Foundation damage is prevalent in St. Louis due to several underlying issues with the soil and the weather. We’ll discuss each of the major contributing factors below.
Expansive soil is one of the leading causes of foundation problems in the United States. Much of St. Louis County is built on soil classified as the Urban Land-Harvester-Fishpot association, characterized by its abundance of clay and poor drainage. All dirt is a mixture of sand, silt, and clay in some ratio, but the high concentration of clay in this part of eastern Missouri creates several problems for home foundations.
The most significant problem is that clay expands when it gets wet. It can grow up to 10% of its original volume when it rains or when snow melts. In compacted soil around your home’s foundation, this expansion typically occurs laterally, putting immense pressure on your foundation walls. Expansive clay soil can crack your concrete block walls and create structural damage.
Conversely, clay loses volume as it dries, presenting another problem for home foundations. As runoff soaks deeper into the ground or evaporates, the soil can shrink away from your foundation and leave it unsupported. Although some downward settling naturally occurs in all homes, clay soil tends to dry at different rates around and under your house, leading to differential settling. This uneven ground movement can wreak havoc on your foundation.
As if that wasn’t problematic enough, clay also retains water in larger volumes and for more extended times than silty or sandy soil. It exposes your foundation to moisture for a longer period, giving it ample opportunity to seep through foundation cracks or the pores in the concrete. As such, water intrusion and moisture build-up under homes are widespread issues in St. Louis that can cause significant property damage.
St. Louis experiences just slightly above-average rainfall annually. However, its location between the Missouri River and the Mississippi River and concentrated precipitation cause frequent flooding.
Flooding increases the risks associated with expansive soil, specifically water intrusion and water damage to affected properties. Runoff entering your home can also cause severe water damage and structural damage.
The city’s steep slopes also leave some homes subject to landslides, especially during periods of flooding. Landslides can move heavy, expansive soil toward your house, sometimes resulting in severe, isolated foundation damage.
Limestone Bedrock and Alluvium Soil
The St. Louis area is underlain by limestone bedrock and alluvium soil, each of which creates potential issues for foundations above.
Limestone is a rigid mineral that can be suitable for construction. However, it dissolves in the presence of water, leaving it prone to erosion. The flooding and proximity to two major rivers in St. Louis can contribute to eroding bedrock, contributing to soil movement above and hazardous differential settling underneath your home.
Alluvium soil is characterized by a high presence of gravel or loose silt and sand. This soil is considered “soft” and prone to erosion, so it can also play a part in differential settling under your house, which can lead to structural damage.
St. Louis residents are prone to frequent earthquakes. Most ground shaking in the area is minimal and doesn’t cause severe property damage as higher magnitudes would.
However, even small instances of movement from earthquakes can cause soil shifting that leaves your foundation unsupported. Since concrete is only minimally flexible, any amount of soil settling can be problematic and lead to structural instability over time.
The Mississippi River floodplain in St. Louis is particularly at risk for crippling ground movement, as it has a higher concentration of soft alluvium soil that is already prone to rapid settling.
Lastly, there are many historic homes in St. Louis built before current building codes were in place. Ground preparation and compaction before foundation pouring are crucial for a stable structure, and older homes built before contractors understood the effects of settlement are more prone to foundation issues.