The type of soil underneath your foundation plays a crucial role in determining its stability. In the Chicago area, around 85% of the soil is composed of clay, with very little loam soil. These soil types can either absorb large amounts of water or evaporate it quickly during mini droughts. You may have noticed that cracks in your walls appear to close during wet conditions and open during dry spells. This movement is characteristic of what structural engineers call a “floating foundation”—a foundation that shifts with the seasons due to weak soils.
It’s important to have your downspouts properly working and keep them at least 10′ feet away from your existing foundation. Another important factor to keep in mind is the grade level pitch must be properly pitching away from you home, otherwise this could cause lateral soil pressure onto your foundation wall causing it to fail and bow.
All homes will settle a little, but some homes will settle more noticeable than others, especially if the foundation of your home or room addition is shallow or above the frost line. Here in the Chicago area all foundations must go below 42″ deep to keep in line with village codes and to be below the frost line (to minimize shifting). If your foundation is shallow (less than 42″ deep) it has a higher probability of settling and sinking.
Why Weak Soils Cause Foundation Problems
The weak soils throughout the entire Chicago area often cause homes to settle and sink, leading to significant structural damage. To prevent this, homes need to be underpinned, transferring the load of the home onto steel pipe piles that reach rock-solid ground deep below the surface.
It's also essential to maintain proper drainage around your home. Downspouts should extend at least 10 feet away from your foundation to prevent excess water from pooling near the base of your home. Additionally, ensure that the grade level is pitched away from your foundation to avoid lateral soil pressure that can cause bowing walls and foundation failure.
Common Types of Soil in the Chicago Area
- Clay – A soil with a highly expansive texture and is easily molded when wet. It is composed of very small particles and can easily shrink and expand. Not suitable for shallow foundations or if the ground gets to wet or too dry. – Not the best type of soil to support foundations especially a shallow foundation (Room additions, Garages, Enclosed Porches Etc.)
- Loam – A soil made up of sand, clay, and silt, with this type of mix and perfect weather conditions this soil is ideal for building homes on, unfortunately only little spurts of the Chicago area have this type soil.
- Peat –A soil type made up of decomposed plant material, which has built up over something like 10,000 years in poorly-drained wetland habitats – NOT ideal for supporting a foundation.
- Silt –A type of soil thatis granular material of a size between sand and clay. Alone not a suitable for drainage purposes can remain mostly moist and cause a foundation to absorb water leading to a very damp and wet basement.
- Gravel – Very large spaces between particles, very little moisture retainment. – Ideal for drainage systems.
- Rock – A type of soil made up of limestone, bedrock, sandstone, or another rock. It is solid and doesn’t absorb moisture. – Ideal for supporting a foundation.