Cambridge is exploring a small scale, low cost metro concept.
Cambridge could soon host a prototype value-engineered metro system running in tunnels under its historic centre.
Local delivery body Cambridge Greater Partnership is currently looking at options to build an affordable very rapid transit (AVRT) system in and around the city centre.
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The study includes a scheme proposed by University of Cambridge Arup professor of transitional energy strategies John Miles, which, he claims, could halve the construction of metro systems.
The system would feature a number of radial and circumferential legs tunnelled under the city centre with suburban sections running above ground.
Most stations and interchanges would be built above ground, to further cut costs.
Work completed to date suggests that if tunnel bores can be kept below 3.7m in diameter, and tunnelled sections kept to between 4km and 6km long, then the costs would be half that expected for a conventional tunnelled metro.
Miles says the “micro-metro” concept has been developed to provide severely congested small to medium sized towns with a much cheaper metro system so that finances stacked up despite smaller passenger numbers than those for major city metros.
Capital costs are estimated at £10M to £15M per kilometre. The government’s December 2010 Infrastructure Cost Review found that unit rates for tunnels vary widely depending on diameter, but that benchmarking by the British Tunnelling Society suggests a figure of around £40M/km for a typical two-lane tunnel.
It’s called a micro metro in a deliberate attempt to go back to the concept and redesign mass transit for cities
“It’s called a micro metro in a deliberate attempt to go back to the concept and redesign mass transit for cities in such a way that it would be affordable for small cities,” says Miles. “It’s responding to a generic issue that there are lots of relatively small cities in the UK, 500,000 people or fewer, which have severe congestion problems.
“But when you come to solve those problems with things like light rail or tram, it is unaffordable. The disruption and the construction cost is so high that the classic solution is out of reach.”
To develop the micro-metro idea, he says the team has looked at each technical aspect of a traditional system and asked whether it could be done in a cheaper way.
It’s responding to a generic issue that there are lots of relatively small cities in the UK of 500,000 people or fewer
To reduce cost, expensive railway systems are replaced with a Guided Bus style system with rubber-tyred vehicles running on an asphalt road surface, and powered by on-board battery technology.
The system could go up to 25km out of the centre of a city with the vehicles travelling hundreds of kilometres before needing to be recharged, claims Miles.
Costs are reduced further by minimising the number of stations, which can interrupt the flow of the system. This also has a knock-on effect of simplifying complex and expensive signalling systems.
Short shuttle service
In addition, he says the concept of vehicles shuttling between fewer stations rather than going the full length of a route, could also lead to even simpler signalling systems, increasing cost savings. Using the precedent set by London Underground, he said people would be prepared to “hop-on and hop-off” if they knew the service was fast and frequent.
To make the system even more affordable and attractive to commuters, he says vehicles would be autonomous, with fast and frequent services running throughout the day.
Initially he says the project took a step back because it was decided the new system would have to run overground on its own segregated lane to ensure services would not be interrupted by other traffic.
He says this approach was generally practical up to the perimeter of a city, but within an already congested centre it may not be possible.
City centre tunnels
In Cambridge, for example, running vehicles in tunnels under the centre was seen as the only viable solution. To keep the cost of tunnelling to a minimum the vehicles were designed to be as small as possible, while remaining comfortable for passengers.
“When you look at the whole system, the cost of the vehicles is a small part, the biggest cost by far is the civil engineering and that includes the tunnels,” says Miles.
“Tunnelling is horrifically expensive, but tunnelling is very dependent on the size of tunnel you’re boring, so if you reduce the size of the tunnel you reduce the cost significantly.
“Therefore the fundamental discipline is to keep the size down so the vehicle is the narrowest and lightest possible that still gives good comfort to the customers. Everything then flows from that. It’s very simple logic at its base level.”
Cross between jet and train
The vehicle Miles’ team has designed is “a cross between small regional jet aircraft and an underground train,” he says.
Although it has not been tested, he says there is a 30 month programme to get it to the point of construction.
While he says it was a new concept, the technology used in the micro metro idea is not new and therefore there is no technology risk.
Miles says he hopes more funding would be available to develop the design further.
Tunnelling in Cambridge may sound costly and risky but ICE president Lord Mair says it is possible.
“An important myth to explode is that Cambridge is inappropriate for tunnelling,” said Mair.
He said at a depth of around 15m the Gault clay was “ideal for rapid and economic tunnelling” with a maximum settlement at the surface of 10mm for a 4m diameter tunnel.