Floods Are Not the Only Risk Buildings Face From Heavy Rainfall

By Franco Faraudo

As the West Coast experiences record rainfall, millions are on high alert for floods. The danger of floods is ingrained in humans, some of the oldest stories ever told are cautionary tales about large flood events. We all have images burned into a permanent place in our memories of floods sweeping away homes and filling once-dry valleys with water. But floods are not the only thing we need to be worried about when it comes to large rains. Sustained rainy periods can make the earth underneath us damp, which with certain types of soil can actually push buildings out of the ground.

There have been many instances of what is called a “floating foundation” or “hydrostatic uplift.” It usually happens to buildings with shallow foundations on soil with a significant amount of clay. One of the most famous examples of uplifting happened in San Francisco, one of the very cities getting drenched at the moment. When The Fillmore Center Parking Garage was built in 1950, the hole excavated for its construction allowed water to soak into the ground beneath neighboring buildings. Some of them, particularly the ones with shallow foundations, were pushed upwards as much as 15 inches. As you would expect, lawsuits and engineering reviews ensued. 

The Fillmore Center Parking Garage was an example of construction related to over-saturation of soil but there are plenty of other buildings that have been damaged by wet clay soil. After Hurricane Katrina, buildings in New Orleans suffered significant damage from uplifting due to how long the water stayed pooled on the ground. This is the worry for what is happening now as a number of “atmospheric rivers” are set to continuously hit certain areas. While one alone might not be enough to cause a large-scale flood, enough water could cause soil to saturate to a level not considered by engineers when the buildings were designed.

It isn’t just uplifting that can be a problem for foundations. Soil is anything but uniform so the forces created when it expands and contracts are never distributed equally. The differences in pressure can cause settlement and lateral movement called “heave.” If we experience alternating extremes, extended wet seasons alternating with long periods of drought, it can cause soil to pull away from foundational pilings and cause them to lose the “skin friction” that they need to support the weight above them.

Building owners are always worried about water. Water damage is the most common insurance claim and floods are the most costly natural disaster. Now they need to worry not just about the water that is in the pipes inside the building and the clouds above it, but also about the water in the ground that sits below. Many places on the West Coast need rain to help alleviate a water shortage but just like medicine, when it comes to rainfall, the poison is in the dosage.



Here are some great maps showing just how hard and how acute the rainfall has been for some parts of Northern California.

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