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. |
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.