H.A. Mang, J. Eberhardsteiner, C. Kropik, G. Meschke, L. Martak, H. Liebsch:
"Tunnelabzweigungen bei kriechaktiven Bodenverhältnissen";
in: "Schriftenreihe "Straßenforschung"", Bundesministerium für Bauten und Technik, Wien, Austria, 1994, S. 129.
On account of its great flexibility, the "New Austrian Tunnel Construction Method" (NATM) is a construction method is suited particularly to drive near-surface tunnels in urban environments or in densely built-up areas, as well as to produce special structures, such as e.g. crossways, tunnel branches and installation cubicles, and to construct cavities under difficult geological conditions. The objective of the research project was to continue the development of numerical methods that allow a near-reality mathematical control of the complex three-dimensional tension and deformation conditions that occur in tunnel construction, as well as to corroborate scientifically NATM application to tunnel driving.
The study under revieq begins with a summary presentation of the theoretical principles of a hybrid calculation method that was developed at the Institute for Material Strengths at the Technical University Vienna. The method is based on a linkage between boundary-element and finite-element discretizations and is suited particulary to solve problems in connection with cavity constructions.
NATM is characterized by a complex interaction in time and space between the structural supporting elements of the tunnel construction - in the first place the shotcrete shell - and the active soil creep conditions. Great significance attaches to the use of realistic laws in connection with the soil and shotcrete materials during numerical simulations of tunnel driving operations, in particular since choosing the strength and stiffness characteristic of the shotcrete has considerable impact on walling resistance.
A three-dimensional constitutive model was chosen to describe the soil material. This elasto-viscoplastic cap model, which was formulated on the basis of return mapping algorithms, has the efficiency that is characteristic of this algorithm class. Special attention was paid to using effective and stable algorithms in formulating the numerical model. The formula has optimum convergence features, within the framework of the incremental iterative solution strategy of the coupled BE-FE method.
In addition, a three-dimensional constitutive model for the shotcrete was developed in the framework of the research project under review, which is based on the multiple-surface viscoplasticity theory. The model takes account of the changes over time in all relevant material parameters, in particular stiffness, as well as tensile and compressive strength, differences in the material and failure behavior of concrete when exposed to multiaxial tensile and compressive strains, and to the creep behavior of shotcrete. In this connection, the theoretical consequences of concrete ageing were taken into adequate consideration.
The results of the FE calculations were compared to data recorded and made available by the City of Vienna. The comparison showed that the main phenomena that occur in urban cavity constructions were reflected in the results of the calculations. In many respects, there is also very good quantitative correlation, although the data were recorded during soil work projects in all parts of Vienna.
Another objective of the project under review was to establish whether the data measured at one cross-section constitute a comprehensive record of the soil deformation behavior in that area. In this connection, the actual results of the calculations provide valuable guidance on future measuring programs.
The report ends with a study on the three-dimensional elasto-viscoplastic BE-FE strength of tunnel branches at different branching angles. The results correlate exceedingly well with reality and provide a good overview of the deformations and stresses that can be expected in the shell and in the soil when driving a tunnel through tunnel intersections and into tunnel branches. The results did not indicate that the branching angle had a significant impact on the relevant status parameters, i.e. the lining of the main tunnel and of the tunnel branch.