Soil Classification and Testing

OSHA sorts soil by how well it holds together, measured as unconfined compressive strength in tons per square foot (tsf):

• Stable Rock — solid natural mineral matter that can be excavated with vertical sides and stay intact.
• Type A — cohesive soil with strength of 1.5 tsf or greater (clay, silty clay, hardpan). The strongest soil.
• Type B — cohesive soil between 0.5 and 1.5 tsf, plus granular cohesionless soils like silt and sandy loam.
• Type C — cohesive soil of 0.5 tsf or less, plus sand, gravel, and any soil with water freely seeping in. The weakest, and the most common on wet or disturbed sites.

Soil that would otherwise be Type A can't be classified that way if any of these apply:

• It's fissured (cracked).
• It's subject to vibration from traffic, pile driving, or nearby equipment.
• It's been previously disturbed — an old trench or backfill.
• Water is seeping into it.
• It's part of a sloped, layered system where the layers dip into the excavation.

In practice, a lot of jobsite soil that looks like solid clay ends up classified B or C once you account for traffic, past digging, or moisture.

A visual test means looking at the excavated soil and the sides of the opening — grain size, cracking, layering, signs of water, and whether it's clumping or running. A manual test checks the soil in hand or with a tool:

• Plasticity — can you roll a moist sample into a thin thread without it breaking? Cohesive soils can.
• Dry strength — a dry clump of cohesive soil resists crushing; granular soil crumbles.
• Thumb penetration — press your thumb into the sample; Type A resists, Type C indents easily.
• Pocket penetrometer or shearvane — gives an actual strength reading in tsf.

If conditions change during the job — it rains, the soil dries out, water shows up — the competent person reclassifies. Yesterday's Type B can be today's Type C.