Complicated piling and ground stabilisation works could add up to £750M to High Speed 2 (HS2) costs if the route goes through a salt mining area of Cheshire, a report commissioned by a group opposed to the line has warned.
The report published by consultant TerraConsult on behalf of campaign group Mid Cheshire against HS2, said that on one section where viaducts had to be built, the geological ground conditions would pose severe challenges to engineers and substantially increase construction costs.
The 20.2km long section in question runs from a tunnel portal north of Crewe to Arley Brook north east of Northwich.
The report – Ground engineering assessment HS2 in Cheshire salt district, Crewe tunnel portal to Arley Brook supplementary report for new Alignment released 15th November 2016 – says that although all of the engineering difficulties can be overcome, the section would cost significantly more per kilometre than a comparable section which passes through rural countryside with simpler ground conditions. It also questions the choice of route taken by the line.
It said that two proposed viaducts with a combined length of 1.32km could be built over the Winsford salt mine near Middlewich. Foundations for the structures would have to pass through drift deposits and the disintegrated brecciated [sedimentary rock type] bedrock to be supported on underlying “intact” rock, presenting considerable technological challenges.
“These piles will be unusually difficult to construct with consequential extremely high construction costs,” says the report. “There is limited comparable experience in the UK of piling into the difficult ground conditions with wet rockhead, particularly where the depth to competent bedrock is so deep (circa >100m) and these piles will probably be the deepest piles ever constructed onshore in the UK,” says the report.
TerraConsult put the estimated cost for “just” backfilling and stabilising mineworkings to enbable viaduct construction at £28M.
This section of the line has already been changed since initial plans were published and is included in the government’s latest preferred route announced on 15 November 2016. The proposed current alignment has been moved up to 800m to the west of the previous route, which the report says considerably increases the elevation of the track requiring longer embankments and viaducts as a result.
The report surmises that the change was made to avoid building 2.5km of the line above caverns at Holford Brinefield near Northwich and gas storage caverns to the south.
“Between 2013 and 2016 it appears that HS2 Ltd has carried out a fundamental review of the stability of the solution mined brine caverns at Holford Brinefield and the associated risks and costs of construction of the original route across this feature,” it said.
The report also says that the route now crosses part of the Winsford mine which is used for the disposal of hazardous waste, some of which it says will require relocation before the line can be built.
It concludes that due to the geotechnical complexities, the additional cost of this alignment will be £750M compared to a problem-free route in rural countryside – an estimated increase of £250M on the 2013 route.
The route is part of Phase 2b of the HS2 project covering the section above where the line splits from Crewe to Greater Manchester and from the West Midlands to Yorkshire. HS2 Ltd said that the routes was still unconfirmed and could be subject to change.
An HS2 Ltd spokesperson said: “Development of HS2 is a thorough and detailed process that draws on information gathered during the course of several years’ work.
“We are now consulting on the route through this part of Cheshire, which was revised following consultation responses to our original proposal.
“We welcome all contributions to the current consultation as part of ongoing work to develop HS2 in the North West, this will help inform the Government’s decision on the route later this year.”