 Complete the following phrase:  The lock was originally constructed of masonry, but in the mid-1960s, its was extended at the seaward end, when a steel caisson gate was installed in a reinforced concrete structure. The gate was designed by Sir William Halcrow  Partners, and weighed 58 tons. It was moved into place by electric motors driving chains, and had a hollow centre, making it partially buoyant, with the buoyancy being adjusted by the addition of ballast. Over time, the steelwork degraded, and the mechanism for controlling the flow of water into and out of the gate to alter its buoyancy ceased to operate, resulting in the rollers on which it moved seizing up. In 2014 Northumbrian Water agreed that the structure was beyond economic repair, and due to the complexity of the work, decided to manage the replacement project themselves. Options were somewhat constrained by the fact that the lock is a grade II listed structure, the site is designated as a Site of Special Scientific Interest, a Special Protection Area, a 