Philadelphia is a hotspot for foundation problems primarily because of several geological and climatic factors. We’ll describe each of these below and how they negatively affect the concrete under your house.
Philly is situated above a very perse arrangement of soils, many of which are less than ideal for construction. The most widespread dirt in the city has a high concentration of clay particles, making the ground expansive. Expansive soil leaves foundations prone to damages in three different ways, all of which are heavily influenced by the weather.
The first problem with Philadelphia’s expansive soil is that it swells in the presence of water. Runoff soaks slowly into the ground, gradually filling the small gaps between the tiny clay particles.
As the dirt absorbs more and more rainwater or snowmelt, it expands and places undue pressure on your foundation. It pushes your foundation walls inward and can force your concrete slab upward to the point where they crack, compromising the foundation’s structural integrity.
The opposite moisture conditions are also a problem, as expansive soil shrinks when it dries. Moisture in the ground naturally drains or evaporates over time, causing the soil to lose the volume it gained from the runoff. The shrinking soil can retract from the concrete under your home, at which point your foundation sinks into the earth until it finds the support it needs to hold up your structure.
This downward movement of the earth and your foundation can happen at different rates under your house in a process called differential settling. The result is sections of your concrete sinking more rapidly than others, eventually causing stress cracks and a loss of structural stability.
The third and final issue with expansive soil in Philadelphia is that the clay is highly absorbent. It can hold water against your foundation for days after precipitation falls.
Your foundation’s long exposure times to moisture increase the likelihood of leaking or water vapor seeping into your home. As such, the resulting water damage and humidity problems are prevalent in Philadelphia.
Philly experiences around 44 inches of rain every year, nearly one and a half times more than the national average. The excessive runoff perpetuates the issues with expansive soil.
The ground is continually moist for long periods due to the above-average precipitation, so the soil is likely to remain at its peak volume and place intense and constant hydrostatic pressure on your concrete.
Additionally, the frequently wet soil holds runoff against your foundation for days or weeks at a time. As a result, water intrusion is a prevalent problem in Philadelphia.
Pennsylvania has the highest flooding rate of any state in the U.S. Philadelphia is prone to several different types of floods, including flash floods when rainfall fails to penetrate the dense clay soil quickly, river flooding, and tidal flooding.
To make matters worse, the sewer system in the city was built in the late 1800s when far less of the absorbent soil was covered by homes and pavement. The system is ill-equipped to handle the current demands during heavy rainfall and flooding of the Delaware River, so it frequently backs up and contributes to floods.
Flooding saturates the ground, increasing the likelihood of severe hydrostatic pressure on your foundation as well as the risk of water intrusion and moisture accumulation under your home.
Limestone Bedrock & Sinkholes
Lastly, Philadelphia is underlain by various types of bedrocks, including schist, limestone, and dolomite. Limestone and dolomite dissolve in the presence of runoff and groundwater, creating underground caverns and channels over time.
These voids can collapse and create soil instability and sinkholes near the surface. Additionally, geologists believe that old stream valleys that are less compacted than typical soil contribute to land subsidence.
No matter the underlying cause, the downward shifting of soils under or around your home can contribute to differential settling and create significant structural problems.