Over the years, I have inspected numerous foundations of homes that are constructed on severe topography, i.e., where the ground surface slopes significantly, such as on the side of a hill or on a creek lot. Generally speaking, what I find is that portion of the foundation that is downhill, has settled. This settlement occurred because the builder/developer brought in fill material to level the lot prior to installing the foundation and the fill dirt consolidated (compressed) over time and allowed the slab to settle. Also, dirt located uphill will tend to move downhill (because of gravity). This action is called “soil creep”.” Soil creep” is a slow gradual action that usually occurs over a several year period.
Obviously, it is possible to design/construct a stable foundation for these types of conditions, however, the design needs to be well thought out. It is recommended that a site specific soil report be obtained that determines the soil properties & characteristics. The design engineer will then use the soil data to design the foundation. Many times, a foundation located on the side of a hill should include piers that are founded in a stable bearing stratum.
Usually, these piers are drilled concrete piers, preferably installed into a stable rock formation or if rock is too far down, the pier shaft is belled at the bottom to a diameter of 2.5 to 3 times the diameter of the pier shaft. In other words, if a 12″ diameter pier shaft is specified, the bell diameter will be 30″ to 36″ diameter (depending on what the design engineer determines).
Yes, this is expensive, but it is usually not as expensive as trying to install piers after the home is built and the foundation moves.
Also, when a home is being constructed on a severe hill, it is many times, advisable to install a retaining wall. I have seen many homes where the soil is moving downhill and causing a void space to appear under the slab. The only way to stop this “soil creep” downhill is to have a stable retaining wall stopping the soil. The foundation design engineer can determine if a retaining wall is necessary.
The object of this blog is to caution folks who are going to build a slab foundation on the side of a hill to be cautious AND obtain the proper engineering advice.