Although stiff London clay is 'ideal' for tunnelling, extreme care is needed in this project, according to Darcy. En route to the Piccadilly line platforms one of the new pedestrian tunnels comes within 6m, or a tunnel's diameter, of the platforms in King's Cross mainline station. 'One tunnel diameter is about the minimum you can have. We are at the limit of what's safe, ' explains Darcy.
'The tunnelling methodology and design aren't overly cautious, but they are robust, ' adds Balfour Beatty Management assistant supervising engineer Nick Butler.
Tunnel diameters range from 6m to 9m and are excavated in three passes, with 250mm of sprayed, steel bre reinforced concrete providing temporary support.
The concrete mix has been designed by Morgan Bemo and achieves 1N/mm 2 strength after one hour.
Using a road header, first a 4.5m-diameter pilot tunnel is created, and a crown beam cast.
Work is just about to start on enlarging the tunnel to its full width and about two thirds of its depth. The invert will be deepened to create the bore's full 4.5m diameter. The nal pass will create the nished tunnel prole (see diagram).
Darcy says the method is standard for London clay. But the fact that the sprayed concrete lining is designed to permanent works standards is unusual. 'The nal segmental cast iron lining [which is to be installed within the sprayed concrete lining] isn't strictly necessary, ' comments Darcy. 'It's belt and braces, which will probably be useful over the century or so these tunnels are designed to last. It's nicer aesthetically, and it's psychologically reassuring.' The reason for such hefty temporary works is settlement control. Intensive monitoring and compensation grouting is also being used.
Bachy has installed horizontal arrays of tubes as a manchette under the Great Northern Hotel and King's Cross mainline station from the access shaft and from a disused Network Rail tunnel. If settlement takes place, grout will be injected into the ground, restoring it to its original level. The temporary tunnel linings are designed to minimise risk of ground movement in the first place, but to withstand high grout pressures if remedial action is called for.
Darcy is hopeful that, under the Great Northern Hotel at least, no compensation grouting will be needed. 'The hotel went down by a few millimetres during piling for the access shaft. We've already carried out compensation grouting there and got it back up to zero - we've pre-heaved it, so may not have to put in further compensation grouting.' A particularly sensitive piece of tunnelling work will be the connection to the Piccadilly line, which is directly under the platforms of King's Cross mainline station. Tunnel diameters are large, and there are junctions to create. 'It's where there is greatest potential for movement - we can't just bang away at this job, ' explains Darcy.
'We have an agreement with Network Rail that trains will stop 38m further back in the station while we're creating the junction and escalator barrel down to the Piccadilly Line platforms. It's a precaution to prevent differential loading on top of the tunnel while we drive it down.'