House Foundations

Do Soils Affect Foundation Design?

As the term would imply, the foundation of a home provides the basis and stability for the rest of the house being constructed.  While the foundation type varies depending whatever region you might be in, a concrete slab foundation is very much the norm here in California.  And depending on the soils in the specific location, the structural engineer will design the foundation to ensure the house is stable for many years to come.

With a concrete slab, trenches will be dug around the perimeter of the proposed foundation and filled with concrete, maybe 18 – 24 inches deep for normal soils conditions.  Then, a 5-inch thick slab is poured across the entire foundation.  And it is typical to put steel rebar rods in this slab to add strength to the concrete.  With some other design features, this would typify a concrete foundation with normal soils conditions.

I recently heard of a project where the soils report indicated liquefaction conditions, which means the soil had some strength issues and is particularly subject to risk during an earthquake.  As a result, the structural engineer recommended a 10-inch thick slab and additional steel rebar.  The cost of these foundations went from a normal $15,000 – $18,000 bid per home to close to $30,000.  In addition to the added rebar and concrete, other factors such as a concrete boom contributed to the increased costs.

Another common issue is having an expansive soils condition.  The typical solution will often be post-tension slabs, which have the added expense of placing cables in the concrete and then being pulled taut prior to the concrete curing.  Again, the structural engineer will recommend these conditions based on the soils report.

Please share with us below your experience, comments, and questions.