An Engineered Solution

The slab was the major issue that made this house virtually unsellable. Like most foreclosures, the house suffered from years of delayed maintenance, like peeling paint on the windowsills, and broken glass in some of the windows. Once it was determined, however, that the cracked slab had heaved over 3″, a structural engineer was called in to tell the sellers (the bank) that the house had to have major repairs. We had several estimates and ideas of what the repairs would cost and what solutions were available, but we bought the house already knowing that we were going to tear out the slab.

The bank reasoned that one of the lowest bids was likely to be the cost of the repair so the house price was reduced by that amount. We were willing to take the chance, because the rest of the foundation system seemed to be without any visible cracks and we loved the earth-bermed, south-facing “bones” of the house as well as the agricultural zoning and almost 2 acres of land in the city. SO I had to set about finding a solution for a slab on expansive soils.

Much of Colorado has expansive soils and there is a great deal of information about the problems associated with them and building on them. Slab technology has changed over the years and this house was built in the early 1980’s. There was a theory then that the slab should not have an underlayment of rock or plastic as it might retain and draw in moisture. The compromised slab was laid directly on the soil. and although a french drain was laid all around the perimeter of the house, there was moisture that got under the slab and the ground heaved and sank over time until there were deep cracks at the kitchen, at both the south and north corners, and along the back wall. The engineer said that the canal behind the house also “leaked” moisture throughout the area and was probably also responsible for the problems.

This Missouri State University report (will open DAMAGE TO FOUNDATIONS FROM EXPANSIVE SOILS.pdf) is a nice explanation of the issues with expansive soils and foundations. Professional organizations often have good technical articles regarding their subjects. I found several short articles on Concrete in Practice (National Ready Mix Concrete Assoication) and the American Concrete Institute’s Standard. Other sources were commercial sites like Prospections in Texas that had a couple of great articles about this common problem in that state and also lead to the author’s personal site with even more articles and links.

Somewhere in all this research I started reading about Mat Foundations as a structural solution to expansive soils which finally led to Waffle Mat, the solution I chose for this project.

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