Construction of Basements in Clay Soils

If you plan to build a basement in an area that has clay in the ground, you should take precautions to prevent structural damage to your home. So here are a couple of tips to consider before you start basement construction on clay soils.

Before you begin basement construction, you need to know what type of soil you are building on. The United States Department of Agriculture has information on soil for most of the United States. Soil tests can also be done so you know what you are working with. Another important step is to make sure the soil is compacted. Not compacting the soil properly can also lead to problems down the road.

Expansive clay soils, such as bentonite and montmorillonite, can cause structural damage to basements and home foundations when proper precautions are not taken during construction. When you decide to build a foundation in an area that has clay soil, you need to consider the nature of the soil you plan to build on. Clay retains moisture, and as the seasons change, so does the amount of moisture in the soil.

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

Before laying the foundation, you must be building on stable ground. With clay in particular, it’s important to make sure the soil is well compacted so the soil doesn’t shift under the weight of your house. It is possible to treat the soil with a lime mix to increase stability and control swelling. This is done by removing a layer of soil and adding several trenches below the area where you want to build your basement or foundation. Once you have the trenches in place, fill them with a lime mix. This treatment adds about a month to the time you will need for construction.

Basements that are built on clay soil must be able to breathe or move slightly up and down with the groundwater in the soil. Concrete floors are not attached to concrete foundations so this movement does not cause structural damage. To allow for this movement, all internal basement walls must 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 get your building permit, you should find out if this is a requirement for your area.

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

US Department of Agriculture

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