Large cross section tunnels in soft ground

Contemporary underground transit systems often require cross sections which are not adaptable to shield tunnelling. Due to the frequently large dimensions of these cross sections, which are normally situated at shallow depths and in soft ground, full face advance is not feasible because of ground control and construction requirements. In these large cross section tunnels, shotcrete is frequently used for initial support.;The thesis is aimed at reviewing the current practice of construction and design of these large tunnels and at formulating preliminary design concepts. Excavation schemes which have been frequently used in practice are reviewed and classified and selection criteria for choosing an appropriate scheme are outlined. A number of case histories are investigated and catalogued. The data collected from these field cases are presented in the form of empirical design recommendations, which have been developed for four distinct soft ground classes.;Aspects of tunnel stability are dealt with using a framework based on the Theory of Plasticity. Solutions based on the Upper and Lower Bound theorems are applied to the problem of selecting the construction procedure for a large cross section tunnel. The theoretical results are compared with results from model tests taken to collapse and from case histories of tunnel failures. It is verified that the theroetical solutions provide reasonable estimates of the parameters at collapse in the model experiments. Comparison with actual field cases indicates that these solutions can be used in engineering practice provided appropriate factors of safety are employed.;A detailed review of a case history of a large cross section tunnel in coarse grained glacial soil is also included. Efforts are placed on relating tunnel behaviour to geological and geotechnical aspects. The field measurements obtained also serve as basis for checking the adequacy of the empirical recommendations derived.