Deep foundations are structural elements that transfer the load of a building or structure deep into the ground, reaching stable soil or rock layers. They are used when surface soils are weak or unstable, ensuring that the structure remains safe and secure. Common types of deep foundations include piles and caissons. These foundations are typically installed using specialized equipment and techniques, such as drilling or driving. Deep foundations help prevent issues like settlement or tilting, which can occur if a structure is built on inadequate soil. They are essential for large buildings, bridges, and other heavy structures.