Optimisation of the TBM drive at the Boßler Tunnel using the observation method and an incremental design and contract development process

The 8.8 km long Bosler 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 Bosler 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.