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How does soil quality affect foundations?

How does soil quality affect the way foundations are built?

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Buildings rely on the soil beneath them to stay put. If the soil under the house moves up, down or sideways, the house is in trouble. Designers of homes may know quite a bit about the soil conditions at a site and may design the building exactly for those. More commonly, soil conditions are assumed to be a certain type, and footings and foundations are designed with a margin of safety to account for adverse soil conditions, within reasonable limits. Occasionally they guess wrong and the building moves, but for an average site, it costs more to find out how good the soil is over the whole site than to design a system that will work on most soils.

You should understand that many soils are a combination of types, and many building sites contain more than one soil type. The soil profile can change across a site from side to side, and can change down into the soil. With all those qualifiers, here is a ranking of soil types from strongest to weakest.

1. Bedrock (strongest) 2. Gravel 3. Coarse sand 4. Fine sand 5. Clay 6. Silt 7. Organic material (weakest)

With the exception of organic material, all of the soil types can be built on, given appropriate consideration for the soil type. The soil bearing capacity changes with moisture levels for most soil types, in some cases dramatically.

The function of footing and foundation systems varies with location. Perimeter foundations have to resist the lateral thrust of soil outside the foundation wall. Interior foundations and footings under columns, for example, see more purely vertical loads.
by Manage My Life
April 26th, 2007
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