When we place a foundation we usually dig into the earth to place our foundation on soil deep enough so it doesn’t freeze and stable enough that it doesn’t shift. It must be competent (strong) enough to support the building. If it is not competent enough we need to dig deeper to more competent soil, or distribute the weight of the building more broadly to reduce the maximum bearing forces on any one small area of the soil.
The sizes of soil elements, from largest to smallest, are boulders, cobbles, gravel, sand (all frictional soils), silt (can be frictional or cohesive), and clay (a cohesive soil), which has plate-shaped particles that can behave unpredictably in the presence of moisture.
Buildings resting on clay heave as the clay saturates and then heave again as it dries out. Soils are usually made up of a combination of these particles and the ratios of the different sizes of particles in the soil determine how the soil will behave. Peat and topsoil are examples of organic soil, which cannot be built on because they shift as they decompose. As water is added to dry soil, it reaches its plastic limit (soil expands) and then its liquid limit (soil flows). --Michael Ermann, Amber Book
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