Soil and Stability
Some soils are more stable than others. The type of soil is one of the factors that determine the chance that an excavation will cave in.
There are three basic soil types that you may encounter:
- Type A - means cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Examples of 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 hardpan are also considered Type A. No soil can be Type A if fissured, subjected to significant vibration, or has been previously disturbed.
- Type B - means cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf. Some examples are: granular cohesionless soils including angular gravel, silt, silt loam, sandy loam, and in some cases, silty clay loam and sandy clay loam. Type B also includes previously disturbed soils except those which would otherwise be classed as Type C.
- Type C - means cohesive soils with an unconfined compressive strength of 0.5 tsf or less. Some examples include: gravel, sand, and loamy sand. Also included may be submerged soil or soil from which water is freely seeping, and submerged rock that is not stable.
Soil has other qualities that affect its stability. These include granularity, saturation, cohesiveness, and unconfined compressive strength.
- Granularity refers to the size of the soil grains; the larger the grains, the less stable the soil.
- Saturation means how much water soil will absorb.
- Cohesiveness means how well soil holds together; clay is a cohesive soil.
- Unconfined compressive strength is determined by a test that shows how much pressure it takes to collapse a soil sample. For example, type A soil must have an unconfined compressive strength of at least 1.5 tons per square foot.
Trench Safety Measures
Trenches 5 feet (1.5 meters) deep or greater require a protective system unless the excavation is made entirely in stable rock. If less than 5 feet deep, a competent person may determine that a protective system is not required.
Trenches 20 feet (6.1 meters) deep or greater require that the protective system be designed by a registered professional engineer or be based on tabulated data prepared and/or approved by a registered professional engineer per 1926.652(b) and (c).
Knowledge Check Choose the best answer for the question.
5-5. At what depth do trenches require a protective system, unless the excavation is entirely in stable rock?
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