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Geologic Hazards in Arizona | Swelling & Shrinking Soils

Arizona’s Expansive and Collapsing Soils – A Geologic Hazard: Why, Where & What you can do?

SOIL (n) :  The unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.  (Source: Soil Science Glossary)
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There are many different soil types.  The basic ingredients of all soils are variable proportions of solid particles (sands, silts, and clays), organic material, water, and atmospheric gases (oxygen, nitrogen, argon, and carbon dioxide).  Arizona’s state soil – each state has a type soil – is the Casa Grande soil from near the city of the same name.

Soil Hazards in the U.S.  According to the American Society of Civil Engineers about half of the homes in the United States are built on expansive soils. And of these homes, nearly half suffer some damage because of the soil.  Each year in the U.S., expansive soils are responsible for more damage to homes than are floods, tornadoes, and hurricanes combined!

The geology and semi-arid climate of the Desert Southwest provide near ideal conditions for the formation of expansive and collapsing soils.  And, unfortunately, problem soils are found throughout Arizona, from Yuma in the southwest to the northeast corner of the Colorado Plateau.

Expansive and Collapsing Soils.  Expansive soils contain clays – microscopic-sized minerals – that are capable of large volume changes in the face of changing water conditions.  Add a little water – say during a monsoon storm -- to expansive smectite clay and it swells to many times its original volume.  Remove that water during the hot, dry summer and the clay component of the soil shrinks.  The resulting changes in soil volume can cause considerable damages to homes, sidewalks, pipelines, and streets. 

Expansive Soils in Phoenix & Tucson. Visit the Natural Resources Conservation Service website for maps showing the distribution of shrink/swell soils (i.e., expansive soils) for the greater metropolitan areas of Phoenix and Tucson.

Collapsing Soils consist of loose, dry, low-density material – i.e., undercompacted – that shrinks in volume when wetted (hydrocompaction), and/or when loaded with a great weight, such as a building or street.  These types of soils are particularly common in the semi-arid southwestern U.S. where wind and ephemeral streams deposit loose, unconsolidated, and undersaturated (re.: dry) sediments that are prone to sudden collapse. 

Recognizing Expansive Soils.  Expansive soils display characteristic visual and mechanical behavior.  Some common visual and tactile indicators include:

  • Desiccation cracks – see the image top right
  • Popcorn soil texture
  • Soil that is sticky when wet
  • Cracked foundations and warped roads

But the surest check for expansive soils is to employ a reputable geotechnical firm to 1) take soil borings, and 2) test the expansiveness of the soil. 
 
And how can expansive/collapsing soils impact you?  Well, if problem soils are identified prior to construction, there are a number of things that you can do to minimize their impact.  For a quick rundown on your options see “So your home is built on expansive soils”, by W.K. Wray and published in 1995 by the American Society of Civil Engineers.

 

If, however, your home was built without engineering for expansive soils, some of the more common effects include (1):

  • Foundation cracks
  • Heaving and cracking of floor slabs and walls
  • Jammed doors and windows
  • Ruptured pipelines
  • Heaving and cracking of sidewalks and roads

Unfortunately, remediating expansive soils after building your home can be an expensive and daunting task.  But there are some things that you can do, and chief among them is behaving miserly in watering around your home’s foundation.  The destructive cycle of expansive soils is driven by alternate wetting and drying.  To break the cycle, avoid placing plants next to your foundation, thereby minimizing the volume of water introduced next to your home. 

For an excellent, well-written, well-illustrated primer on expansive soils, see “A Guide to Swelling Soil for Colorado Homebuyers and Homeowners”.  Written for the general public and published by the Colorado Geological Survey, this text, revised in 2007, sells for just $7.00. 

(1) Assn. of Environmental and Engineering Geologists website

File(s) Available for Download:

Collapsing Soil

Desiccation Cracks in AZ 2004

For More Information:

USDA: Informative and detailed maps of Phoenix and Tucson showing areas of problem soils.

Association of Environmental & Engineering Geologists  

USDA Natural Resources Conservation Service (NRCS)

NRCS Web Soil Survey 

Soil Science Society of America


(above) Colorado Geological Survey, A Guide to Swelling Soils

Casa Grande Soil – Arizona’s State Soil

Status Map of Soil Surveys in Arizona from NRCS Soil Survey Program

 

 

 

 

 

 

 

 

 

 

 

 

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