The article provides a summarised report about the redesign of the 8822 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 2900 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 5800 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. Reproduced with permission (Joham 2018).
In February 2012, the DB Netz AG called for tenders for the construction measure NBS (new line) Wendlingen-Ulm, Planning section 2.2 Albaufstieg Tunnel in a negotiation procedure. The project was split into three contract sections, with the twin-bore Boßler Tunnel, total length 8822 m (contract sections 1 and 2), and Steinbühl Tunnel with a total length of 4847 m (contract section 3). For the driving of the Boßler Tunnel, two starting points were intended, one at the portal in Aichelberg for the driving of contract section 1 to the south and a 950 m long access tunnel in the Umpfental for the driving of contract section 2 in the other direction to the north and to the south to the portal at Buch (Figure 1). The Steinbühl Tunnel was to be built as contract section 3. The client specified drill and blast with shotcrete support as the construction method. Only for the 2800 m of tunnel in contract section 1 from the portal at Aichelberg was an alternative proposal for mechanised tunnelling (TBM) approved. In the tendering in May 2012, the Austrian consortium Tunnel Albaufstieg (ATA), consisting of the companies Porr, Hinteregger, Östu/Stettin and Swietelsky, bid for all three contract sections. In the subsequent negotiation procedure, which continued until October 2012, the ATA won the award with an alternative proposal to construct all three sections with the use of a TBM on section 1 for 2800 m (cf. Figure 1). The ATA was optimistic from the start that with the latest machine technology, it should be feasible to drive most of the Boßler Tunnel with a TBM. The advantages of the much larger apportionment of the TBM and infrastructure costs as well as a reduction of the risk from the geological tunnelling class distribution should enable a win-win situation for the client and contractor. The ATA therefore made an optimisation proposal to the client, after the award but still during 2012, for an extended TBM drive on contract sections 1 and 2 of the Boßler Tunnel (Figure 2). This laid the foundation for a technically innovative but difficult and laborious path.