Deep foundations are used when the ground doesn't have strong support near the surface. They are built deep underground to find a stable layer for buildings or bridges. This type of foundation is complex and requires expertise in both structure and soil. The goal is to ensure stability and durability of structures by reaching a suitable bearing stratum. Deep foundations are not easy to construct and are different from regular foundations. They are necessary when spread footings are not possible due to limited space. In some cases, like building a bridge pier, the foundation must go even deeper to avoid erosion.