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On Unsteady Ground: Homes on Expandable Clay Soils

The front range of Colorado is one of the most picturesque places to live, with dramatic views of the mountains, wide-open skies, and historic tree-lined neighborhoods. But below the surface lies the challenging issue of expansive clay soils that can affect a home’s foundation.

According to the Colorado geological survey, “Innocuous as it sounds, Colorado’s most significant geologic hazard is swelling soil — that is, soil laced with layers of certain types of clays. These clays cause more property damage than any other natural hazard. Bentonite and montmorillonite (weathered volcanic ash) clays underlie many populated areas of Colorado. They can expand up to 20% by volume when exposed to water and exert up to 30,000 pounds of force per square foot.[i]

Common signs a home is affected by these expandable soils include:

  • Foundation cracks, wall or floor cracks and other types of fractures.
  • Foundation settling, sinking, or upheaval, sagging or uneven floors.
  • Doors and windows that don’t open and close properly and have between the frames .

Typical solutions when building over bentonite include certain types of piers drilled into “competent bedrock” to a recommended depth that will hold structures in place. Creating void space between the piers, under the foundation wall, protects the assembly from the upward movement of the soil. Building recommendations include perimeter drain design and landscaping recommendations, as well as foundation design criteria[ii].

If you suspect there is a serious issue, a foundation expert can help sort through what is really happening. A buyer can also review engineering reports, site sample reports, and any other documentation about how the builder in a given area mitigated the potential damage from expansive soils in an area. Proper excavation, backfill, compaction, drainage, etc., are all critical to a successful build-out for a structure. Some new builders offer warranties on their homes should you perceive shift after you move in.

What Now?

If an older home you love shows some signs, do not despair as there are many options for addressing clay-related shifts. Sealing is one, where waterproofing can be used to combat moisture and drainage issues. Stabilizing is another, where carbon fiber and steel can be used to stabilize the walls of a home[iii].

Yet another set of options includes mudjacking or polyurethane injection. These work to fill the voids in the soil under a structure, and keep further risk at bay. Mudjacking, also known as leveling and slabjacking, “lifts the concrete foundation upon a densely-packed, swelling wave of slurry (a mixture of hard fill including mud, sand, cement, crushed limestone, etc.), while polyurethane injection utilizes dense, durable, environmentally-safe polyurethane foam[iv],” according to the experts at Olshan Foundation Solutions.

They further explain that “in each case, holes are drilled in the floor that penetrate through the concrete foundation and into the sunken spaces below (mudjacking holes are bigger and harder to patch). The slurry or foam is then injected in massive, concentrated quantities, enough to fill the underlying cavities and gradually lift the foundation (and all the weight above it) back to an adequate position. Once the technicians monitoring the procedure are satisfied they’ve added enough foam or slurry to support the concrete slab indefinitely, they will fill and repair the injection holes so the home’s floor looks as good as new.”

The More You Know

Going into home-buying eyes wide open is a priority at Neat Homes. That’s why we believe in providing buyers with inspections upfront on our listings, so both the buyer and seller can determine what works best for their particular situation based on full information.

Not all homes are affected, and many homes have stabilized since their initial shift. There are many experts in the front range accustomed to working with the nuances of front range soils. What matters most is finding the home you love in the neighborhood that works best for you and your family, and then learning about the particularities of that home and then budgeting and planning based on your priorities. 


[i] http://coloradogeologicalsurvey.org/geologic-hazards/swelling-soils/

[ii] https://coloradobuildermag.com/build/the-dangers-of-building-on-bentonite/

[iii] https://www.homeadvisor.com/cost/foundations/repair-a-foundation/

[iv] https://www.olshanfoundation.com/blog/mudjacking-or-polyurethane-injection/