SETTLEMENTS ABOVE TUNNELS IN THE UNITED KINGDOM - THEIR MAGNITUDE AND PREDICTION

Settlements can be a problem with soft ground tunnelling in urban areas where buildings, both modern and ancient, can be put at risk; services, too, can be endangered and at times it has been deemed necessary to divert services before tunnelling is begun. These environmental considerations have led to a considerable research effort being devoted to the study of settlements caused by tunnelling through soft ground; much of the research work has been undertaken either directly by or under contract for the Transport and Road Research Laboratory. Measurements of settlement and ground movement made on tunnelling projects located, in the main, in built-up areas are reviewed. The ground conditions studied included stiff-fissured clays, glacial deposits and recently deposited silty clays, as well as cohesionless soils of low density, weak rocks and made ground. Many of the tunnels were driven in free air by use of shields, but compressed air was used in the weaker soils to maintain stability; the bentonite shield and chemical treatment of the ground were also used in loose sands. The data from these case studies are used to provide simple analytical tools that enable better prediction of the magnitude of settlements and ground movements caused by tunnelling through soft ground to be made.