The North Downs Tunnel will be the first bored tunnel constructed on the CTRL project. In fact it is the only one in the first section of the rail link. With a cross sectional area of 160m2 and a maximum clear span of 14.9m, it is claimed to be the biggest running tunnel ever constructed in the UK.
The horseshoe-shaped structure will have a sacrificial spray concrete primary support lining, reinforced with rock bolts and mesh and be completed with a permanent heavily reinforced concrete lining up to 500mm thick.
The single structure, which will accommodate two high speed lines side by side, is matched in dimensions only by the crossover caverns in the Channel Tunnel itself.
Actual tunnelling on Contract 410 will not start until April next year after initial site preparation work, preliminary earthworks and then construction of the two portals - the London Portal to the north and the Country Portal to the south. Once the work gets under way, Miller's chief tunnelling engineer Colin Eddie, expects to achieve progress rates of up to 8m a day on the 3.2km tunnel.
'The programme is fairly tight,' says Eddie. 'Therefore to advance the tunnel from both ends is very important.' The plan was formulated in the main by Austrian tunnelling experts from Beton & Monierbau, although Miller's considerable experience from projects like the Heathrow airport baggage tunnel stand it in good stead. The plan is to construct the entire spray concrete shell before starting on the permanent insitu concrete lining.
Tunnellers will start by excavating and spray concreting the first 100m of tunnel from each portal using a crown, bench and invert sequence with a wet process shotcrete. This will create protected marshalling areas at each end of the works.
Excavation of the remaining 3km of tunnel will then proceed from each end with construction of the tunnel crown over its entire length to create a spray concreted pilot bore around 6m high. When the two faces meet in the middle, equipment will be moved back to each portal to commence the bench excavation in the middle of the bore. This again will be taken right through. Finally the tunnel's primary lining will be completed with the invert support expected to be in place by the end of 2000.
'For firms of the experience of Beton & Monierbau this will be bread and butter work,' insists Eddie. He maintains that they were able to reduce construction costs by not having to operate more than one face from each portal - something he believes that none of the other bidders were planning to do.
'The experience within the joint venture means that we are confident in our predicted advance rates and that we will have sufficient time,' he adds.
Work will start on the permanent lining in September 2000, beginning from the southern portal using two 10m long shutters. The invert excavation will then be completed two months later with the contractor working solely from the northern portal. Installing the permanent lining will then proceed right through the tunnel over 12 months.
The contractor is currently working with engineers from Ove Arup - the part of RLE responsible for the tunnel's structural design - to see if it can reduce the thickness of the permanent lining, given that the spray concrete lining will be so strong. Despite this, permanent insitu concreting will be a major task.
'The sheer scale of the concreting will be immense,' comments Eddie. 'We will be placing around 300m3 of concrete a day which will be a substantial challenge.' He expects this work to be completed well ahead of the tunnel handover in November 2001. Trackworks and the electrical and mechanical fit out will then be carried out under a separate contract.