Soil Tests: Thumb, Penetrometer & More
Before deciding how steep to slope a trench or which shoring to use, a competent person must classify the soil. OSHA requires at least one visual test and at least one manual test. If in doubt, treat the soil as the weakest type, Type C.
- Competent persons
- Construction
Photo: competent person pressing a pocket penetrometer into a soil clump from the trench wall
Soil types
| Type | Unconfined compressive strength | Examples |
|---|---|---|
| Stable rock | — | Solid rock that stays intact when exposed |
| Type A | 1.5 tons per square foot (tsf) or more | Clay, silty clay, hardpan |
| Type B | More than 0.5 but less than 1.5 tsf | Silt, angular gravel, previously disturbed soil not Type C |
| Type C | 0.5 tsf or less | Gravel, sand, loamy sand, submerged or seeping soil |
Soil cannot be Type A if it’s fissured, subject to vibration, previously disturbed, layered so it slopes into the trench, or has seeping water. See Soil Classification.
Visual tests
- Look at the soil as it’s dug: does it come out in clumps (cohesive) or run (granular)?
- Look for cracks (fissures) in the walls and at the surface near the edge
- Look for layers, and whether they tilt toward the trench
- Look for water: seeping, standing, or a wet layer
- Look for signs of previous digging, utilities or old backfill
- Note sources of vibration: traffic, equipment, pile driving
Manual tests
| Test | How | What it tells you |
|---|---|---|
| Thumb penetration | Press your thumb into a fresh clump of soil | Type A: indented by the thumb only with very great effort. Type C: thumb easily goes in several inches |
| Pocket penetrometer | Push the spring-loaded tip into the soil; read the scale | Estimates unconfined compressive strength in tsf |
| Shearvane (torvane) | Press the vanes into the soil and turn | Estimates shear strength |
| Plasticity (thread) test | Roll a moist sample into a thread about ⅛ inch thick | If a 2-inch thread holds together when held by one end, the soil is cohesive |
| Dry strength | Crumble a dry sample in your hand | Crumbles easily: granular. Hard clods: cohesive |
Test where workers will be. Take samples from the trench wall and spoil at the work location, and test again as you go. Soil can change every few feet, and with weather.
When in doubt
You can always choose to treat soil as Type C and use the slopes or protective systems for Type C. You can never treat it as stronger than your tests show.
Standards & references
- 29 CFR 1926 Subpart P, Appendix A: Soil classification
- See also: Protective Systems · Competent Person Inspections
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