Title: Soil Classification and Testing
Type: Lesson
The type of soil you’re digging in directly affects trench stability — and determines what kind of protective system is required. OSHA divides soil into three main types, based on how easily it collapses.
Clay, silty clay, or hard-packed, uncracked soils
Can stand up vertically for short periods
NOT Type A if: it’s been disturbed, is cracked, or contains water
Angular gravel, silt, previously disturbed Type A
Includes soil with fissures or minor water seepage
Most common soil on active job sites
Sand, loamy sand, submerged or saturated soil
Includes soil with water or soil that is freely seeping
Must always use protective systems
| Soil Type | Description | Example |
|---|---|---|
| Type A | Hard-packed, dry, no cracks | Clay, silty clay |
| Type B | Somewhat stable | Angular gravel, disturbed clay |
| Type C | Least stable, wet or loose | Sand, saturated soil |
A competent person must classify soil before work begins using at least one of the following:
Plasticity Test: Roll soil into a thread; if it holds shape, it’s likely cohesive (Type A or B)
Thumb Penetration Test: Push your thumb into a fresh sample; easy penetration usually means Type C
Pocket Penetrometer: Measures soil strength (in tons/sq ft)
Shearvane Tester: Measures torque required to shear a soil sample
Test results should be documented on-site as part of the trench safety plan.
Soil can change from one area of a trench to another — especially with water, depth, or traffic nearby. Always test, classify, and use the right protection based on the weakest soil present.
Click “Mark Complete” to continue to Lesson 4: Protective Systems: Sloping, Shoring & Shielding, where we’ll cover how to protect workers from cave-ins.