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Dry Expansive Clay - Expansive Soil

I came across a web page entitled “EXPANSIVE SOIL IS NOT THE PROBLEM”. This claim is from a 30 year “foundation expert”, who’s only qualifications are to have been in the concrete industry for that time. In fact, this person has never taken responsibility for his own fix to soil heave or settlement and had to worry about a warranty for soil related fixes. Understanding expansive soils requires an understanding of soil mechanics and has little to do with concrete. Here in this blog, I would like to explain why expansive soils are the root of many home foundation problems.

Expansive soils are made up from expansive clays. Clay particles are microscopic in size… in fact, to see them you need not just an ordinary microscope, but a scanning / tunneling microscope. Since clay particles have a slight negative charge and water has a slight positive charge, they are attracted to each other and because of their complimentary charges try to position each other between themselves…. Causing the clay to expand with water and shrink when dried.


“Expansive soils are indeed the root of many home foundation problems.”

– Bob Brown (Industry Expert, LEED Accredited & Certified Foundation Repair Specialist by the Foundation Repair Association)


If structures are on top of these soils when they get wet, they can swell. Or if the soil dries out can shrink… causing the structure to either heave up or shrink down. Slabs have little weight and cover a larger area and heave easily. Footings, not usually (bot not impossible).  Check out:

A novice will always point to the weight of the house on footings and the soil’s inability to support those loads. In the grand scheme of things, 10 feet of soil on top of a weak layer can weigh just as much as a house or more. Surface water may never reach those depths and have almost no influence.


Putting gutters on a house will not solve foundation problems by itself for the following reasons:
  • The house may be settling from deep soil weaknesses that have little to do with surface water…. this is common in the Gilbert area where deeper clays 10-15 feet deep are finally drying from earlier farming.
  • If the home is located on expansive clays and you dry them out with rain gutters, you will likely induce settlement from those clays drying and shrinking.
  • If a floor slab is heaving from accumulating moisture, it tends to accumulate near the middle. Drying out the edges exacerbates the differential.

Metro Phoenix Clay Soil Map - Expansive SoilSoils are complicated……. with many different layers that are not uniform in depth. Those layers have different mineral contents, density, and moisture content. Water moves through those layers at different speeds and each of those strata react differently to moisture intrusion. Many of the characteristics of soil behavior are counter intuitive. Foundation movement analysis should not be attempted by those without a thorough understanding of soil mechanics.

To ensure that your home is being analyzed properly don’t be fooled by a salesmen who touts their years of experience “as better than an engineer”. Insist that your home be analyzed by a licensed professional engineer and that the repair plan recommended flows from that analysis. 

You & your home deserve it.