On every job site where employees must work in excavations, an important responsibility of the Competent Person is the accurate classification of the soil. OSHA’s Subpart P, Appendix A, describes methods of classifying soils based on site and environmental conditions.
The Competent Person is required to perform at least one visual and one manual test as a basis for classifying the soil. The soil type is then used to select the proper shielding or shoring protective system (or the proper sloping technique) that will protect workers. The second installment of this article will discuss the details and the “how to†of these visual and manual tests.
OSHA Recognizes 4 Types of Soils.
Stable Rock
OSHA defines Stable Rock as natural, solid mineral matter than can be excavated with vertical sides, and will remain intact while exposed. Stable Rock is extremely rare. In fact, less than 2 percent of the soil in the U.S. is classified as Stable Rock. In addition, the process of excavating with saws, breakers, dynamite, etc., will likely fracture and destabilize the material that was initially considered stable.
Type A Soil
Cohesive soils with an unconfined compressive strength of 1.5 ton per square foot (tsf) (144 kilopascals (kPa)) or greater. Examples of cohesive soil include 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:
- The soil is fissured; or
- The soil is subject to vibration from heavy traffic, pile driving from heavy traffic, pile driving, or similar effects; or
- The soil had 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 to other factors that would require it to be classified as a less stable material.
Type B Soil
- Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa); or
- Granular cohesionless soils including: angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam.
- 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 Soil
- Cohesive soil with unconfined compressive strength of 0.5 tsf (48 KPa) or less; or
- Granular soils including gravel, sand, and loamy sand; or
- Submerged soil or soil from which water is freely seeping; or
- Submerged rock that is not stable, or
- Material in a sloped, layered system where the layers dip into the excavation or slope of four horizontal to one vertical (4H:1V) or steeper
IMPORTANT: Soil Conditions Can Change. It is important to remember that, for a variety of reasons, soil conditions can change as time passes. The on-site Competent Person must constantly monitor soil conditions and assess whether additional precautions are necessary. A project might start in Type B soil, and then change to Type C, and then back to Type B.
How to Avoid Classifying Soils.
In many instances the Competent Person can simply classify the encountered soil as Type C, the least stable, and then shield, shore, or slope accordingly.
Matt Timberlake