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C. Kropik, G. Hofstetter, H.A. Mang:
"Viscoplastic Three-Dimensional Hybrid BE-FE Stress Analysis of the Excavation of a Tunnel";
in: "Computational Mechanics '92 - Proceedings of the International Conference on Computational Engineering Science", Technology Publications, Hong Kong, 1992, 103.

A realistic mechanical simulation of the excavation of a tunnel under complex geological conditions and an economic design of non-regular cross-sections of tunnels, as occurring at a station or at tunnel bifurcations, require the use of three-dimensional numerical analysis procedures. The application of hybrid BE-FE analysis techniques enables exploiting the complementary advantages of the BEM and the FEM. The "near-field" including the interior of the tunnel, the shotcrete shell and its outer vicinity, where stress concentrations and viscoplastic deformations occur, is discretized by the FEM. The boundary of the elastic "far-field" and the coupling surface of the BE subregion are discretized by the BEM. The constitutive model for the "near-field" is a viscoplastic cap-model. The main difference of this model form the well-known Drucker-Prager material model is consideration of plastic material behavior also for stress paths along or close to the hydrostatic axis. The time-dependence is taken into account according to Duvaunt and Lions. Immediately after application of the load the material is assumed to behave elastically. Thereafter, it approaches the rate-independent solution exponentially such that this solution would be reached for t > \infty.