OSHA Soil Classification — Type A, B & C
The classification system behind every slope and shoring table, per OSHA 1926 Subpart P App A — federal law. The competent person classifies with at least one visual and one manual test, the weakest layer governs a layered system, and the honest headline is the disqualifier list: most field soil is Type B or C long before strength enters it. State plans can be stricter.
The four classifications
| Type | Strength | Definition | Examples |
|---|---|---|---|
| Stable rock | — | Natural solid mineral matter excavatable with vertical sides that remain intact while exposed | Sound bedrock |
| Type A | ≥ 1.5 tsf | Cohesive soil at full strength with NO disqualifiers (see the list — most field soil fails one) | Clay, silty clay, sandy clay, clay loam; cemented soils (caliche, hardpan) |
| Type B | 0.5 – 1.5 tsf | Mid-strength cohesive soil; granular cohesionless soils; previously disturbed soil (unless Type C); would-be Type A that is fissured or vibration-exposed | Angular gravel, silt, silt loam, sandy loam; unstable dry rock |
| Type C | ≤ 0.5 tsf | Weak cohesive soil; granular soils; ANY submerged or freely-seeping soil; layered systems dipping into the cut at 4:1 or steeper | Gravel, sand, loamy sand; anything under water |
The Type A disqualifiers — read these before claiming A
No soil is Type A if any of these holds: the soil is fissured; subject to vibration — heavy traffic, pile driving, or similar; previously disturbed (nearly all utility corridors); part of a sloped layered system dipping into the excavation at 4h:1v or steeper; other factors requiring classification as less stable material. OSHA's interpretation letters are blunt about it — soil at full Type A strength next to pile driving is not Type A. The practical consequence prices bids: claiming A buys a 53° slope, but a wrong claim buys a collapse, and the slope chart’s no-classification default (34°) exists precisely so a crew never has to guess.
The field tests
| Test | How it works |
|---|---|
| Plasticity (thread) test | Roll a moist sample into a ⅛-inch thread — cohesive if a 2-inch length holds by one end without tearing |
| Thumb penetration | Type A: readily indented by the thumb, penetrated only with very great effort. Type C: easily penetrated several inches, molded by light finger pressure |
| Dry strength | Dry soil that crumbles to individual grains or fine powder under moderate pressure = granular |
| Pocket penetrometer / shearvane | Instrument estimates of unconfined compressive strength (penetrometers can read ±20–40% — a guidance figure, not CFR) |
Common questions
What is the difference between Type A, B, and C soil?
Strength plus circumstances. Type A is cohesive soil at 1.5 tsf unconfined compressive strength or better with none of the disqualifiers; Type B is the middle band (0.5–1.5 tsf) plus granular cohesionless soils and demoted would-be Type A; Type C is anything at or under 0.5 tsf, plain granular soils like sand and gravel, and — automatically — any submerged or freely seeping soil. The classification drives the allowable slope: 53°, 45°, or 34°.
Why is real Type A soil so rare on utility work?
The disqualifiers. Soil that tests at Type A strength still cannot be classified A if it is fissured, subject to vibration (traffic, pile driving), previously disturbed, or part of a layered system dipping into the cut at 4:1 or steeper. "Previously disturbed" alone demotes nearly every existing utility corridor — which is why experienced crews treat most street work as Type B at best and price protective systems accordingly.
What tests does the competent person have to run?
At least one visual and at least one manual analysis. The manual options: the thread test (an eighth-inch thread holding a 2-inch length = cohesive), the thumb test (Type A is readily indented but penetrated only with very great effort; Type C is easily penetrated several inches), dry strength, or an instrument — pocket penetrometer or shearvane. And classification isn't permanent: rain, vibration, or drying triggers reclassification.