There are numerous factors in Nashville that contribute to foundation problems. We’ll discuss each of these in-depth below, as well as how they adversely affect the concrete under your home.
The soil in Nashville is considered sandy and silty loam, meaning it has a high presence of sand and silt and a relatively low concentration of clay. Clay particles retain water and slow drainage, and the lack of these absorbent soil constituents in Nashville leads to excessively fast drainage and overall loose soil. This type of earth is problematic for foundations for two primary reasons.
The first is that it’s prone to erosion. Without the clumping effect of clay particles, the ground in northern Tennessee is easily washed away by runoff. This regularly occurs at the surface, but it’s significantly more dangerous when precipitation causes erosion underground.
The rapidly-draining soil can easily be shifted by runoff under or around your home, potentially leaving sections of concrete without the foundation they need to hold up your house. If the concrete loses support from eroding soil, the unsupported portions can crack away from your home and sink more rapidly into the earth. This process, called differential settling, can quickly cause structural damage.
The second reason Nashville’s loose soil is problematic for homes is that underground erosion can occur against your home’s foundation, leaving voids next to the concrete over time. These voids can fill with water during precipitation, leaving your foundation at severe risk of water intrusion. As such, water problems and moisture build-up are prevalent issues for Nashville homeowners.
Sandy and silty soils are prone to erosion, which means that runoff is particularly hazardous for home foundations built on them. Unfortunately, Nashville residents not only have to contend with the problematic soil, but they also experience an above-average annual rainfall.
The city gets approximately 50 inches of rain each year, which is about 50% higher than the national average. It rains, on average, about twice per week throughout the year in Nashville.
The frequent and heavy rainfall in Nashville makes the issues with the rapidly-draining soils significantly more dangerous. More rain means more runoff, which means a higher chance of the ground around your house eroding and causing differential settling.
It also contributes to problems with voids forming against your concrete, filling with water, and leading to water intrusion in your house.
Tennessee is considered to be at moderate risk of drought, experiencing abnormally dry conditions about 15% of the year. A lack of rainfall can cause the soil to lose volume as it dries out, contributing to differential settling.
The intermittent periods of drought that are punctuated by heavy rainfall lend themselves to cycles of swelling and shrinking. Even though the ground movement is relatively minimal due to a low concentration of expansive clay in the soil, even small ground movement can create severe settling issues for homes in Nashville.
Shallow Frost Line
Nashville experiences hot summers and cold winters, with temperatures regularly dipping below freezing in the colder months. The moderately temperate climate results in a very shallow frost depth throughout much of the city, sometimes as shallow as 12 inches.
Frost depth can significantly impact foundations because it determines how deep the water in the soil freezes. Shallow frost depths like those found in Nashville mean that only a small portion of the earth will freeze over during the winter months.
Water expands when it freezes, which means only a very shallow section of the ground will freeze and expand during the Nashville winter. This places uneven stress on the concrete under your home, similar to the hydrostatic pressure that can occur from water pooling outside your foundation walls. It can cause cracking and the loss of structural integrity over time.
Limestone Bedrock & Sinkholes
Lastly, the bedrock in Nashville is comprised mostly of limestone, dolomite, and gypsum. These minerals can provide adequate stability, but they are prone to erosion when exposed to water.
The bedrock in Nashville erodes at relatively high rates because of the above-average rainfall in the area. When erosion occurs, the soil above the bedrock loses stability, which in turn can cause ground movement below or around your home.
This is a phenomenon called karst, which forms caves underground and sinkholes on the earth’s surface. Karst can potentially leave your foundation without the support it requires to hold up the structure above.