The Definition of Soil Classification

Definition of Soil Classification

Soil classification is the process of identifying the type and stability of soil in an excavation so that the right protective system can be chosen. Under OSHA's excavation standard, a competent person classifies each soil and rock deposit as stable rock, Type A, Type B or Type C. The classification determines how steeply trench walls can be sloped, which shoring tables apply and which trench shields are rated for the job.

Getting the classification right matters because soil can look solid and still collapse without warning. A cubic yard of soil can weigh well over a ton, and cave-ins are one of the deadliest hazards in construction. Classification is the starting point for any trench safety plan that relies on sloping, benching or manufacturer tabulated data.

OSHA Soil Types

The classification system is set out in Appendix A to 29 CFR 1926 Subpart P. It uses unconfined compressive strength, measured in tons per square foot (tsf), along with other conditions.

• Stable rock: Natural solid mineral matter that can be excavated with vertical sides and stay intact while exposed.
• Type A: Cohesive soils such as clay, silty clay and sandy clay with an unconfined compressive strength of 1.5 tsf or greater. Soil cannot be Type A if it is fissured, subject to vibration from heavy traffic or pile driving, previously disturbed, or affected by other factors that make it less stable.
• Type B: Cohesive soils with a strength greater than 0.5 tsf but less than 1.5 tsf, granular cohesionless soils such as angular gravel, silt and silt loam, previously disturbed soils that are not Type C, and Type A soils that are fissured or subject to vibration.
• Type C: Cohesive soils with a strength of 0.5 tsf or less, granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable.

In layered soils, the deposit is generally classified by its weakest layer, although each layer may be classified separately where a more stable layer lies below a less stable one.

How Soil Is Classified

Visual and Manual Tests

OSHA requires classification based on at least one visual and at least one manual analysis. Visual checks include looking at the soil as it is dug, checking for cracks, fissures, layering, water seepage, previously disturbed ground and nearby sources of vibration. Common manual tests include:

• Thumb penetration test: Type A soils can be indented by the thumb but only with great effort, while Type C soils can be easily penetrated several inches.
• Pocket penetrometer or shearvane: Hand tools that estimate unconfined compressive strength.
• Plasticity test: Rolling a moist sample into a thin thread to see whether it holds together, which indicates cohesive soil.
• Dry strength test: Checking whether a dry sample crumbles easily or breaks into clods that are hard to crush.

Allowable Slopes

Appendix B to Subpart P links each soil type to a maximum allowable slope for excavations less than 20 feet deep:

• Stable rock: Vertical (90 degrees).
• Type A: 3/4 horizontal to 1 vertical (53 degrees).
• Type B: 1 horizontal to 1 vertical (45 degrees).
• Type C: 1 1/2 horizontal to 1 vertical (34 degrees).

Reclassifying

Soil conditions change. Rain, freezing and thawing, nearby vibration and water seepage can weaken soil during the job. If conditions change, the competent person must reclassify the soil and adjust the protective system.

Good Practices for Soil Classification

• Classify before entry: Complete and record the classification before anyone enters the excavation.
• Test more than one spot: Take samples from different depths and locations, since soil can vary along a trench.
• When in doubt, go weaker: Many contractors treat all soil as Type C unless testing clearly supports a stronger rating.
• Watch for warning signs: Cracks at the edge, bulging walls, water seepage and falling chunks of soil all call for immediate reassessment.
• Match the protective system: Make sure slopes, shoring tables and trench box ratings are all based on the current classification.

Example of Soil Classification

A sewer contractor begins a trench in what looks like firm clay. The competent person runs a thumb penetration test and a pocket penetrometer reading, which suggest Type A strength. However, the trench runs next to a busy road with frequent truck traffic, and the soil shows fine fissures near the surface. Because soil subject to vibration or fissuring cannot be Type A, the competent person classifies it as Type B and lays back the walls at a 1:1 slope.

After overnight rain, the crew finds water seeping from one wall. The competent person reclassifies that section as Type C, flattens the slope to 1 1/2:1 and brings in a trench box for the deepest part of the run before work resumes.

Frequently Asked Questions: Soil Classification

Who can classify soil on a job site?

A competent person who is trained in soil analysis and protective systems and has the authority to correct hazards must classify the soil.

Can soil be Type A if it has been dug before?

No. Previously disturbed soil cannot be classified as Type A. It is generally Type B or Type C depending on its properties.

Is soil classification needed if a trench box is used?

Yes. Trench boxes are rated for specific soil types and depths, so the classification is needed to confirm the box is suitable.

How often should soil be reclassified?

Whenever conditions change, such as after rain, freezing, new vibration sources or signs of instability. Daily inspections by the competent person should include a check of soil conditions.