Co-operation between contractors on Contracts 220, 230 and 240 has been remarkable and unprecedented. Large areas of the Stratford site are being shared as are site offices and a joint 220, 240 production plant set up there to manufacture lining for both tunnelling contracts.
'Normally if you ask four tunnellers their views on lining, you'll get six different ideas, ' jokes NCS JV project director Terry McDonald.
But after the contractors on all four tunnelling projects had revisited RLE's precast concrete tapered lining design they decided to go with the original concept plus a few minor tweaks. A minimum 120 year life is assumed for the segments, which will be sealed against water ingress by EPDM rubber gaskets backed up by hydrophilic (swelling) gaskets.
A dozen fixing inserts will be cast into every segment to cover for every sort of attachment that might be required at some time in the future.
Casting the lining at Stratford for the 24km or so of tunnels in Contracts 220 and 240 will be a huge operation. In effect the work is similar in scale to the huge production facility for the Channel Tunnel.
The nine plates and key in each 7.85m external diameter ring will be 350mm thick in dense concrete reinforced with both steel and polypropylene fibres. Each 1.5m wide ring will weigh 32t and some 16,000 of them are needed in all.
O'Rourke company Malling Precast in joint venture with Holtzman has a contract to set up the plant and supply the two contracts. Many of the people involved will be personnel from the former Taylor Woodrow tunnel lining outfit.
Four separate carousels will carry the moulds around a central batching plant with steam curing to speed demoulding. The operation will run in a huge soundproofed shed. Crushed limestone aggregate will be brought in on five 1,200t trainloads per week while 1,100t of cement will arrive by road. This will be a blend of ordinary Portland, pulverised fuel ash, and ground granulated blastfurnace slag.
Production is driven by the need to progress an average of at least 90m per week at each of the four tunnel faces. The plan is to start manufacturing in November and build up a three month supply.