NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 649

Figure 20.32 Structural model and characteristic parameters of the Lias α (Wittke & Züchner 2008)

20.3.4 Fundamentals of the Design
During the excavation of construction lot 92, the groundwater was lowered to the invert
of the construction pit using drawdown wells. Measurements of the groundwater level at
the airport and the runway area showed that the drawdown cone had a range of approx.
450 m (Fig. 20.33). To avoid settlement due to loading and consolidation of the valley de-
posits mentioned above, it became necessary to recharge the groundwater using injection
wells located north of the runway (Erichsen & Tegelkamp 1998, Wittke et al. 2002).

In the area of the airport runway, the tunnel roof is located approx. 21 m below the
ground surface (Fig. 20.28). The Flughafen Stuttgart GmbH (FSG) demanded that the
tunneling-induced subsidence and settlements in this area had to be limited to 15 mm.
The differential vertical displacements and the resulting inclinations at the ground sur-
face were to be limited to 1:1000.
These requirements proved to be quite demanding because of the several meters thick
settlement-sensitive, soft valley deposits existing in this area (Fig. 20.30). Lowering the
groundwater table during tunneling would have led to settlements of the ground surface
in the area of the runway due to loss of the hydrostatic uplift. In addition, shrinking of
the organic valley deposits due to loss of water would have led to settlements. Lowering
the groundwater table during tunneling could therefore not be permitted. This was why
the gradient of the tunnel was selected in such a way that the tunnel cross-section over
most of its height is located in the layer consisting mainly of claystone with low per-
meability. Furthermore, in the zone north of the runway, where the tunnel crosses the