Multiple factors contribute to the dangers of structural problems in Chicago. We’ll discuss the most significant issues below and provide an explanation of the impact they have on your home’s foundation.
The soil in Chicago belongs to the Alfic Udarents subgroup of soil classification. This effectively means that the dirt in the area has an abnormally high ratio of clay to other particles and drains very slowly.
Expansive clay soil puts concrete foundations at risk for three primary reasons.
The first problem is that the ground readily absorbs water, which soaks into the gaps between the tiny clay particles. The more runoff that comes in contact with the soil, the heavier and more voluminous it becomes.
The ground swells and puts added stress on your concrete block walls and slab. The hydrostatic pressure is often intense enough to crack the concrete, creating weaknesses that require structural repair.
The second issue is the opposite of the first: the soil becomes less voluminous when it dries. Following rainfall and ground swelling, the runoff gradually drains into the earth, decreasing the soil’s moisture content.
The dirt will shrink away from your concrete, relieving the hydrostatic pressure on the walls and causing the slab to settle deeper into the ground.
This downward movement of the slab is hazardous because it doesn’t always occur uniformly. The soil around your home often dries unevenly, leading to differential settling that can cause significant structural damage.
The last major problem with Chicago’s expansive soil is that it retains water more successfully than typical dirt. As such, runoff and floodwater are held against the concrete for extended periods, giving it ample opportunity to soak through cracks or pores in the foundation.
It’s not uncommon for Chicago homeowners to experience a moist or wet basement or crawlspace.
Above-Average, Concentrated Rainfall
Chicago gets hit with just under 43 inches of rain annually, almost one and a half times the national average. Above-average rainfall leaves the clay soil saturated for much of the year, so hydrostatic pressure is continuously a problem for the concrete under your home.
The rain is somewhat steady from month to month year-round, but most of the precipitation in the summer and fall occurs in large storms that bring most of the month’s rainfall.
The concentrated rain not only contributes to the issues with expansive soil but also means there is plenty of time for the soil to dry between incidences. The frequent wet-dry cycle in Chicago lends itself to differential settling and undue stress on your foundation.
Flash Floods & Deep Frost Line
Flash flooding is a significant problem for homeowners in Chicago for two main reasons. First, the clay soil drains very slowly, so the concentrated and heavy rainfall quickly accumulates on the surface and slowly dissipates.
To make matters worse, Chicago has a combined storm drain and sanitary sewer system, meaning there is a constant and heavy load on the drains throughout the city. They are often unable to keep up with the demand during heavy rain.
Most Chicago homes have a basement because the frost line is very deep, around four to five feet below the surface, on average. Plumbing lines need to be run below this depth anyway, so most builders continue to dig out for a full basement for the added convenience and living space.
The resulting problem is that basements need to be set deep in the ground, but the frequency of flooding puts all underground rooms at severe risk of water intrusion and moisture accumulation.
Finally, many areas throughout Illinois are underlain by limestone bedrock. Limestone is supportive, but it dissolves in the presence of water.
When groundwater or draining runoff interacts with the bedrock in Chicago, it gradually wears it away, creating underground voids.
The result is referred to as a karst landscape, which eventually causes sinkholes to form on the earth’s surface when the caverns and channels eventually collapse.
Sinkholes are more common in other parts of Illinois, but they still occur in Chicago and other urban areas throughout the state.
The karst landscape creates ground instability and contributes to differential settling under your house, often resulting in costly structural damage.