Their construction was complicated by the need to keep the railway bridge operating without interruptions, the Bakerloo line tunnels passing only a few feet under the river bed, and the potential danger of unexploded World War II bombs in the Thames mud. Despite extensive surveys of the riverbed, London Underground was unwilling to accept these risks and preliminary works were stopped in 2000. The design was modified so that the support structure on the north side, which would have been within of the tube lines, was moved out of the river bed and onto Victoria Embankment. Excavation near the tube lines was carried out when the tube was closed, and foundations were hand-dug for additional security.