London's latest underground railway will be an overhead monorail running 20km on a northsouth alignment, with vertical 'stations' every 3km.
Trains will travel at 10km/hr at a depth of 20-30m, carrying a maximum of three people each.
It is easily guessed that this is not some addition to the Tube network. Instead it is the latest power cable tunnel in the city and at 3m internal diameter much the biggest built so far, although the future could bring even longer versions.
'The trains will only carry maintenance engineers, ' explains David Whittaker, project manager for designer Brown & Root. ' They are needed because the distance between shafts is too great for unassisted evacuation within the 30 minute safety limit.'
The small cars, made by German firm Scharf, will also have automatic monitoring systems on board to allow unmanned cable inspections. 'It keeps man entry to a minimum, ' adds principal engineer Michael Francis.
Brown & Root has notched up 30km of experience on power tunnels for London Electricity, with the largest so far a 10km Wimbledon to Putney link of 2.6m diameter. Such tunnels have environmental and social benefits in reducing overhead line and are more secure.
But they are expensive. The new £200M, 20km tunnel will run from Elstree to St John's Wood.
Client this time is the National Grid Company (NGC), spending £45M on civils and M&E.
Tunnel parameters are set by the cable requirements. 'And that means heat extraction, ' Whittaker says. The power line will be three 200mm diameter cables carrying a three phase 400kV link. At full capacity it will generate a huge amount of heat - even more when a planned second line is installed later. Fan cooling ventilation means shafts every 3km and dictates the diameter. Even then the maximum temperature will rise to 50degreesC.
The design team has worked on the cable type with NGC and is using cross-ply polyester insulation. 'It has only been used to 132kV before although higher voltages have been used abroad, ' says Whittaker.
Normally a cable of this power would use oil bathed paper layer insulation. But these cables need special joints where the oil bath is sealed from the next length and the tunnel must have a wider chamber to accommodate the joint. 'You must allow for expansion and topping up, ' explains Francis.
There is design provision to revert to oil bath insulation if the new cable type proves less durable than hoped.
The second design constraint is whole life costing. Inspection and man entry is expensive so maintenance is kept down with as few working components inside as possible.
Radio links will use a passive feeder rather than active boosters and there will be no internal lighting and so no bulbs to change. Automated train inspection and telemetric heat incident detection will monitor cables.
Above ground locations must be found for the shafts in a busy city, and until the locations were finalised the tunnel alignment could not be settled. Each shaft requires an M&E building at ground level and each meant exploring for options and gaining permissions. Brown & Root is planning and environmental consultant as well as designer and supervising engineer.
Green belt restrictions have applied in two shaft positions and one chosen site will be in a park disguised as a pavilion.
Another will be in a sports centre, the result of a planning trade with the local council.
Alignment has also been affected by the clutter of services at the south end of the route where Tube trains, water pipes gas lines and even unrecorded government 'obstacles' are encountered. The last section is mainly beneath road too, as allowed by the Streetworks Act, and together these factors mean tight curves.
'For most of the route we have a 200m minimum radius, but under the road it is 50m, ' says Whittaker. The Lovat tunnel boring machine backup train must be reduced for the tight curves so this section is being driven last by contractor Murphy which was awarded the work in January. Three machines will make the drive, two out of the mid point shaft which is in a railway goods yard at Cricklewood and one from Elstree. The tunnel is predominantly through London Clay and will have a precast concrete segment and key lining.