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

A pier foundation is particularly effective for structures built on unstable soil due to its unique design and functionality. Essentially, pier foundations involve deep supports that extend through unstable soil layers to reach more stable soil or bedrock below. This method helps distribute the load of the structure more evenly and reduces the risk of settling, which is a common issue associated with unstable ground.

When unstable soils are present, traditional shallow foundations might not provide the necessary support and can lead to structural issues over time. Unlike shallow foundations, which only reach a limited depth and may not be effective in providing stability, pier foundations go deeper to anchor the structure more securely.

Mat foundations are essentially large concrete slabs that support structures over a wider area, distributing weight more broadly, but they might not always be feasible on very unstable soils due to the potential for uneven settling. Spread foundations, while helpful for distributing loads on well-compacted soil, would face similar challenges as shallow foundations in addressing instability. Thus, pier foundations are often the preferred choice in these scenarios, ensuring that the structure remains safe and stable despite the poor quality of the surface soil above.