Several aspects of the geology and climate in the Chapel Hill area put home foundations at risk of structural problems. We’ll discuss each of these below and how they affect your house.
Expansive Soil
North Carolina’s geology is unique to the rest of the country due to an arc of exotic soils believed to have been left behind by glacial movement several hundred million years ago. The arc consists of two distinguishable sections called the Triassic basin and the Carolina terrane. The southeastern tip of Orange County is located in the basin, but the rest of the county is built over the terrain.
The soil within the Carolina terrain is characterized by an unusually high concentration of clay particles. These prevalent soil particles are tiny, creating highly-absorbent dirt that retains moisture between the clay particles. The expansive soil under Chapel Hill creates three primary issues for foundations.
Most significantly, clay-rich soil expands when it gets wet, sometimes up to 110% of its original volume. The upward and horizontal movement created by the swelling puts intense hydrostatic pressure on your foundation walls and slab. The concrete can only flex to a minimal degree, after which it cracks under the stress, compromising the foundation’s structural integrity.
Clay soil also loses volume when it dries. As runoff soaks deeper into the earth or evaporates, the ground around your home can shrink away from your foundation, diminishing the support it typically provides when it’s moist. Areas of your concrete that are unsupported commonly crack under your home’s weight and sink more rapidly into the soil than the rest of the foundation. This process, called differential settling, can cause severe structural damage.
Lastly, clay is more absorbent than silt or sand, so dirt with a high clay content retains more water and holds it for longer periods. This means that your foundation will be exposed to runoff for an extended time after rainfall.
Combined with the pressure exerted at the surface of your walls by swelling soil, this leaves your foundation prone to water intrusion and moisture build-up, as the water has more time to soak through the pores or cracks in your foundation.

Split Foundations
North Carolina homes feature split or multi-level foundations more often than homes in many other parts of the United States. Partial basements and cellars offer convenience, additional living space, and a cheaper installation cost than full basements or crawlspaces. However, they lend themselves to foundation damage.
Soil dries at varying rates depending on the depth, which means the earth under the deepest part of a split foundation will often have a different moisture level than that under the shallowest point.
Since the clay soil in North Carolina is especially prone to shrinking depending on the moisture level, the split foundations prevalent in the area contribute to differential settling problems.
Frequent Rainfall & Drought
Chapel Hill receives an average of around 45 inches of rain a year, which is nearly 50% higher than the annual average in the United States. The city experiences some type of precipitation about once every three days on average during normal circumstances.
The frequent rainfall keeps the ground relatively moist, which means the soil is regularly swelling and pushing against your foundation. The regular precipitation increases the chance of foundation damage from hydrostatic pressure.
Additionally, much of North Carolina experiences drought from time to time. Extended dry conditions allow the typically moist soil to dry out completely, causing the shrinking of the earth and the potential for differential settling.
The sometimes rapid changes from wet ground to dry and back again during periods of frequent rainfall and drought create an abundance of soil movement that can be crippling for rigid foundations.
High Relative Humidity Year-Round
Along with much of North Carolina, Orange County is characterized by a humid, subtropical climate, meaning the relative humidity is comparatively high all year.
Humidity is a problem for basements and crawlspaces because their location underground leaves them prone to moisture build-up. Humid air from outside can seep in and get trapped under your home, contributing to a long list of related issues, including mold growth and water damage.