Ambitious plans to build what would be one of the longest highway tunnels in the world are outlined in a UK Government study into the feasibility and constructability of a trans-Pennine underground link connecting the major cities of Manchester and Sheffield (Fig 1).
According to a 56-page interim report published last week by Highways England on behalf of the UK Department of Transport, a twin bore tunnel of up to 15m in diameter and “between 20–30km long” is envisaged for the central section of a longer 40–50km link between the two cities. Each tunnel bore would be required to accommodate a “minimum of two lanes”.
For the TBM sections, a precast concrete segmental lining of between 500-700mm thick is anticipated, although conventional drill+blast is considered feasible given a predominant geology that consists of the coarse-grained sandstones, limestones and mudstones of the Millstone Grit Group.
The report says: “Based on information available at this stage, we consider that EPBM, slurry and open-face TBMs are likely to be required. However, the type [of machines] ultimately selected will depend on the tunnel alignment and its ground conditions (rock mass strength and hydrogeological conditions).
“According to geological and geotechnical data available, a major part of the tunnel should be excavated in moderately strong rock and locally weak rock, including fault zones; soils should not be encountered, except for a short section of the tunnel close to the portals.”
In order to benefit from synergies with current proposals for a trans-Pennine rail link – which would link the two separate “Y” arms of the UK’s proposed HS2 programme (Fig 1) – a heavy rail link along a similar alignment is also suggested. This could mean four separate tunnels of total length between 80–120km.
A selection of more detailed alignment proposals will be outlined in a follow-up report due to be published in March next year (2016). A final report to consider the transport, socioeconomic and environmental benefits of the alignments that are considered “strategically and economically viable” will be presented in a final report in October 2016.
A tunnel is considered strategically important at this location because the cities of Manchester and Sheffield are separated by a range of hills known as the Pennines, which themselves form part of the protected Peak District National Park over which a new surface-level connection is impossible. Although Manchester and Sheffield are only 45 miles apart, journey times between them average approximately 85 minutes using the more direct highway route across Snake Pass, which is often closed during bad weather. The alternative and less direct motorway route adds 30 miles to the journey and takes an average of 95 minutes to complete.
The Pennines also act as a road and rail choke point for connections between a range of large towns and cities across the whole of the north of England and north Wales, including Liverpool, Leeds, Hull, Chester, Warrington and Bradford. A road tunnel is considered strategically important if the underdeveloped region is to fully benefit from the UK’s £50 million HS2 program – which does not currently include a cross-Pennines link to address the problem.
The report concludes: “We are at too early a stage in the design of the potential scheme to present robust analysis on any of the economic costs and benefits of a scheme. However while there needs to be detailed transport and economic modelling, the indications are that there is the potential for significant benefits.”