Cofferdams and bulkheads, structures to hold back water, are sometimes also considered retaining walls. The piers are protected by sheet pile cofferdams and filling. To divert the water, two roller-compacted concrete cofferdams will be constructed. The foundations were constructed inside deep cofferdams and steam-driven pumps kept out water. Frame and fabric cofferdams are erected in the water and covered with watertight fabric. Contractor for the cofferdams was Messrs. Lynn (Liverpool). Cellular cofferdams were constructed to retain the material. To facilitate the river's diversion, two cofferdams were built. Portable cofferdams can be inflatable or frame and fabric cofferdams that can be reused. Ultimately, the cofferdams would become peripheral portion of the main earth-cum-rock fill dam. Common uses for cofferdams include construction and repair of offshore oil platforms. The "Gazelle"-class cruisers were also equipped with cork cofferdams to increase buoyancy. The cost of using the cofferdams was $300,000. The ships were also equipped with cork cofferdams. The cofferdams for the bridge were the largest ever built at the time.