Beton und Monierbau (BEMO) has been awarded a US$43M contract by German rail company Deutsche Bahn (DB) to strengthen and renovate the two-part Alten Mainzer tunnel in Frankfurt.
Works to be performed by BEMO include tunnel reconstruction and ground consolidation measures to limit settlement risk to key buildings along the route of the twin-track tube. Consultant on the DB tunnel renewal project is ILF Beratende Ingenieure.
The Alten Mainzer tunnel was originally built in 1881-4 and was a 1196m long structure lined with sandstone brickwork, running between the main station at Mainz and Mainz South station. However, damage from acidic flue gases and humidity led to the central section being demolished to leave an 291m long open cut section, the remaining tunnels being of 661.9m and 287.7m long, respectively.
Later, a second bore was built, the 1295.6m long Neuer Mainzer tunnel, which runs almost parallel between the main station at Mainz and Mainz South station. The latest works, to be undertaken by BEMO, will see the original stretches of tunnel enlarged and widened to similar dimensions and structure as the new tunnel.
The brickwork lining of the original tunnel will be removed and in its place a sprayed concrete lining shell is to be built, housing twin rail tracks spaced wider at 4m instead of the current 3.5m. In a two-stage construction process, the contractor will build the top shell with a temporary base before final stage works to excavate deeper and form the permanent invert.
Preparatory works for the tunnel renewal activities include sinking a series of 6m diameter shafts up to 14m deep for soil treatment. The injection wells will enable jet consolidation adjacent to, and below, some buildings in the settlement zone over the Alten Mainzer tunnel.
BEMO won the job after beating off two rival bids the highest of which was a quarter more, at US$53.9M, excluding VAT.
