 The construction of the deep-level station involved the excavation of a 39-metre (128 ft) deep void around and beneath the sub-surface station to house the escalators to the Jubilee line platforms. The void, known as the station box, was the deepest ever excavation in central London and was carried out between thick reinforced concrete diaphragm walls stiffened and braced horizontally for stability. Under Bridge Street, on the south side of the station box, the two Jubilee line platform tunnels are arranged with the westbound platform below the eastbound. The depth of the station box excavation and the proximity of the tunnels represented a significant risk to the stability of the foundation of the Houses of Parliament's clock tower (popularly known as Big Ben), which stands only 34 metres (112 ft) from the edge of the station box. To protect the foundation and manage settlement in the sub-soil, a series of 50-metre (160 ft) long steel tubes was installed horizontally around and beneath the clock tower's 3-metre (9.8 ft) thick foundation. The tubes were provided with a control system through which grout was injected into the ground to compensate for settlement detected from detailed measurements of the clock tower's position. As excavation of the station box and the tunnels took place, grout was injected on 22 occasions between January 1996 and September 1997. The process limited the movement of the clock tower to an acceptable maximum of 35 millimetres (1.4 in). Without the grout injection, the movement would have been at least 120 millimetres (4.7 in) causing cracking to the structure of the tower and the Houses of Parliament. The Jubilee line platforms were opened on 22 December 1999, although trains had been running through the station without stopping since 20 November 1999.
