The importance of exploration, stability analyses and monitoring during construction of tunnels is shown by means of three practical examples. For the subway tunnel underneath the Stuttgart airport a lowering of the ground-water table, which would have lead to extensive subsidence of the runway, was avoided. Further the shotcrete support of the circular tunnel was designed to withstand the external water pressure and the high horizontal in situ stresses in the mudstone. The results of monitoring with only small deviations were in agreement with the prediction. The reasons for this coincidence were the extensive explorations, the stability analyses and the experience gained from other tunnels constructed in the same rock. For the very surface near Elite tunnel in Ramat Gan, Israel, a more comprehensive exploration and an extensive monitoring would have been helpful during construction. Nevertheless also for this project the favourable load carrying action of the shotcrete support has been proven, provided the curvature of the tunnel cross section is adequate. The example of the portal near zone of a highway tunnel In Thuringia has shown the capacity of NATM to adapt to difficult unforeseen geological conditions. A more comprehensive exploration of the ground conditions during the design phase could however have lead to a higher degree of safety and to a more economical solution.
A safe and economic design plays a paramount role specially with shallow tunnels, since in many cases these tunnels cross under buildings or transportation routes. According to the Recommendations of the Working Group on Tunneling (ETB) of the German Geotechnical Society (DGGT 1995), the ground conditions are of central importance for choosing a suitable construction method and for achieving the stability of the excavation. Furthermore, a step by step design process is characteristic for tunneling, and individual design phases are repeated if necessary (Fig. 1). The design is based essentially on: the results of a comprehensive geotechnical exploration of the ground conditions, stability analyses coherent with the ground properties and the construction method, and the diligent verification of the stability by in situ monitoring during construction and in Some cases even afterwards (see Fig. 1). The extent and depth of the tasks resp. working steps listed in Figure 1 depends on the conditions of the particular project.
On September 29, 2001 the tunnel under Stuttgart airport was inaugurated, which provides a rapid transit connection between the towns and villages south of the airport and the city center. The tunnel lies at a depth of 10 to 30 m in the layers of the Black Jurassic or, more precisely, the Lias α formation (Fig. 2). In the portal areas, it cuts through a rather more permeable alternating sequence of jointed limestones, calcareous sandstones and mudstones, in which the groundwater is confined locally below surface layers of a low permeability. Over most of its length, the tunnel is situated in the low permeability mudstones of the Lias α.