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Basement Construction in Clay Soils

If you plan basement construction in an area that has clay in the soil, you need to take precautions to avoid structural damage to your home.

Before you start basement construction you need to know what type of soil you are building on. The U.S. Department of Agriculture has soil information on most of the United States available at http://websoilsurvey.nrcs.usda.gov/app/ . Soil testing can also be done so you will know what you are working with. Another important step is to make sure the ground is compacted. Failing to compact the ground properly can also lead to trouble in the future.

Expansive clay soils, such as bentonite and montmorillonite, can cause structural damage to basements and home foundations when proper precautions aren’t taken during construction. When deciding to build a basement in an area that has clay soil you need keep in mind the nature of the soil that you plan to build on. Clay holds moisture and as the seasons change the amount of moisture in the soil also changes.

If you build on clay when it is holding large amounts of moisture, you can have your basement sink as the moisture content of the soil decreases. If you build while the moisture content is low, your basement may be pushed up when the moisture content increases. This can end up causing structural cracks. There are several ways to deal with the seasonal changes in the moisture content in clay when you are constructing a basement.

Before putting in the basement, you need to be building on stable ground. With clay in particular, it is important to make sure you have the ground compacted properly so you do not end up with the ground shifting under the weight of your home. It is possible to treat the soil with a lime mixture to increase stability and control swelling. This is done by removing a layer of the soil and adding several trenches under the area you want to construct your basement or foundation. Once you have the trenches in place you fill them with a lime mixture. This treatment adds about a month on the time you are going to need for construction.

Basements that are built in clay soil need to be able to breathe or move up and down slightly with the ground water in the soil. Concrete floors are not attached to the concrete foundation so that this movement doesn’t cause structural damage. To allow for this movement, all internal walls in the basement need to be “floating walls” or raised walls. Floating walls are suspended from the first floor and have a gap between the wall and the concrete floor slab. When you build in some areas you are actually required to use floating walls in basement construction. When you obtain your building permit you need to find out of this is a requirement for your area.

When basement construction includes plumbing, a reverse of this floating wall process is required. The floating walls are attached to the floor to create a solid base and the space is at the top of the wall in these cases. Expansion joints and flex lines are used for basement plumbing when working in clay soil to allow for the natural movement with ground moisture changes. Space must be left for pipes to reduce the risk of pipe breakage.