In tunnelling nothing is ever certain until the last muck is out and all the permanent lining is in place. But with only about 700m of chalk left to explore with its top heading the huge 3.2km long North Downs Tunnel is running well ahead of schedule and looking less and less critical to the Channel Tunnel Rail Link.
Every day and every night, seven days a week, two 12 hour shifts working from the London portal are gouging out another 8m to 10m of Bluebell Hill.
Eurolink's tunnellers are driving to meet the top heading from the country portal which was sealed off after 1,470m - when it was 12 weeks ahead of programme - so that work could concentrate at that end of the tunnel on pulling out the main bench.
This task of opening the bore up to its full 166m 2cross-section is currently running twice as fast as planned, despite a fire in a hydraulic excavator in February which led to an emergency evacuation and the loss of half a day's work.
The scale of the North Downs Tunnel is big by normal UK tunnelling standards, dwarfing the few large machines and very few people involved in the excavation and shotcreting work. Each tunnel face is worked by just six miners and a shift boss backed up by a fitter, an electrician and an engineer. There are three teams each working six days or nights followed by three off. Rail Link Engineering's tunnel team consists of a section manager, field engineer for each shift, tunnel designer, an assistant and a field geologist.
Although not quite matching up to the diameter of the Channel Tunnel's crossover chambers, the North Downs Tunnel overall is an immense excavation job generating some 600,000m 3ofspoil.
By the end of 2003 it will carry both tracks of the CTRL with trains cresting a switchback and passing each other at 290km/h in each direction. Plenty of space is needed around the dynamic envelope of the trains to allow for the aerodynamic effects. Hence the large tunnel.
In fact it will now be marginally larger than was originally proposed. The primary shotcrete rock support was originally to be lined with a 500mm thick tube of reinforced concrete cast insitu around a travelling shutter.
Value engineering based on experience of actual chalk excavation and a revised structural analysis proposed by contractor Eurolink led to the decision to slim the lining to a 350mm thick unreinforced insitu concrete shell. It will save ú5M ($8.25M) against the target cost:
earning ú1.25M for Eurolink and saving the client ú3.75M.
The scale of the tunnel enabled Eurolink to invest heavily in equipment, much of it brand new, which has the effect of making the work look deceptively easy. For instance, opting for a conveyor system to carry all the spoil out of the two adits made for much safer working because it eliminated the stream of muck wagons running around in the tunnel and turning or reversing in the most congested areas.
Face excavation is carried out with a Paurat 242 roadheader. Liebherr 932 Litronic excavators with swivel mounted arms are trimming and loading the conveyors. A two boom Atlas Copco Boomer is working in advcance of the excavation, installing an array of grouted spile pins above the heading.
Wet shotcrete from one of the Steelfields 60 batchers at each portal is delivered to the exposed chalk by MBT pumps and Normet robot arms.
The tunnelling support is not true New Austrian Tunnelling Method as it is 'not the observational method', says RLE contract manager Alan Myers. The arch truss spacing, rock reinforcement and shotcrete is fully designed in advance rather than being continually adjusted in response to measurements of ground movements.
But there is an intense concentration on the behaviour of the ground and the lining. 'Before we were allowed to start the tunnel we had to demonstrate to the Government's adviser and to Railtrack that we had the controls in place to ensure it was done safely, ' says Myers.
A mass of instrumentation is included to monitor all aspects of the construction process, from rock bolt pullout tests to surface settlement.
Every day at 4pm the NATM engineer, the production team and RLE's designer review all data from the previous 24 hours, agree the support regime for the next 24 hours and sign up to it.
Typical truss spacing, and hence the advance cycle, is between 1m and 2m depending on the ground. Now that the London portal drive has reached the middle chalk it is up at the higher end.
Trusses were set much closer at the beginning of the drive when the weathered upper chalk was much more uncertain, with swallow holes up to 40m deep. Swallow holes at the portal put work six weeks behind programme at one time. Under Buckmore Park kart track there was just 6m cover to the bottom of a filled valley there was 40mm surface settlement. In the good chalk deep beneath Bluebell Hill surface settlements are down to 4mm to 6mm at the most.
As soon as the chalk is cut to profile, a 50mm thick sealing layer of shotcrete goes on. The arches are fixed along with mesh reinforcement followed by 150mm thickness of shotcrete. Rock dowels are then drilled and grouted through the fresh concrete shell in arrays of 14 or 15 holes. The 4m deep holes are filled with thixotropic grout and 25mm diameter rebars pushed in.
A second layer of shotcrete follows to give an overall thickness of about 250mm.
A complex cocktail of admixtures is required to ensure that the shotcrete goes off and hardens very quickly. As construction proceeds on the five hour construction cycle load from the newly excavated ground is shed onto the front end of the tubular shotcrete shell - putting very high loads into the concrete.
But a puzzling concrete failure occurred during the country portal drive. For no obvious reason, the shotcrete changed its behaviour and began to set and harden in a fashion more akin to ordinary concrete.
Detailed investigations eventually discovered that the cement fineness had changed and it was arriving on site less finely ground. The material was still within its manufacturing specification but the small change had a substantial effect on the specialised mix being used in the tunnel.
The unrelated accident last year in which a miner was crushed and severely injured prompted a safety shutdown for a week.
A large lump of rock fell from the face during placing of the trusses - the time at which the miners are most vulnerable to unexpected behaviour of the ground.
Falls are more likely during excavation, with boulders dislodged by the roadheater, not half an hour or so later when the face sealing layer is in place.
Project teams Rail Link Engineering is designer and project manager for Section One of the Channel Tunnel Rail Link, working on behalf of Railtrack which will become the owner of Section One on its completion.
Contractor for the North Downs Tunnel is Eurolink; a joint venture of MillerCivil Engineering/Dumez-GTM/Beton und Monierbau.