Soil Classification
Soil Classification for Excavations
How OSHA's four-category soil classification system works, and how a Competent Person determines which one applies on site.
Soil classification is the foundation of excavation safety planning - it determines which protective systems are allowed and at what slope angles. OSHA classifies soil into four categories, from most to least stable.
| Classification | Description |
|---|---|
| Stable Rock | Solid mineral material that can be excavated with vertical sides and remain intact while exposed. |
| Type A | The most stable soil type, such as clay, silty clay, or clay loam, with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Never classified as Type A if fissured, previously disturbed, subject to vibration, or if water is seeping through it. |
| Type B | Moderately stable soil, such as silt, sandy loam, or unstable dry rock, with an unconfined compressive strength between 0.5 and 1.5 tsf. |
| Type C | The least stable soil, including granular soils (gravel, sand, loamy sand), submerged soil, soil with seeping water, or previously disturbed soil, with an unconfined compressive strength of 0.5 tsf or less. |
The Four Soil Types, Explained
What Is Stable Rock?
Stable rock is solid mineral material that stays intact with vertical sides when exposed - it's the only classification that can be left unsupported at a full vertical cut without a protective system, and it's also the least common condition most crews actually dig into.
What Is Type A Soil?
Type A is OSHA's most stable soil classification: cohesive material like clay, silty clay, sandy clay, or clay loam with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Soil can never be classified as Type A - no matter how it tests - if it's fissured, previously disturbed, part of a sloped layered system, subject to vibration from traffic or heavy equipment, or has water seeping through it. Any one of those conditions drops it at least to Type B.
What Is Type B Soil?
Type B is moderately stable soil - silt, sandy loam, medium clay, or unstable dry rock - with an unconfined compressive strength between 0.5 and 1.5 tsf. Soil that would otherwise test as Type A gets reclassified as Type B once fissuring, vibration, or water seepage is present.
What Is Type C Soil?
Type C is OSHA's least stable classification: granular soils like gravel, sand, and loamy sand, submerged soil, soil with actively seeping water, or previously disturbed soil, with an unconfined compressive strength of 0.5 tsf or less. Type C soil requires the flattest sloping angle of the three soil types and can't be benched at all - shoring or shielding are the only compact-footprint options.
Visual Tests
Observing the soil and excavation for signs of instability: fissures, layered soil types, water seepage, previously disturbed ground, spoil pile placement, and vibration sources nearby (traffic, equipment).
Manual Tests
At least one manual analysis test is required, such as a plasticity (thread) test, dry strength test, thumb penetration test, or pocket penetrometer reading, to confirm the soil's unconfined compressive strength.
Water & Vibration Impacts
Water seepage and vibration (from traffic, equipment, or nearby construction) both reduce soil stability and can downgrade a classification - soil that would otherwise test as Type A cannot be classified that way if water is seeping through it or it's subject to vibration.
Layered Soil Systems
Where a trench exposes more than one soil type in layers, OSHA requires the excavation to be classified based on the least stable layer present, unless each layer is classified and protected individually.
Competent Person Responsibility
Soil classification must be performed by a competent person based on actual site conditions. Training provides the knowledge framework, but field conditions must be evaluated on site - classroom or online instruction cannot substitute for physically testing the soil in the trench being dug.
Official reference: OSHA 1926.652, Appendix A - Soil Classification →
See also: Protective Systems, which explains how classification determines which sloping, benching, or shoring options are allowed.