Determination of Soil Type
By grouping different types of soils (described in the previous sections of this module) according to requirements for safe excavation, the excavation standard has defined four soil classifications (provided below). For a detailed explanation of OSHA classification system, please see Appendix A of the excavation standard.
OSHA groups soil and rock deposits into four classifications: Stable rock, Type A soils, Type B soils, and Type C soils.
Characteristics of OSHA Soil Types
Stable Rock
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. It is usually identified by a rock name such as granite or sandstone.
Determining whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or not the cracks run into or away from the excavation.
Type A Soils
Examples of Type A cohesive soils are clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type A.
However, no soil is Type A if:
- Cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (144 kPa) or greater.
- The soil is fissured; or
- The soil is subject to vibration from heavy traffic, pile driving, or similar effects; or
- The soil has been previously disturbed; or
- The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or greater; or
- The material is subject to other factors that would require it to be classified as a less stable material.
Type B Soils
Examples of Type B soil include angular gravel, silt, silt loam, and soils that are fissured or near sources of vibration, but could otherwise be Type A.
Type B soils are defined as meeting any of the following:
- Cohesive soils with an unconfined compressive strength greater than 0.5 tons per square foot (48 kPa) but less than 1.5 (144 kPa)
- Angular cohesionless soils including: angular gravel (similar to crushed rock).
- Previously disturbed soils except those which would otherwise be classed as Type C soil.
- Soil that meets the unconfined compressive strength or cementation requirements for Type A, but is fissured or subject to vibration; or
- Dry rock that is not stable; or
- Material that is part of a sloped, layered system where the layers dip into the excavation on a slope less steep than four horizontal to one vertical (4H:1V), but only if the material would otherwise be classified as Type B.
Type C Soils
Type C soil is the least stable type of soil and include granular soils in which particles don't stick together and cohesive soils with a low unconfined compressive strength; 0.5 tons per square foot or less. Examples of Type C soil include gravel, and sand. Because it is not stable, soil with water seeping through it is also automatically classified as Type C soil, regardless of its other characteristics.
Type C soils are defined as meeting any of the following:
- Cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kPa) or less>
- Granular soils including gravel and sand;
- Soils that have been disturbed;
- Submerged soil or soil from which water is freely seeping;
- Submerged rock that is not stable; or
- Material in a sloped, layered system where the layers dip into the excavation or a slope of four horizontal to one vertical (4H:1V) or steeper.
Knowledge Check Choose the best answer for the question.
2-4. Which of the following is considered Type A cohesive soil?
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