'It's not often that you stick a soft ground machine in the ground and expect it to do 7.5km, ' says Nishimatsu/ Cementation/Skanska joint venture project director Terry McDonald.
Such drives are certainly a novelty for London. The two tunnel boring machines on Contract 220 will each have to go twice as far as a typical drive for the Tube railways, as well as cutting holes about four times the cross section of the Underground's newest and largest running tunnels on the Jubilee Line Extension.
Detailed studies by RLE of machine problems on other projects such as the Channel Tunnel and JLE identified features which led to component failures. McDonald explains that his joint venture was given a very clear specification with statements such as: 'Your gearbox shaft will be supported like this so that it doesn't fail.'
The Kawasaki machines chosen for the job each have six gearboxes. Each TBM weights 650t with the back up system bringing the weight to 100t overall. They have been deliberately overdesigned with huge reserves of torque available. ' Not much less than a hard rock machine, ' says McDonald. Nishimatsu and Kawasaki have a long and close association, with the latter supplying well over 200 TBMs to the contractor.
Reliability of the machines is absolutely essential if the programme is to be met. The aim is for 92% availability of the TBMs. Each is designed to take 25 minutes digging in closed mode for an advance of one, 1,500mm wide, lining ring or 20 minutes in open mode.
A vacuum suction grip erector arm has to build 10 segments of a 32t ring in 25 minutes. Machines will work two shifts, 24 hours a day, seven days a week in higher risk areas where continuous working is specified.
Spoil will be removed from the cutting chamber by a massive 1.1m diameter screw conveyor. This is designed so it can rapidly be moved forwards and pressurised when converting from open to closed driving mode. This transition when unstable material is met, can be completed in seconds simply by shutting 'a gate here and there'.
Management of settlement control has been a typically thorough Japanese approach says McDonald. 'A lot of thought and money has gone into trying to minimise settlement.'
Thixotropic gel grout will be pumped into the tunnels from a batching plant at Stratford and injected with a catalyst at the tailskin to fill the void around the lining. The gel sets in seconds rather than the minutes for conventional cement grouts. The system also eliminates the need for mixing bags of powdered materials in the tunnels. Nishimatsu used this technique on the Dockland Light Railway's Thames tunnels.
Both Kawasaki designed machines will be manufactured by Markham in Chesterfield and assembled by Qualter Hall at an offshore fabrication yard in Middlesbrough. This includes back up equipment designed by Bennett Associates, before being broken down for shipment and final erection on site at Stratford ready to start digging in June next year.