Arizona &
the Desert.
Desert ground can hold a house steady for decades and then drop the first time water reaches it. Underneath whole basins, pumping has lowered the land and torn open fissures a mile long. Arizona’s foundation problems start with water, even where there is almost none.
Dry ground that moves.
The Arizona Geological Survey notes that problem soils are found throughout the state, from Yuma in the southwest to the northeast corner of the Colorado Plateau.
Collapsible soils. Wind and short-lived desert streams leave behind loose, dry, low-density sediment. It can stand firm for years, but it is undercompacted. When water finally soaks into it, or when it is loaded heavily by a building or pavement, it compacts and drops. Geologists call the water-triggered version hydrocompaction, and it is particularly common in the semi-arid Southwest.
Expansive clays. Arizona also has clays that swell when wet and shrink as they dry. The Natural Resources Conservation Service publishes shrink and swell maps for both Phoenix and Tucson.
Caliche. Many desert soils contain caliche, a layer cemented by calcium carbonate. It can be hard enough to stop an excavator and still sit on softer soil below. Water that soaks in from above can slow and linger at that layer instead of draining away.
Put those together and the pattern is clear. Desert foundations rarely fail because the ground is wet all the time. They fail when water arrives somewhere it never used to be: a leaking line, a downspout, a new pool, landscape irrigation against the wall, or a monsoon storm with nowhere to go.
Three desert failures.
Two happen under one house. The third happens under an entire valley.
Hydrocollapse
Loose, dry desert soil compacts the first time it is soaked or heavily loaded. The settlement is concentrated where the water went, so one corner or one side of a house drops.
Expansive clay
Where the soil holds swelling clay, water reaching it pushes the slab up instead of letting it down. Same trigger, opposite movement.
Earth fissures
When pumping lowers the water table, basin sediments compact and the land subsides. Where the basin thins against buried bedrock, the ground is pulled apart and cracks open at the surface.
Pumping that sank valleys.
The Arizona Geological Survey describes earth fissures as an anthropogenic hazard: they form when groundwater pumping causes basin subsidence and tension cracks the unconsolidated sediments.
According to the Survey, subsidence and fissures affect more than 3,000 square miles of Arizona. Fissures can run up to two miles long, open 15 to 25 feet wide and reach 90 feet deep, and their true depth may extend toward the water table hundreds of feet down. They have been documented in Maricopa, Pinal, Pima, Cochise and La Paz counties. One complicated construction of the Loop 202 in the Phoenix area, others crossed the Chandler Heights area, and one near Apache Junction damaged Houston Road.
Overdraft had been a problem since the 1930s. On June 12, 1980, Arizona’s Groundwater Management Act became law, creating the Arizona Department of Water Resources and Active Management Areas for Phoenix, Tucson, Pinal, Prescott and Santa Cruz, with a long-term goal of balancing what is pumped against what is recharged.
Warning signs.
In the desert, look for movement that lines up with water. If the cracks are near the irrigation, the pool, the downspout or a known leak, that is a clue.
- 01Cracks in walls, stucco or block that start after a leak, a new pool or new landscaping
- 02One corner or one side of the house settling while the rest stays put
- 03Doors and windows that begin to bind
- 04Cracks in the slab, tile or flooring, or floors that are no longer level
- 05Separation where patios, garages or additions meet the house
- 06Water pooling against the foundation after monsoon storms
- 07Long straight cracks or depressions in the ground, pavement or desert floor nearby
- 08A property in or near a mapped earth fissure or subsidence area
Find the water, test the soil.
A desert foundation investigation asks two questions: what is this soil going to do when it gets wet, and where is the water coming from.
Borings and density
Soil borings and samples show how loose and dry the soil is and how deep collapsible or expansive layers go, including what sits below any caliche.
Collapse and swell tests
Laboratory tests load a sample and then soak it, measuring how much it drops or swells. The result tells an engineer whether wetting is the problem.
Fissure and subsidence maps
The Arizona Geological Survey publishes earth fissure maps. Check them before buying land or designing a foundation in a subsiding basin.
Stop the water, reach stable soil.
A desert repair has to reach soil below the zone that water can wet, and the water source that started the movement has to be found and stopped.
Seeing the signs in Arizona?
Tell us what is happening at the property and whether you already have a soils report or a repair proposal. We reply by email.
Send a question →Arizona Geological Survey
Earth fissure and subsidence information, including the Survey’s fissure mapping for Arizona’s study areas.
azgs.arizona.edu →More regions
Florida karst, Texas clays and the rest of the Atlas. The ground fails differently everywhere, and so does the right repair.
Explore the Atlas →The Arizona library is expanding, with more on Phoenix, Tucson and the basins of southern Arizona to come.
- Arizona Geological Survey, Earth Fissures and Subsidence
- Arizona Geological Survey, Problem Soils: collapsible and expansive soils
- Arizona Municipal Water Users Association, The 1980 Groundwater Management Act
- University of Arizona, Land Subsidence, Earth Fissures, and Water-Level Change in Southern Arizona
The Foundation Repair Network publishes general educational information about geology, soils and foundation performance. It is not an engineering evaluation of any specific property. Conditions vary site by site; consult a qualified engineer before making repair decisions.