Excavation Safety: The Competent Person, Soil Types & Trench Shoring

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Excavation Safety: The Competent Person, Soil Types & Trench Shoring. Don't let the plain title fool you. Here's the part that actually matters on the job: Every trench is a decision made by a competent person who classifies the soil, picks the protective system from the tables or the tabulated data, inspects it every shift and after every change, and has the authority to pull people out. Learn it well and you will never be the operator who dug the trench that buried someone, and you will be the person the crew trusts to say when it is safe.

The intro course gave you the big numbers: protective systems at 5 feet, spoil 2 feet back, ladders within 25 feet. This lesson is the competent person's job in depth under OSHA 29 CFR 1926 Subpart P: classifying soil, selecting and installing sloping, shoring, and shields, and what the daily inspection has to find.

Who the competent person is

Subpart P defines the competent person as someone capable of identifying existing and predictable hazards in the excavation who has the authorization to take prompt corrective measures to eliminate them. Both halves matter: a foreman who knows soil but cannot stop the work does not qualify, and neither does a superintendent with authority who cannot classify soil. The employer designates the competent person, and the duties include classifying the soil, selecting the protective system, inspecting the excavation before each shift, as conditions change, and after every rainstorm, and removing workers until hazards are corrected.

Classifying the soil

Appendix A of Subpart P sorts soil into four groups by unconfined compressive strength and other conditions:

• Stable rock — natural solid mineral that stays intact with vertical sides.
• Type A — cohesive soil at 1.5 tons per square foot or more, such as stiff clay. Soil cannot be Type A if it is fissured, subject to vibration from traffic or equipment, previously disturbed, part of a layered system dipping into the trench at four horizontal to one vertical or steeper, or seeping water.
• Type B — cohesive soil between 0.5 and 1.5 tons per square foot, granular cohesionless soils like silt and sandy loam, and previously disturbed soil that would otherwise be Type A.
• Type C — cohesive soil at 0.5 tons per square foot or less, granular soils like gravel and sand, and submerged soil or soil with freely seeping water. Classification uses at least one visual test and one manual test. Visually, look at spoil particle size, fissures, layering, seepage, nearby vibration, and signs of previous excavation such as old pipe or fill. Manually, use a pocket penetrometer or shear vane for strength, the thumb test (a thumb indents Type A only with great effort and pushes easily into Type C), and a ribbon test to confirm cohesion. The practical result on most developed sites: old utility corridors are previously disturbed, so Type B at best, and anything with water in the bottom is Type C. In a layered system, classify by the weakest layer.

Choosing the protective system

Subpart P requires a protective system in any excavation 5 feet or deeper, and in shallower ones where the competent person sees a potential cave-in, unless the excavation is entirely in stable rock. There are four routes:

• Sloping and benching per Appendix B. Maximum slopes for excavations under 20 feet deep are three-quarters horizontal to one vertical for Type A, one to one for Type B, and one and one-half to one for Type C. Benching is permitted only in Types A and B. Width is what kills sloping on a tight site: a 10-foot Type C trench needs 30 feet of extra width.
• Timber shoring per Appendix C and aluminum hydraulic shoring per Appendix D, selected from the tables by soil type, depth, and width.
• Trench shields, or boxes, used within their tabulated data prepared by a registered professional engineer. A box rated for Type B at 8 feet is not rated for Type C at 10.
• An engineer's design for anything over 20 feet or anything the tables do not cover.

Working with trench shields

Shields do not hold the trench open; they protect the people inside while the trench caves around them. The rules follow from that: workers stay inside the box and never in the trench outside or ahead of it; no one is in the box while it is installed, removed, or moved vertically; when the box is combined with sloping above, it extends at least 18 inches above the top of the vertical sides; the excavation below the box may extend no more than 2 feet, and only if the box is rated for the full depth and there is no sign of soil loss from behind or below; boxes stack and spread with the manufacturer's pins and spreaders only; and the box is inspected for cracked welds and bent spreaders before every use. The operator sets and pulls the box with rated slings on the lifting points and never swings a load over anyone inside.

Installing and removing shoring

Hydraulic shoring is installed from the top, from outside the trench, cylinder by cylinder, at the pressure in the tables; the crew never enters an unshored trench to set the first shore. Removal is from the bottom up, with backfill placed as the shores come out so no one is exposed to an unsupported wall. Timber shoring follows the same principle. Any shore that is damaged, leaking, or not bearing is replaced, not reused.

The rest of the daily inspection

Beyond the protective system, the inspection covers surface encumbrances such as sidewalks, poles, and footings near the edge; underground utilities located and protected; access and egress within 25 feet of lateral travel in trenches 4 feet or deeper; water accumulation, which requires pumping and makes most soil Type C; hazardous atmospheres tested in excavations deeper than 4 feet where they are reasonably expected; loads never suspended over workers; spoil and equipment 2 feet back; a warning system such as barricades or stop logs where mobile equipment works near the edge; and fall protection where walkways cross the trench. Everything is re-inspected after rain, after a change in the trench, and at shift change.

Going Deeper (Intermediate)

The hardest part of the job is the trench that is almost fine. Soil dries and fissures in sun; a vibrating roller fifty feet away turns Type A into Type B; a water main break overnight makes the bottom Type C; a crew extends a box two feet into an unrated depth to finish a joint. The competent person re-classifies when conditions change, not once a job. Good crews write the classification and the system on the daily excavation inspection form with the depth, the test used, and the inspector's name, and they keep the tabulated data for the box at the trench, because it is required there and it answers the question when the box is pushed past its rating.

Advanced / Pro-Level

Pros plan excavations before the bucket moves: a trench plan for each run with the soil type from the geotechnical report, the depth from the profile, the system, the box sizes, the ladder locations, the spoil side, and the haul route. They know sloping gets expensive at depth and a box with a sloped top is often the practical answer. They look at adjacent structures and get an engineer's review before a trench goes near a footing, and they back the laborer who refuses to go into a trench that does not look right.

Practice Challenge

A pipe crew is laying 8-inch sewer at 9 feet deep along an old street where the water main was replaced ten years ago. The pocket penetrometer reads 1.8 tons per square foot on stiff clay and the foreman wants to slope at three-quarters to one as Type A. What does the competent person say? (Answer: the soil is previously disturbed from the water main work, so it cannot be Type A regardless of strength; it is Type B at best, which means one to one sloping, which the street width will not allow, so the run goes in with a shield or hydraulic shoring rated for Type B at 9 feet or deeper, with the ladder, spoil distance, and daily inspection in place, and re-classification to Type C if any water shows.)

In Practice

A competent person who classifies the soil at the start of every shift, writes it down, matches the box to the tabulated data, and walks the run after every rain sends the whole crew home every day. One who classified the soil on Monday, left the form in the truck, and let the crew push the box an extra two feet on Friday gets a phone call nobody recovers from.

Common Mistakes to Avoid

• Calling disturbed soil Type A because it feels stiff
• Choosing the protective system by habit instead of by soil type and depth
• Entering the trench outside or ahead of the box, or riding in it while it moves
• Skipping the re-inspection after rain, vibration, or a change in depth
• Keeping the tabulated data in the office instead of at the trench

Takeaway: Every trench is a decision made by a competent person who classifies the soil, picks the protective system from the tables or the tabulated data, inspects it every shift and after every change, and has the authority to pull people out.

Educational content only — not engineering, geotechnical design, or safety advice, and not a substitute for hands-on training, your employer's competent-person program, or a licensed engineer. Follow OSHA 29 CFR 1926 Subpart P and the other standards as adopted in your jurisdiction, the project drawings, specifications, and geotechnical report, the tabulated data for your protective systems, and the manufacturer's operator's manual for every machine. Editions and local adoption vary.