Soil Classification – Part 1: The Basis for Employee Safety in Excavations

by David V. Dow on June 1, 2011

Looks can be deceiving. This photo clearly demonstrates the importance of properly classifying soil on every job site. It would be easy to assume that this rock, which is shale, is stable. But shale is a layered material, and very unstable. The guys working in this trench shield were glad the Competent Person on site had taken the proper steps to insure their safety.

NOTE: This is the first in a 2-part series on the importance of proper classification of the soils that can be encountered on job sites. Such classification is crucial where excavations are part of the work. In this installment we talk about soil types. In the second part — coming in a couple of weeks — we will discuss the various tests the Competent Person on the site can perform to help determine soil type.

A Crucial Responsibility of the “CP”. . .

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

This is cohesive soil with an unconfined compressive strength of 1.5 tons per square foot (TSF) or greater. Examples include clay, silty clay, sandy clay, clay loam, hardpan, and cemented soils. No soil can be considered to be Type A if it is fissured, if it is subject to vibration, if it has been previously disturbed, if it shows seeping water, or if it is part of a sloped, layered system that slopes into the trench at an angle of four horizontal to one vertical (4H : 1V), or greater.  Type A is also very rare because of vibration or having been previously disturbed.

Type B Soil

Type B soil includes:

  • Cohesive soil with an unconfined compressive strength greater than .5 TSF, but less than 1.5 TSF.
  • Granular cohesion-less soils, including angular gravel, silt, silty loam, sandy loam, and (in some cases) silty clay loam and sandy claim loam.
  • Previously disturbed soils, except those that would otherwise be classified as Type C.
  • Dry rock that is not stable.
  • 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), or steeper.

Type C Soil

Type C soil includes:

  • Cohesive soil with an unconfined compressive strength of .5 TSF, or less.
  • Granular soils including gravel, sand, and loamy sand.
  • Submerged soil or soil from which water is freely seeping.
  • Submerged rock that is not stable.
  • Material in a sloped, layered system where the layers dip into the excavation on a 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.

Watch for the next installment on the details of soil testing.

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