Structure: Soil Considerations for Foundations and Footings

The most successful structures stay still.  Building designers and engineers must, in essence, try and see into the future and determine how to keep a home “still” for the life of the structure.  They begin by building a home on solid footings and foundations.

We can look at footings and foundations as having two functions:

First, they must transfer the live and dead loads of the building to the soil over a large enough area so that neither the soil nor the building will move. A dead load is the weight of the building materials and the soil surrounding the foundation. A live load includes the weight of people, furniture, snow, rain, and wind.

Second, in areas where frost occurs, they have to prevent frost from moving the building. A live load may also be exerted by water in the soil around the foundation. Wet soil exerts much more force than dry soil. Frozen soil exerts much more force than wet soil. Most frost failures in buildings include horizontal movement (foundation walls cracking, bowing, or collapsing inward) and frost heaving (upward movement of the building as the soil under the building expands due to frost).

soil test

Ultimately, soil quality is key. 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 conditions. 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, designers guess wrong and the building moves, but for an average site, it costs more to find out how good the soil is over the entire site than to design a system that will work on most soils.

The strongest soil type is bedrock, followed by gravel, coarse sand, fine sand, clay, silt, and the weakest of the bunch: organic material. With the exception of organic material, all of the soil types can be built on, given appropriate consideration for the soil type. North Carolina is divided geographically into three regions: the coastal plain, the piedmont plateau, and the mountains. The soil of these regions varies. The coastal plain has a grey sandy soil called Portsmouth sand. It is very porous and drains fast. The mountains have a rock base on the slopes while the valleys and pockets have a rocky organic mix. The piedmont plateau, which is where we do most of our home inspections, is a red clay base with a thin layer of organic matter.  The soil- bearing capacity changes with moisture levels for most soil types, in some cases dramatically.

The function of footing and foundation system 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.

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