Engineering consequences of ground conditions for NATM works at London Bridge station, Jubilee Line extension

The ground conditions encountered during construction of an extension to th e London Underground Jubilee Line are reviewed and the geomechanical model derived from field observation of ground and support behaviour is explained . An important aspect of construction in accordance with the New Austrian T unnelling Method (NATM) is the evaluation of ground and support behaviour a nd the feedback of this information to the designer for verification during tunnelling. The NATM work at London Bridge comprises running, station, ven tilation and concourse tunnels as well as temporary access tunnels with int ernal spans of between 4.8 and 11.3 m, excavated predominantly in London Cl ay. The engineering properties of the London Clay were seen to vary with strati graphy. The relationship between stratigraphy and fissuring has been quanti fied and the subsequent influence on tunnelling is discussed. The behaviour of the ground in the short term is caused by plastic deformation and elast ic stress redistribution, whereas the long-term behaviour results fi om por e-water pressure equilibration. The evaluation of the short- and long-term behaviour of London Clay around advancing tunnels is justified on the basis of the encountered geotechnical characteristics of the ground, geotechnica l measurements and back-analysis using empirical methods. The time-scale fo r transition from short- to long-term behaviour is postulated. The engineering consequences of this short- and long-term behaviour of the ground were seen to be minor, requiring no alteration of the designed suppo rt. Where appropriate some alterations to the excavation and support sequen cing were, however, made.