Clay for foundation?

[fixybopixy]

If a developer wanted to use a site consisting of mostly sandy soil, with a 6 foot top layer of expansive clay, what are the two best strategies to employ? Choose 2
A. remediate the soil by adding a layer of organic fill and recompact this layer.
B. remove and replace the soil with a gravel sand mixture.
C. place the foundations below the 6 foot layer on the underlying sandy soil.
D. nothing, top layer is appropriate for foundations.

Answer is B & C but I don't see why C is correct - how could it ever be ok to leave clay under a building?

[DaveW]

I don't see how C intends to build on the clay. The intent of C is to build the foundation deeper beyond the bad soil (clay). In other words, the intent is to dig and build on the sandy soil.

[fixybopixy]

I understand that C means that the foundation is below the soil, but this implies that the building is above the clay.

And since clay is subject to frost action and shrinking and swelling, wouldn't that cause differential settlement to the building?

[DaveW]

Yes the building itself is above the clay, but the foundation is not above the clay. To dig six feet below the clay soil is the intent of answer C.

Please understand that placing the foundation below the clay is a good step towards constructing a building on more stable soil. Placing the foundation on stable soil should help minimize differential settlement. I don't see how answer C implies that the foundation will rest on clay.

Expansive clays as they relate to lateral pressure exerted on the foundation wall is another problem though, but that is outside the scope of this question and answer.

[fixybopixy]

yes but i imagine that the first floor of a building will crack if it sits directly atop the clay. am i misunderstanding?

[DaveW]

The first floor will not crack because it will not rest on the clay. It will rest on the foundation which will transfer the load six feet deep onto the more stable sandy soils.

Most times, basements are made out this type of scenario.

[verygnar2meetU]

It helps to sketch the problem, and follow the forces through the structure. Simple schematic diagrams go such a long way towards understanding the concept.