Expansive Soils

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.

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.

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. https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/az/soils/?cid=nrcs144p2_065083