Know Your Soil Types

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DPC of TExas

Different soils can have a drastic impact on how a project area is excavated. Knowing these soil types and how excavation practices affect the area helps you approach your project safely.

There are three classes of soil: type A, Type B or Type C.

Type A soils include clay, silty clay, sandy clay and clay loam. This soil type is the most stable soil as far as excavation is concerned. The only exception would be if the soil is fissured or has been previously disturbed. It can also become unstable if water is seeping through it or if it has been subjected to vibrations from heavy traffic or pile drivers.

Type B Soils include angular gravel, silt, and silt loam. These soils often have been cracked or disturbed causing the soil to not stick together as well as a Type A soil would. Type B soils also include soils that would be classified as Type A, but they have been fissured or near sources of vibration.

The least stable of the soil types is Type C soils. These include gravel and sand. Soil with water seeping through creates instability and therefore are automatically classified as Type C soils regardless of soil type.

It’s important to remember that excavation areas can have all three soil types in different layers of the soil. The least stable layer determine the classification of that excavation area.  As you begin your excavation project, perform a visual test of the construction site. With a visual inspection of the area will help determine any factors that will lower the strength of the soil. For example, if a soil comes out in clumps as it is being excavated, it is a cohesive soil and creates a stable environment for excavation. Other things to look for include sources of vibration near the excavation site, previously disturbed soil, or signs of water seeping through the soil. You can also look for crack-like openings or chunks of soil that crumble off. If any of these signs are present, the soil is an unstable soil type.

Choosing a good soil sample is key to performing a soil test. A sample should be typical of the surrounding soil in the excavation area. Additional soil tests should be conducted as the excavation deepens. Test the soil from the excavation pile, provided it is fresh soil and has not been compacted in any way. Dried soil samples produces different soil test results so testing with fresh soil is highly recommended.

A thumb penetration test can be used for all three soil types. However, Type A soils typically only show an indentation is the soil with a thumb penetration test. On the other hand, a Type C soil can be easily penetrated with a thumb penetration test. As conditions change, soil must be reclassified.

You can also test the soil using the plasticity method or the pencil test to see if the soil is cohesive. Another testing method is through the use of the pocket penetrometer. The pocket penetrometer provides a numerical number to the soil sample.

For a more comprehensive review of soil testing, you can view this video provided by OSHA:

Jennifer Pratt

Program Manager, DPC of Texas