COMPUTATIONAL MECHANICS OF THE EXCAVATION OF TUNNELS

Contains a report on three-dimensional finite element (FE) analyses of deformations and stresses resulting from the excavation of shallow un derground railway tunnels. Multisurface elasto-viscoplastic material m odels are employed for consideration of the mechanical behaviour of th e soil and the shotcrete shell supporting the excavation. Both are for mulated within the framework of closest point projection algorithms. F or soil a cap model is used, consisting of a curved failure surface, a tension cut-off and an elliptical cap. The latter allows consideratio n of the evolution of plastic strains even for the limiting case of a purely volumetric stress state. The movement of the cap is governed by a hardening law, describing the relation between the hydrostatic pres sure and void ratio. The shotcrete model is a rotating crack model, ta king ageing of the maturing concrete into account. It consists of a st rain-hardening Drucker-Prager cone and three Rankine (crack) surfaces. Demonstrates the usefulness of the cap model to predict the mechanica l behaviour of the soil by means of tests on remoulded, saturated clay . The model parameters of the clayey silt of Vienna, where the analyse d tunnel is located, are fit to standard test results. The parameters of the shotcrete model are fit to test results published in the litera ture. Compares the analysis of a single-track tunnel with the results of field measurements from sliding micrometers. Furthermore, the stres ses in the shotcrete lining are examined. In view of the inhomogeneity of the material and of unavoidable deficiencies of the measurements i t is fair to say that the mechanical effects resulting from the excava tion of tunnels are modelled reasonably well.