How Cambridge could get a tunnel system for £500million

A system of tunnels under Cambridge was discussed in considerable detail at the recent Future of Transport event.

Details of how Cambridge’s underground transport system would be built – including the design, cost and engineering challenges – were outlined by Prof Lord Robert Mair at the Future of Transport event in the new Postdoc Centre on Thursday.

Speaking to an invited Cambridge Network audience, Prof Mair, the country’s leading tunnelling expert and a Cambridge resident, outlined the prospect of a tunnel four metres in diameter offering a “basic” service which would include stations at the city centre, the Biomedical Campus/Addenbrooke’s and Science Park at a cost £15million per mile – “not a huge sum in the scheme of things”.

The total expected outlay, including stations, would be £500million. A more elaborate system (with additional stations) would push the cost up to £754million, “just to give you a taste”.

Prof Mair, current president of the Institution of Civil Engineers, and emeritus professor of civil engineering and director of research at the University of Cambridge, was an advisor on London’s £15billion Crossrail project.

“The tunnels would take no more than three to four years to construct,” Prof Mair told the delegates at the Future of Transport conference which was sponsored by AstraZeneca, Smart Cambridge and the Greater Cambridge Partnership.

“The good news – and it’s a myth it’s important to explode – with regard to the land is that, as most of you with gardens will know, there is Gault clay very close under the ground in Cambridge,” said Prof Mair. “Gault clay is not dissimilar to London clay, which is ideal for tunnelling, that’s why Victorian engineers built the first underground system in the world there.”

The clay is also relatively stable. “We’re also able to predict the settlement for the tunnelling remarkably well so if for example we had 4 metre diameter tunnels 15 metres below ground level, that would result in settlement of 15millimetres, around half an inch.”

Prof Mair’s analysis didn’t involve a preference the type of transport used inside the tunnels.

“I want to keep a completely open mind about this – the tunnel could contain a bus or smaller pod system, or a rail system”. However it would, of course, be preferable to decide what system would be deployed in the tunnels before building them, if only to be sure that you’re building the right size tunnel in the first place.

The system described at the Future of Transport by Prof Mair could run “driverless, rubber-tyred, electric battery-powered pods in four-metre diameter tunnels carrying 40 passengers”.

The diameter is the first decision to be made, and size matters.

“If you halve the diameter of the tunnel you quarter the costs,” he said.

The Crossrail project involved digging machines for a tunnel seven metres in diameter. Whether freight would also be carried through the Cambridge tunnels was not a topic covered in the study.

The gault clay under Cambridge is uncluttered with infrastructure.

“Unlike London there is very little underground, Cambridge is pretty much virgin territory under the ground so the opportunity for having many possible schemes is enormous,” said Prof Mair.

The game-changing project, which is also advocated by the mayor of Cambridgeshire and Peterborough James Palmer, took a big step forward as Prof Mair showed pictures of the Crossrail tunnelling equipment which could be deployed in Cambridge. The Crossrail project involved “20 metres of completed tunnel a day, or 40 metres when it’s really going well”.

Prof Mair’s comments build on advocacy initiated by Connecting Cambridgeshire’s AVRT (Advanced Very Rapid Transport) study earlier this year, and his analysis is broadly supported by research professor in transitional energy strategies at the University’s of Cambridge’s Department of Engineering, Prof John Miles.

Speaking at the Greater Cambridge Partnership’s first-ever conference this summer, Prof Miles spoke of an underground public transport system that does not run on rails, but even if no clear favourite for the type of vehicle to be used has emerged, the logistics and broad financial parameters of the tunnel-building project are now far clearer.

Nor is this scheme solely suitable for Cambridge. “It’s a low-cost, affordable scheme which is applicable to other medium-sized cities where, if you keep the diameter small, you can get the tunnelling costs to be achievable.”

Secondary decisions in the use of pods would involve the technology required to power the vehicles. Options include a live track or other self-charging mechanism, or battery power with sophisticated charging ports.