What type of foundation is typically used for structures on unstable soil?

A pier foundation is typically used for structures built on unstable soil due to its ability to transfer the load of the structure deep into the ground where the soil is more stable. Pier foundations consist of long, slender columns (piers) that are drilled or driven into the ground, reaching stable strata far beneath the surface. This deep foundation system reduces the risk of settlement or movement experienced by structures built directly on unstable soil.

In comparison, shallow foundations, such as spread and mat foundations, are generally not suitable for unstable soils, as they distribute the load over a larger area at or near the surface, which can still be prone to movement and differential settlement. While mat foundations can support larger structures by spreading the load over a wider area, they may not effectively address the underlying issues of unstable soil in the same way that pier foundations can. Thus, the design choice of a pier foundation is specifically tailored for such conditions, making it the correct answer for this question.