After 18 months of tunnelling beneath the UK’s capital, all six of the London Tunnels have been completed on Section 2 of the Channel Tunnel Rail Link.

The 8.15m o.d. concrete lined tunnels run an impressive 35km. A total of six TBMs were used to excavate ground conditions that varied from London Clay, to dewatered Thanet Sand and chalks.

With a total value of over US$1bn, the London Tunnels were originally awarded as three separate contracts. Early set-backs relating to the Stratford (Station) Box works, before tunnelling had even commenced, prompted the London Tunnels contractors, the Stratford Box contractor and the client to agree to an innovative commercial alliance. The alliance made collaboration easier and gave greater incentives for co-operation.

At a ceremony held in Stratford to mark the completion of tunnelling work, UK Transport Minister, Tony McNulty, said: “This incredible engineering achievement is a strong example of yet another transport success in London. The tunnels were completed on budget, on time and only 1mm wide of their final target.”

The Nishimatsu/Cementation Skanska joint venture, contractor for C220, the 7.5km twin tunnels west from Stratford to King’s Cross railway lands just north of St Pancras station, used 2 x Kawasaki TBMs for its contract, which was completed three months ahead of schedule and under budget. The other contract to originate from Stratford, C240, saw contracting joint venture Costain/Skanska/ Bachy Soletanche opt for 2 x Wirth TBMs. This contract was for 4.5km of twin tunnel east to the Barrington Road ventilation shaft site. Along the way, the joint venture had to contend with significant subsidence in residential gardens, attributed to unrecorded deep wells, but still managed to complete the drives ahead of schedule.

The third contract, C250, was constructed by the Nuttall/Wayss & Freytag/Kier joint venture using 2 x Lovat TBMs for the 5.3km twin drives. The tunnels also terminated at Barrington Road and despite passing underneath mainline and underground rail lines, did so without any disruption to services.

Overall, expected advance was predicted at 75m per week, but actual progress across all six drives was 100m per week, with a best seven days performance of 282m.

The upline and downline running tunnels are linked by cross passages for emergency egress and maintenance purposes at approximately 500m centres. In contrast to the running tunnels, these were mainly constructed with traditional London hand tunnelling methods and spheroidal graphite iron rings – also largely built by hand. Despite the mix of high technology TBMs and traditional methods, the tunnels have achieved an enviably low accident rate compared to similar projects, with no serious injury to workers or the public.

McNulty recognised the teamwork that had been necessary to construct the tunnels, saying: “to have completed them on schedule is a tremendous achievement by Rail Link Engineering, our project manager, our contractors and of course the tunnelling gangs”.