Soil Classification
The type of soil decides how steeply a trench can be sloped and what protection is needed. A competent person classifies the soil using at least one visual and one manual test.
- Construction
Soil types
| Type | Description | Maximum slope (H:V) for excavations under 20 ft |
|---|---|---|
| Stable rock | Natural solid mineral matter that stays intact when exposed | Vertical (90°) |
| Type A | Cohesive soils like clay with high strength (1.5 tons per sq ft or more) | 3/4 : 1 (53°) |
| Type B | Medium strength cohesive soils, angular gravel, silt, previously disturbed soil that isn’t Type C | 1 : 1 (45°) |
| Type C | Granular soils (sand, gravel), submerged soil, soil with water seeping, low strength | 1½ : 1 (34°) |
Soil can’t be Type A if it’s fissured, subject to vibration, previously disturbed, or has water seeping. When in doubt, treat it as Type C.
Soil strength
Soil types are based partly on unconfined compressive strength, the pressure it takes to make a sample fail:
| Type | Unconfined compressive strength |
|---|---|
| A | 1.5 tons per square foot or more |
| B | More than 0.5 but less than 1.5 tons per square foot |
| C | 0.5 tons per square foot or less |
Tests
The competent person must do at least one visual test and at least one manual test.
Visual tests
- Look at the excavated soil: fine-grained (clay, silt) or coarse (sand, gravel)? Does it stay in clumps?
- Look at the trench walls and nearby ground for cracks, fissures, layers, and spalling.
- Look for water seeping from the sides or standing in the bottom.
- Look for previously disturbed soil: old utility trenches, fill and backfill.
- Look for vibration sources: roads, railways, pile driving and heavy equipment.
Manual tests
- Thumb penetration: Type A soil can be dented by a thumb but only penetrated with great effort. Type C soil can be easily penetrated several inches.
- Pocket penetrometer or shear vane: gives a strength reading in tons per square foot.
- Plasticity: roll a moist sample into a thread about 1/8 inch thick. If a 2-inch thread holds together, the soil is cohesive.
- Dry strength: dry clumps that break only with difficulty suggest clay; clumps that crumble easily suggest granular soil.
- Wet shaking and drying tests: help tell clay from silt and find fissures.
Layered soils
When the trench wall has layers of different soils, the whole excavation is classified by the weakest layer. A layer of sand under clay can make the whole wall Type C, and a sloping layer that tilts toward the trench is especially dangerous.
Benching
Benching (cutting steps into the trench wall) is allowed in Type A and Type B cohesive soils, within the limits in Appendix B. It is not allowed in Type C soil.
Conditions change. Rain, freezing and thawing, vibration and time can all weaken soil. The competent person must re-classify if conditions change.
Standards & references
- 29 CFR 1926 Subpart P, Appendix A: Soil classification
- Appendix B: Sloping and benching
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