On 20 December 2003, the largest diameter TBM in the world broke through, marking the completion of boring of the 7.2km long Groene Hart tunnel in Holland.

The 14.87m diameter slurry TBM, nicknamed Aurora, was designed and built by NFM Technologies, part of the Wirth Group, (T&TI January 2002) for the Bouygues Construction/Koop JV which completed the tunnel several months ahead of the planned schedule.

In 24 months of tunnelling, 1.3Mm3 of spoil was removed at a maximum advance rate of 40mm per minute. The average daily progress was 16m, with a best monthly performance of 616m using the machine that has a 10% larger face area than the world’s previous biggest TBM. Dismantling the 120m long 3,100t machine commenced in mid-January and is expected to take three months.

  A large diameter single bore with a dividing wall down its centre was chosen over two smaller diameter tunnels as it was felt to offer significant financial, safety and environmental benefits. It halved the approach works required and the need to excavate cross passages was eliminated by equipping the dividing wall with escape doors at intervals. Also the project says the risk of settlement is reduced with a single as opposed to twin drive through the protected “Green Heart” in Holland’s centre.