Soil classification and excavation

 

Soil classification is the systematic grouping of soils according to their physical properties, grain size, plasticity, strength, and engineering behavior. In HSE/construction work, classification is particularly important for assessing the stability of excavations and trenches.

OSHA Soil Classification for Excavation Safety

For excavation and trenching, soils are generally classified into:

Soil Class	Description	Stability
Stable Rock	Natural solid mineral material that can be excavated with vertical sides and remain stable	Highest
Type A	Most stable cohesive soil, such as clay, silty clay, or sandy clay under qualifying conditions	High
Type B	Medium-stability soil, including many silts, sandy soils, and previously disturbed Type A soils	Medium
Type C	Least stable soil, including many granular soils such as sand and gravel	Low

1. Stable Rock

Material that can be excavated with vertical sides and remain intact.

Example: Solid, competent bedrock.

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2. Type A Soil

Generally the most stable soil classification.

Examples can include:

• Clay
• Silty clay
• Sandy clay

However, soil should not automatically be classified as Type A just because it contains clay. Conditions such as cracking, vibration, water seepage, previous disturbance, and other factors can change the classification.

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3. Type B Soil

Intermediate stability.

Examples include:

• Silt
• Sandy loam
• Previously disturbed cohesive soil
• Soil with moderate cohesion

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4. Type C Soil

The least stable classification.

Examples include:

• Sand
• Gravel
• Saturated or submerged soil
• Certain freely flowing granular soils

Type C requires particularly strong excavation controls because of its tendency to collapse.

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Important Factors Affecting Soil Classification

Before allowing workers into an excavation, consider:

1. Soil type
2. Water/seepage
3. Previous excavation or disturbance
4. Cracks or fissures
5. Vibration from machinery or traffic
6. Nearby structures
7. Excavation depth
8. Weather/rainfall
9. Adjacent loads or spoil piles
10. Evidence of previous cave-ins

Example

If a trench contains apparently cohesive clay but has been previously disturbed and is subject to heavy equipment vibration, you should not simply assume it is Type A. The competent person must evaluate the actual conditions and select the appropriate protective system.

Excavation Protection

Depending on the soil and excavation conditions, protection may include:

• Sloping
• Benching
• Shoring
• Shielding/trench boxes
• Safe access and egress
• Keeping spoil/material away from the edge
• Controlling water accumulation
• Competent-person inspections