Highways England is “concerned” about the durability of a tunnel proposed for the M25, which will run beneath Heathrow’s planned third runway.
In its initial deliverability report, Highways England has raised “concerns about several aspects of the design” of the proposed tunnel.
The report, obtained by The Times through a freedom of information request and seen by New Civil Engineer, raises concerns about the durability and robostness of the tunnel designs.
Heathrow’s proposal suggests moving the M25 150m west and dropping it 7m into a tunnel beneath the new runway. Highways England’s concerns over the plans include the durability of the tunnel structure, complex temporary supporting works, and drainage in the high water table area.
The report examined two design options for the six-lane M25 tunnel; a buried ground bearing multi-cell concrete “box” and a buried pile supported concrete multi-span portal frame ‘bridge’.
Both tunnel designs would feature a roof slab / bridge deck comprised of a post-tensioned concrete slab with void formers to minimise the dead load of the concrete roof.
The designs also propose fully integral connections between deck/top slab superstructure and box walls/piers and abutments substructure which would negate the need for bearings. But the report suggests that this could complicate future repairs.
The tunnel will be 600m long and will house 14 lanes of motorway.
Heathrow specified that neither design should have expansion joints, in order to reduce the need for structural maintenance. However, the report says that that the lack of expansion joints over such a long length and span was “untested” and would require a “specialist review team […] to ensure both its short-term constructability and its likely long-term performance”.
The structure will also need to be able to support code F aircraft, weighing 900t, but Highways England suggests that the structure will have to be able to support loads of up to 1,800t from the dynamic and cyclic pressures associated with aircraft take-off.
These loads mean that “fatigue of the post-tensioned tendons at mid-span and hogging regions over intermediate supports should be of key concern,” the report states. The design’s durability must be “robustly challenged and proven” as a result.
Highways England also raised concerns about the “complex temporary works” which would require a cut-off wall around the site to prevent water ingress due to a high water table. It suggests that either a bentonite slurry wall or a steel sheet pile wall would have to be used. Temporary retaining walls will also be need during the tunnel construction, which will be complicated by their proximity to the existing M25 route in places.
The high water table will also complicate the design as well as the supporting works, the report recommends that extra care be paid to how drainage systems in the tunnel will work and be maintained, as well as ensuring that run off from the slab/deck, which could be contaminated with hydrocarbon fuels and detergents, be kept out of the tunnel structure.
The report also highlights the fact that the resources required to build such a tunnel will be significant and overlap with “a time when other nationally important projects are being undertaken”.
Highways England noted that the overlap with other projects on the M25 poses “a big risk to both schemes in terms of customer satisfaction, network access and traffic management availability”.
Highways England network planning director Jeremy Bloom concludes that the plans are “deliverable given appropriate mitigation”.
“Highways England have been clear throughout that Heathrow’s proposals for the M25 and the wider Strategic Road Network are deliverable, given appropriate mitigation,” he said.
“Our engagement with Heathrow Airport Limited has been productive and has focused on seeking to agree solutions that consider both the construction and long term operation and maintenance of proposed changes to the Strategic Road Network.
“We’re also ensuring that the proposals can complement our own plans for improving this section of the M25. By identifying and resolving potential issues at this early stage in design, we can make sure the final proposals will keep journeys on the M25 safe and reliable, both now and in the future.”
“The options we’ve proposed for crossing the motorway are tried-and-tested at other airports around the world,” said Heathrow Airport spokesperson said.
“Highways England has reviewed our early design and confirmed that our plans are robust and deliverable. As our designs develop, we are working with Highways England to agree the engineering techniques being adopted and we will work to ensure that the M25 remains open throughout the build.”
“We will be consulting on our preferred masterplan in June and welcome further public feedback to make sure we get the best plan to expand Heathrow.”
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