Dealing with Expansive Soils

Much of the soil comprising the Front Range of Colorado is expansive in nature. These are clay soils that expand when exposed to moisture and contract as they dry out. The presence of “mud bed cracks” is an indication of this type of soil.

Fluctuations in the moisture content of the soils underlying a structure can cause the structure to move. This is often apparent as unlevel concrete flatwork that was cast in place directly on the soil. Examples of this are driveways, porches, and sidewalks. Water migration under the floor slab is often the cause of unlevel floors, especially in older homes where the basement floor was built directly on the native soil.

What should you do to keep your house in good structural condition? The key to reducing movement of clay soils is the keep the moisture content constant. The most important thing that you should do as a homeowner is monitor and correct the drainage as necessary around the home to reduce water next to the foundation. Avoid plantings next to the foundation that will require frequent watering. Insure that downspouts have extensions to carry water several feet from the house before it is discharged. And provide a positive slope away from the house so water that drops near the house travels away from instead of toward the foundation.

Construction techniques have been developed since the 1980’s to help mitigate the effects of soil movement on structures. One that is commonly seen in the Pikes Peak region is know as “floating slab construction”. The interior basement walls are built with a gap at the bottom to allow the floor slab to move to a small extent. The slab itself is not structural, so small amounts of movement or cracking of the basement floor is not detrimental to the overall structural integrity of the house.

Floating slab construction apparent in unfinished basement. Note gap at bottom of stud wall. Structural support is provided by the steel columns and beam along with the foundation walls.

Another more recent technique used for both new construction and repairs of homes on expansive soils is installation of a raised basement floor. The basement floor, built of either wood or concrete, is above a short crawlspace.

Retrofitted wood basement floor (left) and new raised concrete basement floor (right).