads.
• Workers must be protected from cave-ins while entering and exiting the shielded area.
• No one may be inside a shield while it is being installed, removed or moved vertically.
• In trenches, digging up to 2 feet below the bottom of the shield is allowed only if the shield is designed for the full depth of the trench and there is no sign of soil loss from behind or below it.

Damaged shields and shoring components must be examined by a competent person. If that person cannot confirm the equipment is still fit for use, it comes out of service until a registered professional engineer evaluates and approves it (1926.652(d)(3)).

The competent person: knowledge plus authority

OSHA defines a competent person in 1926.650(b) as someone capable of identifying existing and predictable hazards and who has authorization to take prompt corrective measures to eliminate them. Both halves count. A knowledgeable foreman who cannot stop production is not a competent person under this definition.

Under 1926.651(k), the competent person inspects the excavation, adjacent areas and protective systems daily before work starts, as needed throughout the shift, and after every rainstorm or other hazard-increasing event. If they find evidence of a possible cave-in, protective-system failure or a hazardous atmosphere, exposed workers must be removed until the hazard is controlled.

The same section covers the conditions that often decide whether a protective system works:

• Utilities: estimated locations must be determined before digging, and owners must be contacted (1926.651(b)).
• Egress: trenches 4 feet or deeper need a stairway, ladder, ramp or other safe means of egress within 25 feet of lateral travel (1926.651(c)(2)).
• Atmosphere: where oxygen deficiency or a hazardous atmosphere could reasonably exist, test before entry into excavations deeper than 4 feet (1926.651(g)).
• Water: no work in accumulating water without adequate precautions (1926.651(h)).
• Spoil and equipment: keep materials at least 2 feet from the edge, or use retaining devices (1926.651(j)(2)).

Making it work on the job

Practical trench safety comes down to a few habits. Pick the protective system during planning, not when the excavator is already in the hole. Keep the tabulated data or engineer’s design on site. Make sure the competent person is named, present and empowered. And treat every change in the weather, the soil or the depth as a reason to look again.

Sources

• OSHA, 29 CFR 1926.650 (Scope, application and definitions): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.650
• OSHA, 29 CFR 1926.651 (Specific excavation requirements): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.651
• OSHA, 29 CFR 1926.652 (Requirements for protective systems): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.652
• OSHA, Subpart P Appendix A (Soil classification): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926SubpartPAppA
• OSHA, Subpart P Appendix B (Sloping and benching): https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926SubpartPAppB
• OSHA Fact Sheet, Trenching and Excavation Safety: https://www.osha.gov/sites/default/files/publications/TRENCH_EXCAVATION_FS.pdf
• OSHA Trenching and Excavation topic page: https://www.osha.gov/trenching-excavation

James Trench writes for Trenchsafety (trenchsafety.org), an equipment reseller based in Floral City, Fla.