OSHA Soil Classification: Stable Rock, Type A, Type B and Type C

Before a competent person can choose a slope or read a shoring table, they have to answer one question: what kind of soil is this? OSHA's answer comes from Appendix A to Subpart P, which sorts soil and rock into four categories. The more stable the category, the steeper the walls can be.

The four categories

tsf = tons per square foot. Slopes are horizontal to vertical, so 1½:1 means 1½ feet back for every 1 foot down.

Unconfined compressive strength

Unconfined compressive strength (UCS) is the load per unit area at which a soil sample fails when squeezed with no side support. It is the backbone of the A, B and C system. In the field it is estimated with a pocket penetrometer, a shear vane or the thumb penetration test, covered in the next lesson.

Type A soil

Type A soils are cohesive soils with a UCS of 1.5 tsf or greater. Examples are clay, silty clay, sandy clay and clay loam, and in some cases silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also Type A.

But no soil is Type A if any of the following is true:

• The soil is fissured.
• The soil is subject to vibration from heavy traffic, pile driving or similar effects.
• The soil has been previously disturbed.
• The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of 4H:1V or greater.
• The material is subject to other factors that would require it to be classified as less stable.

Type B soil

Type B includes:

• Cohesive soil with a UCS greater than 0.5 tsf but less than 1.5 tsf.
• Granular cohesionless soils including angular gravel (similar to crushed rock), silt, silt loam, sandy loam, and in some cases silty clay loam and sandy clay loam.
• Previously disturbed soils, except those that would otherwise be Type C.
• Soil that meets the strength or cementation requirements for Type A but is fissured or subject to vibration.
• Dry rock that is not stable.
• Material in a sloped, layered system where the layers dip into the excavation on a slope less steep than 4H:1V, but only if it would otherwise be Type B.

Type C soil

Type C is the least stable category:

• Cohesive soil with a UCS of 0.5 tsf or less.
• Granular soils including gravel, sand and loamy sand.
• Submerged soil or soil from which water is freely seeping.
• Submerged rock that is not stable.
• Material in a sloped, layered system where the layers dip into the excavation on a slope of 4H:1V or steeper.

Layered soils: the weakest layer wins

In a layered system, the entire excavation is classified according to the weakest layer. If a stiff clay sits on top of a seam of running sand, the trench is Type C. Appendix A does allow each layer to be classified individually when a more stable layer lies below a less stable one, but the safe and simple rule for crews is to design for the worst layer you see.

Classification is not a one-time decision

The competent person must classify the soil based on at least one visual and one manual analysis, and must reclassify it when conditions change. Rain, freezing and thawing, drying of the walls, new vibration sources and water seepage can all turn yesterday's Type B trench into today's Type C trench.

Key takeaways

• OSHA's categories, from most to least stable: stable rock, Type A, Type B, Type C.
• Type A is 1.5 tsf or more, Type B is between 0.5 and 1.5 tsf, Type C is 0.5 tsf or less.
• Fissures, vibration and previous disturbance keep soil from being Type A.
• Submerged soil or freely seeping water makes soil Type C.
• Layered soils are classified by the weakest layer, and soil must be reclassified when conditions change.

Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.