What is sometimes done to the bottom of an open caisson to provide the required bearing?

• A. Belled out
• B. Ground smooth
• C. Drilled deeper
• D. Piled

Answer: (A)

In open caissons, the capacity to carry the load often comes from end bearing on the soil at the bottom. Making the bottom “belled out” enlarges the base area where the structure sits. By widening the bottom to a bell shape, you transfer the load over a larger contact area and, often, onto stronger soil reached at depth. This reduces settlement and increases the ultimate bearing capacity, which is why belled-out bottoms are used when the surface soils are weak or the required bearing is high. Smoothing the bottom doesn’t add bearing area, drilling deeper just changes depth rather than base size, and piling introduces a different type of foundation. Belled out is the method that directly increases bearing capacity for an open caisson.