 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 . 
