Massive fault zones are a major challenge for deep tunnel excavations. Several fault zones were investigated by three dimensional numerical analyses, which considered the excavation process in detail. In the calculations the examined periods range between one and two years. All obtained results such as deformations, strains, stresses, pore pressure, utilisation, forces in the deformation elements and lining sectional forces etc. are time dependent. In order to achieve an acceptable system behaviour, the support measures had to be adjusted to the considered rock conditions. These are, amongst other things, adjustments to the steering gap for the TBM and the ring closure distance and additional drainage drillings required in advance to minimise the adverse effects of seepage forces and in the case of conventional excavation to also increase the number and/or capacity of deformation elements.
Comprehensive geological and geotechnical reconnaissance and determination of parameters are prerequisites within the scope of design. The focus is on the confirmation of the system behaviour, based on the proposed excavation method and support measures in interaction with the prevailing rock mass. The rock mass behaviour type is influenced by the current state of stress, groundwater, spatial setting of the rock mass types including fault zones and tectonic setting.
Based on the foreseen excavation method, regular and auxiliary support measures are applied in order to attain acceptable system behaviour. For conventional tunnel excavation the aim of the analyses is the determination of support measures such as shotcrete lining, rock bolts, deformation elements and drainage drillings. In case of shield-driven tunnels the question arises if the steering gap is sufficient to avoid the closure of the gap and as a consequence the development of excessive pressure upon the shield. For the design of regular and auxiliary support measures of a particular tunnel, project extensive numerical 3D-finite element calculations using program system Z_Soil were carried out.
The analyses of effective stresses in relation to time were performed by taking the consolidation process into account. Unlike in soil mechanics water cannot be assumed as incompressible, this would yield a grossly incorrect value for un-drained bulk compressibility (Jaeger 2007). In a consolidation analysis both equilibrium (1) and continuity (2) have to be fulfilled in each time step: