May 18 is the scheduled start date of TBM excavation on the 4.2km long, twin-tube bored section of the $306.8m, 8km long Sophia railway tunnel, the longest tunnel on the Betuweroute high speed freight rail line in Holland.

The Herrenknecht 9.8m diameter Mixshield TBM is currently being assembled in the 20m deep starter shaft at Oud-Alblas by contractor Tubecon 1, a JV of Dutch contractors Royal Boskalis Westminster NV/Heijmans NV/NBM-Amstelland NV and German contractors Hochtief AG/Philipp Holzmann AG.

Upon launching, the TBM will bore 4.2km to the reception shaft, which is under construction at Hendrik-Ido-Ambacht. (The reception shaft is currently 6m deep and, when completed, will be 20m deep.) When the first tube is complete, the machine will be dismantled and transported back to the starter shaft to begin construction of the second tube.

The TBM will bore continuously while simultaneously erecting the lining to leave an i.d of 8.6m. Each lining ring will be 0.4m thick x 1.5m wide, comprising seven concrete segments and a key. This method is being used in an attempt to realise advance rates of 15-23m/day. In total, an estimated 40 000 concrete tunnel segments are required to line the two tunnel tubes. The segments are produced in Berlin and transported to Holland by train.

For safety reasons, the tunnel tubes will be connected by cross passages every 600m.

The remaining tunnel length, comprising approach tunnels either side of the bored section, will be constructed by traditional cut+cover techniques.