The 8.8 km long Boßler Tunnel was tendered in 2012 as a NATM tunnel, although a variant with a tunnel boring machine (TBM) was permitted for the first 2,800 m of the tunnel from the north side. The joint venture Tunnel Albaufstieg ATA won the contract in a negotiation process with the variant TBM for the first 2,800 m. Out of the conviction that a large part of the Boßler Tunnel should be feasible for a TBM, the ATA made an optimisation proposal, according to which extensive additional investigation should be undertaken in the preceding NATM tunnel and to demonstrate the feasibility of extended TBM operation. This concept was successively implemented, with a 55 m deep investigation shaft being sunk and a 20 m long investigation tunnel excavated. The investigations brought the hoped‐for improved estimation of the rock mass behaviour and finally convinced all those responsible for the project (client and the contractor) that a TBM drive along the entire length of the tunnel was feasible. This procedure demanded extreme flexibility from all parties involved since the design work mostly had to be undertaken at the last minute.

The article provides a summarised report about the redesign of the 8,822 m long Boßler Tunnel from the originally intended shotcrete method to a mechanically driven tunnel. The twin‐bore Boßler Tunnel is the main contract on the Albaufstieg (Alb ascent) section of the new DB line from Wendlingen to Ulm as part of the major DB project Stuttgart–Ulm. In 2012, the tunnel was tendered by the DB AG for construction by conventional methods with shotcrete support, but a bid for the Boßler Tunnel was received from the consortium Porr, Hinteregger, Östu/Stettin and Swietelsky with an alternative proposal to drive a 2,900 m long section of the tunnel with a machine. The contract was awarded with the alternative proposal and during the construction of the Boßler Tunnel, a change was then made to complete mechanised tunnelling in a unique technical and operational process. The report provides an insight into the challenges of a change to the overall construction system, undertaken after the award during the construction period, for the driving of altogether 17,500 m of tunnel bores. Construction operations, logistics, design for construction, additional investigation measures and contractual parameters were reorganised and redesigned on a tight schedule. Starting with the original 5,800 m TBM drive and the start of tunnelling in April 2015, the entire Boßler Tunnel was actually constructed as a mechanically driven single‐pass tunnel by June 2018.

The central problem with tunnelling in karstified rock mass is the karst structures, which can be empty, partially or completely filled. The filling can consist of soil, water or a combination of both. Before the start of tunnelling in the Steinbühl Tunnel, a section of the new DB line between Stuttgart and Ulm, the various structures had to be expected at any time. The associated risks could be reduced through good work preparation, high qualitative investigation measures and good miners and engineers. The geotechnical aspects of tunnelling were of great significance, from the tunnel excavation to the provision of a durable lining.