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Exposed soil cut slope with erosion

Soil

Erosion

The type of soil and climate are taken into consideration by engineers when designing a foundation for a residential structure.  Soil is either native or man-placed.

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Soil samples have to be obtained and tested before engineering properties can be determined.

Structural Damage Caused by Unstable Soil

Structural damage shows itself in the following ways:

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  • Drywall Cracks and Separations

  • Foundation Cracks

  • Trim and Millwork Distress

  • Tile and Flooring Distress

  • Moisture Intrusion

  • Diselevation​​

Determining Soil Types in Your Area

The United States Geological Survey (USGS) and the USDA provide valuable soils information.

USGS

The United States Geological Survey USGS) is a federal agency that compiles soils information.  On its user-friendly website, the USGS has many resources, such as publications, maps, and information regarding soils and geology.

Go to the USGS Map View Site and type in your address to access available soil map data.  You will find plenty of helpful information that has been compiled by geologists.

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The National Resources Conservation Service (NRCS) is a federal agency that primarily focuses on agricultural lands.  However, because of the importance of soil, it has made many technical contributions to soil surveying, classification, and water quality improvement.  The website has a Web Soil Survey page that generates a printable soil report for a specific area defined by the user. 

On the NRCS site, you can print your own general soils report that applies to the area you specify.  Go to:  NCRS Web Soil Survey

Helpful tutorial for the NCRS site:

American Farmland NCRS Web Soil Survey Tutorial 

HOW ENGINEERS SAMPLE AND TEST SOIL

Diagram showing process of drilling and soil sample retrieval for the purpose of soils testing.

STEP 1:  DRILLING

Soil samples are obtained from below-ground at various depths.  Holes are drilled by a drill rig, and soil samples are retrieved.

Copper liner containing soil sample.

STEP 2:  SAMPLES SENT TO LABORATORY

The location, depth, and density of the samples are recorded on the liner, and they are sent to the laboratory.

Soil laboratory testing machines.

STEP 3:  LABORATORY TESTING

Soil samples are sent to a specialized geotechnical engineering laboratory for testing.  Tests done, through various means and methods, on the samples determine the strength and properties of the soil.

Information about different soil types is available here.

WHAT IS
EXPANSIVE OR COMPRESSIVE SOIL?

  • Swelling soil is “expansive.”

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  • Shrinking soil is “compressive.”


Whether or not your area has expansive or compressive soil depends on the local geology.

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This U.S. Geologic Survey map shows areas of expansive soil in the United States:

Department of the Interior, US Geological Survey, Swelling Clays Map of the Conterminous United Stat

Expansive soil has physical qualities that attract and react to moisture and, as a result, can cause the soil mineral particles to react to moisture by increasing in volume (expanding).   As this type of soil dries out, it shrinks.

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The process of determining whether soils are expansive or compressive is achieved by drilling borings at the site to obtain samples at various depths and sending the samples to a specialized soils laboratory for testing.  Samples of the soil are loaded with weight, water is added to the soil, and the soil's behavior is recorded. 


Data obtained from soil testing allow geotechnical engineers to provide structural design guidelines for the construction of the structure.  These recommendations are based on the potential severity of the expansive or compressive behavior of the soil.


In a perfect world, the structural design of the house is specifically tailored to the expected amount of heave or consolidation, and the foundations, slabs, and other components are designed to tolerate the movement or resist the upward loads caused by the expansive soils, or the expansive soil shall be removed or stabilized around and beneath the structure.


In the not-so-perfect world, mitigation of expansive and compressive soils can be neglected or purposely omitted to save money.   As a result, expansive and compressive soils can cause significant and costly damage to structures.

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