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Wednesday, July 12, 2006

Drought causing structural damage

I wrote this article in 2003 and it is still valid today.
Mike

Is the drought affecting your home’s foundation?
by - Michael D. Coyle, P.E.
Coyle Inspection Engineers, Inc.
January 27, 2003

RECENT MOVEMENT: As a professional engineer specializing in home inspections and structural repair design, I have seen a dramatic increase in structural movement over the past year, especially in urban area housing. But why now? Could it be the drought? At first this seems to befuddle common engineering logic. After all, the dictate for construction in expansive soil areas has always been - "keep the drainage away from the foundation and do not place landscaping near the foundation that requires water." All the engineering and real estate literature emphasizes this requirement. However, with the drought there has been very little drainage to speak of, and with the watering restrictions our foundations have been high and dry. So what gives? Have the engineers been wrong all these years?

EXPANSIVE SOIL BEHAVIOR: You are probably familiar with the term "expansive soil." It is prevalent along the Colorado Front Range and by some estimations is present in 50% of the land in the USA. In brief, when the moisture content of expansive soil increases, the soil swells, or increases in volume, hence its name. This can result in upheaval of foundations built on the expansive soil. With development of raw land into housing developments, the pre-existing drainage and soil characteristics are altered by the introduction of "hard surfaces" (roofs, driveways, etc.) and landscape irrigation. In semi-arid climates like Denver, this change from historical conditions to "developed" conditions almost always results in an increase in moisture content in the soil at foundation depths. And with the increase in moisture content the potential for structural movement/upheaval increases. Hence the logic of keeping drainage away and minimizing irrigation is sound and the engineers have been correct in this approach. But drought conditions have not been part of the equation - until now.

THE EFFECTS OF THE DROUGHT: We are now into a new cycle in the life of foundation soils in developed areas. Originally (prior to development) the native soils were fairly dry. However, development resulted in elevated moisture levels that stabilized over time at the elevated levels. And as long as that stable moisture content was maintained, all was well. But along comes the drought, and the resulting watering restrictions, and our foundation soils are now drying out. Well, the devious nature of expansive soil is that it will shrink if it dries while under a heavy load (such as a house), causing settlement. This is especially true when a home is built on a spread footing foundation (most homes built prior to 1970), as opposed to drilled piers (caissons). Homes on caissons are typically anchored into deeper bedrock and therefore are less effected by the drought. Homes on footings, however, typically bear onto the soils above bedrock and tend to "float" with the changing moisture conditions of those soils. And as the soils have been drying recently, the homes have been settling. In almost every inspection I hear the following, "this home hasn’t moved in 30 years and now all of a sudden..."

SYMPTOMS OF MOVEMENT: Look for cracking in exterior brick walls and interior plaster/drywall. Cracks tend to develop first at window and door corners. Sloping floors, and binding doors are also indications of movement. Most lots are naturally sloped and settlement frequently occurs at the downhill side of the home. Of particular interest is the degree of cracking and floor slope and the rapidity of its development. Any new cracks of 1/8" or more indicates trouble. Homes with crawlspace depth foundations are particularly susceptible as the foundation is shallow relative to the ground surface. These soils dry out faster than basement depth soils. If the drought continues, however, I would expect basement depth foundations to become increasingly affected.

SOLUTIONS: Depending on the magnitude and rapidity of the movement, it may become necessary to underpin the foundation with steel piers to prevent further settlement. Minor movements however, can be monitored and addressed on an as-needed basis. With a return to more normal climatic and landscape watering conditions, some minor settlements may actually reverse themselves, at least to some degree. This is hopeful thinking however, and I would not count on it, especially if the drought continues for the next year or more. The longer the drought lasts, the greater our problems will become as settlement will continue and damage will progress. The first step is to have a professional engineer assess the current condition of the home, to determine if monitoring is a legitimate option at this time, or if underpinning is necessary to minimize further damage. If underpinning is warranted, the engineer will need to prepare a structural repair design for use by a structural repair contractor. Make sure to use an engineer with specific experience in residential structural inspection and repair design. In the meantime... pray for rain!

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