The Tunnel Albaufstieg consortium won the contract with an alternative proposal to drive a section about 2.8 km long of the 8.8 km long Boßler Tunnel with a tunnel boring machine (TBM). The next section, where squeezing conditions were forecast, was to be tunnelled by the shotcrete method according to the tender documents. Manifold and extensive additional investigations delivered the basis for the extension of the TBM drive along the entire length of the Boßler Tunnels. The driving of the section through the Aalenian 1 geology was problem free due to the low strengths and good cuttability of the rock mass combined with its impermeability. The next section in the intercalation of claystone and sandstone of the Aalenian 2 was much more differentiated but managable. The breakthrough of the east bore of the Boßler Tunnel confirms to the responsible people for the client and contractor that the decision to follow the aim of consistent mechanised tunnelling had been correct.

In rock formations prone to karstification there are only limited geological/hydrological principles that allow a safe prognosis of the existence and location of karstic and fault structures. A combination of geophysical borehole radar measurements utilizing both reflection and crosshole probe setups can be used to reliably detect anomalous structures, especially air‐filled karstic cavities and fault zones, when a sensible measuring concept for karstified or crystalline rocks is applied. Besides the determination of the anomalous structure's location it is also possible to distinguish its type of filling. Using examples from the major railway project Albaufstieg between Wendlingen‐Ulm the investigation concepts ahead of the tunnel construction are presented. The borehole radar measurements were successfully utilized during different phases of the tunnel excavation and could be integrated into the construction process without significant delays, thus contributing to a safe construction process and later operation of the tunnels. The reliability and high efficiency of these borehole radar investigation concepts places this technology of karst detection at a significant advantage over the commonly applied seismic methods, especially in karst‐prone and crystalline rocks.

In the course of the construction of the new Wendlingen–Ulm line, the Boßler Tunnel with a length of 8,806 m is under construction between Aichelberg and Filstal. Both single‐track main tunnel tubes are for the most part excavated by TBM and lined with segments. The main tunnel tubes have an overburden of up to 280 m and are separated by a rock pillar ranging in width from 5 to 30 m. For safety reasons, seventeen cross passages connect the main tunnels, each at a spacing maximum of 500 m. Cross passage excavation requires the opening of the main tunnel lining and therefore the removal of lining segments. In order to ensure stability of the tunnel lining during the construction phase, special lining segments are used for the main tunnel in cross passage areas. Special lining segments are adapted with additional reinforcement and built‐in components. Shear forces in the circumferential joints are transmitted by shear dowels.