Taking on Expansive Clay Issues

February 13, 2008
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Q: We were asked to dry a crawlspace that had been flooded. We have now been accused of damaging the interior of the building by over-drying, causing it to shift on its foundation. Is this possible?

A: Yes, it is possible, but it could also be the result of other unforeseen conditions. John Banta, in his book “Extreme Weather Hits Home: Protecting Your Buildings from Climate Change,” addresses the issue of expansive clay soil. He states, “Expansive soils are those types of clay soils that expand and contract when they get wet or dry out. This causes them to swell or shrink dramatically. Whether a soil is expansive or not depends on the type of clay as well as other non-expansive materials that are mixed with the soil like sand, certain mineral salts or loam. Expansive soils expand and contract slowly, based on the amount of moisture in the soil. Still, buildings that are built over expansive soils can end up being as damaged as if they’d been in a typical California earthquake.”

In order for this condition to damage a building, a number of conditions need to exist simultaneously. The building must be built over expansive soils, the soils must experience shifts in moisture content sufficient to permit them to shrink or swell and, finally, the shifts in moisture must be severe enough to extend below at least some of the foundation’s footings or piers. This means buildings with deep foundations are less likely to suffer damage from expansive clays than those with shallow foundations.

The Federal Emergency Management Agency (FEMA) has estimated that expansive clay soils are currently responsible for more than $7 billion in damage to buildings, roads and infrastructure in the United States each year. The United States Geological Survey has published risk maps showing “The states of Colorado, Texas, North Dakota, South Dakota and Montana have extremely high concentrations of swelling soils. Whereas the states of Alabama, Arkansas, California, Illinois, Iowa, Kansas, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New Mexico, Ohio, Oklahoma, Oregon, Tennessee, Utah, Washington, Wisconsin and Wyoming have areas that should be checked carefully for expansive soils. The remaining states are less likely to have expansive soils, but it is still a possibility and should still be considered.”

In his book Banta discusses the problems being encountered with buildings constructed on top of these types of soils, and ways to prevent damage from occurring. In parts of the country that are experiencing climate-change-related drought conditions, he points out that studies have shown some of these soils can generate over 15,000 pounds per square inch of pressure as they move. This is more than enough to cause a structure to shift.

Building damage from flooding may or may not be related to drying the soil. Depending on the circumstances, the flow of water from catastrophic flooding may have enough force to result in shifting portions of the building by changing the level of portions of the perimeter foundation, stem walls or piers. The soil under the building may not have been adequately compacted during construction, which could result in rapid settling when the floodwater helped complete the compaction. The flow of water may have resulted in undermining the building’s supporting structure. These are conditions that would have nothing to do with the work that your company performed; they would be directly related to the catastrophic flooding that occurred and would hopefully be discovered early on by an initial inspection.

An additional possible scenario might be that you did not over dry the soil. Instead, in areas of drought that have flooded, tree roots may have grown into the wet soil under the building. When the source of the water that flooded the crawlspace was cut off, the tree roots continued to suck moisture from the soil. A large tree, when stressed by drought, can remove up to 100 gallons of water per day from the soil. This can cause shrinking of the soil localized in the portion of the building that has been inundated by the roots. In order to determine the cause of damage, an experienced structural and soils engineer should be consulted. These specialized experts can determine the cause of the damage and make recommendations to repair the damage.

In some cases, the interior damage from expansive clay soils can be reversed by returning the soil’s moisture content to its original condition prior to the flood. The moisture must be carefully, gradually, added back in a way that doesn’t wash soil or other debris into the characteristic cracks that develop in soil as it dries and shrinks. If extraneous materials get into the cracks, it may prevent the soil from swelling back to its original position, which will not allow the damage in the structure to reverse. A careful engineering evaluation can determine if this approach is likely to succeed, and also provide a plan for soil hydration. Automatic drip watering systems that monitor soil moisture levels and add a measured amount of moisture to the soil as needed have been developed. It may be necessary to have an arborist cut and cap roots of nearby trees to prevent the damage from reoccurring whenever the tree is stressed for water.

You might want to find out if expansive soils are a problem in your area. You can access USGS maps of regions with expansive clay soils at: www.surevoid.com/surevoid_web/soil_maps/risk_areas_cons.html.

If you live in an area where expansive soils are a problem, you might want to consider documenting all cracks in both interior and exterior plasters, and stucco, as well as cabinets separating away from walls as a pre-existing condition.

There are a number of steps that can be taken to help prevent damage to foundations from expansive clay soil when drying buildings:
  • Carefully monitor drying activities.
  • If cracks begin to form in the soil under or around the building you may need to adjust or reduce the aggressiveness of your drying activities. Soils under buildings are usually never completely dry. While it is important to get the moisture level low enough to prevent damage, you do not want to get to the point where soil movement begins.
  • Get professional advice from a soils and building engineer if needed.


When drying soil, it is common for some areas to dry faster than others. This can result in the dry areas experiencing movement while the wet areas remain in place. After drying the soil to a point where it is almost dry, installing a vapor barrier may help to stabilize the soil moisture levels and allow them to equalize without the consequence of excess moisture condensing on the substructure. In addition, adequate unobstructed foundation vents can help prevent excessive humidity from developing above the vapor barrier.

Although the IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration doesn’t specifically address the condition of expansive clay soils, it does provide some guidance. If problems develop as a part of the drying process, disclose and disclaim the potential problem. This would be a good time to review Chapter 12 in the S500 reference guide titled “Limitations, Complexities, Complications and Conflicts.” The presence of potential expansive clay under a buildings falls under “Complexities”. It is a condition “causing a project to become more difficult or detailed,” but does not prevent work from being performed adequately. If the condition of expansive clay soils is not identified until after drying has begun, then the presence of these swelling clays would become a complication and may require “a change in the scope of activities.”

Expansive soils are one of many challenges facing today’s restorers. Being aware of potential situations and responding early can help turn accusations of wrong doing into acknowledgment for a job well done.

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