UK revives Stonehenge road tunnel proposal 21 Nov 2014

Seven years after being dropped, plans to reroute underground a major traffic artery that passes within metres of the ancient UNESCO heritage site at Stonehenge are to be revived. As part of a £15 billion national road improvement programme announced in the UK Budget on Monday (1 December), UK Chancellor George Osborne confirmed that £2.2 billion will be spent on seven highway projects in the south west of the UK, including investigating the possibility of an A303 TBM-bored tunnel bypass around Stonehenge that was dropped in 2007.

Letter confirms reappraisal of Stonehenge tunnel

In a letter to supporters of a TBM bored tunnel solution, and as seen by TunnelTalk, Transport Minister Patrick McLoughlin confirmed that representatives of the UK Highways Agency, the National Trust as land owner, and English Heritage as guardian of the Stonehenge site, have held a series of meetings about revisiting tunnel plans that were dropped on cost grounds in 2007. A feasibility study is currently under way.

In his letter, McLouglin confirms that the Ministry of Transport is working with key organisations in the area and a that a twin-bored tunnel of between 2.5km and 2.9km, “if designed well”, would have a transformational impact on the landscape of Stonehenge and that a business case for a range of proposals along the corridor, including a tunnel option at Stonehenge, is being developed.

As a result of these considerations, the 2.9km tunnel solution is said to be emerging as the most likely option to be carried forward as part of a package of critical A303 improvement measures. This is 800m longer than the twin running TBM-bored proposal that went before a Public Inquiry in 2004 and was subsequently adopted as the preferred scheme (Fig 1), before being dropped after its estimated cost escalated from £125 million in 2002, to more than £500 million by the time that a specially convened government review panel reported its findings in July 2006.

The National Trust said in a statement last month (October 2014) that it “would like to see the longest tunnel possible” but conceded that “early results from our work to assess various options for the A303 at Stonehenge suggest that a tunnel of 2.9km may bring significant benefits for this special place.”

Fig 1. Original (2002) 12.5km-long A303 upgrade section (red), with 2.1km bored tunel

However, the pressure group Stonehenge Alliance, to which the Transport Minister addressed his letter, is urging the National Trust to hold out for a longer 4.5km tunnel, which would start at Longbarrow Crossroads just outside the boundary of the World Heritage Site and National Trust-owned land (Fig 1). The Alliance argues that even a 2.9km tunnel is “far too short” and would leave less famous parts of the World Heritage Site, including the Winterbourne Stoke barrow cemetery on its western boundary, exposed to busy traffic. It is also concerned that siting the portals within the heritage site itself will potentially compromise future archaeological digs.

Any future tunnel design will, however, have to contend with the possibility of encountering radioactive radon along the predominantly chalk geology. TunnelTalk has inspected many of the reports that were in the public domain prior to the tunnel plan being dropped in 2007, but none appear specifically to mention the discovery of radioactive radon.

A303 runs adjacent to ancient Stonehenge site

A January 2006 review of costs ordered by the Government and carried out by Halcrow/Gifford as appointed engineering designers, and Balfour Beatty/Costain as appointed construction contractors, blames the increase from £192 million in 2003, to £510 million at the time of the report’s publication, on “unforeseen ground conditions.” These included “weak phosphatic chalk being detected on a scale unprecedented in the UK and a groundwater table that can rise to the surface in Stonehenge Bottom at times of heavy rainfall. This necessitates extensive additional measures and a strengthened design to facilitate the construction of the tunnel and slows the rate of tunnelling.” The same report, which also looked at alternatives to the then-preferred 2.1km bored tunnel that passed the Public Inquiry stage two years earlier, costed a cut-and-cover alternative at £391 million, and a 2.9km bored tunnel at £697 million. It is expected that a 2.9km tunnel will cost in excess of £1 billion at today’s prices.

These geological discoveries of 2005-2006, however, were only made four years after the Highways Agency awarded construction of the 2.1km tunnel, plus 12.5km of highway improvements at either end of the portals, to the joint venture of Balfour Beatty and Costain, with Halcrow and Gifford as engineering designers. The contract, the first example of an Early Contractor Involvement procurement by the Highways Agency, was valued at £125 million; but by 2003 this had more than doubled to £292 million. With costs escalating, the UK Government convened a review panel to report on five alternatives, which included both the preferred 2.1km TBM-bored scheme, a 2.1km cut-and-cover solution, and three at-grade alternatives. Following a period of public consultation the final July 2006 report put the cost of the bored tunnel at £510 million, assuming a construction start date of 2008. On 7 December 2007 the plan was abandoned.

At a meeting lecture of the British Tunnelling Society last month (October 2014), Professor Rory Mortimore of Brighton University, as a leading geologist with specialist knowledge of chalk geology and Managing Director of ChalkRock Ltd, provided further insightful information for the spiralling costs above the initial construction estimates.

Geologist Professor Rory Mortimore

According to Mortimore, the preliminary design and cost estimate of the tunnel in the late 1990s was established before a detailed investigation of the geology had been carried out. The assumption, he reported to the BTS audience, was that the chalk geology of the Stonehenge area was same as other familiar chalk deposits in the UK; but in reality, as revealed by the detailed site and ground investigation studies, it is significantly different and complex.

“The big surprise,” said Mortimore, “was discovery that the geology on the tunnel route contains a large deposit of phosphatic chalks which contain weak and poorly banded sand and silt layers and a high register of radon radiation. Such a large deposit of phosphatic chalks were unknown in Wiltshire and, indeed, in Europe and their impact on the proposed tunnel project were profound.”

When the site investigation programme began, the prime objective was to know how extensive is the 15m thick layer of phosphatic chalk, and to investigate how it was formed. Following that, the impact of the engineering properties and behaviours of the deposits had to be incorporated into the project estimates.

“As a tunnelling medium, the weak chalk, with its poorly banded layers of sand and silt presented the potential of loose running ground in the face, which was not favourable for the assumed open face NATM or sequential face excavation method, similar to the now completed twin-tube highway tunnel for the A3 route at Hindhead in Surrey that was being developed at the same time,” said Mortimore. “A tunnelling method that provided positive face support would be required to reduce construction risk and this increased the cost of the tunnelling operation.”

Another aspect that increased the estimated cost of the project was the special handling required for the radon-contaminated tunnel material and the potential of working in the material to cause phosphate contamination of the groundwater. “Disposing of radon contaminated phosphatic chalk in a landfill presented major concerns and special handling of groundwater and construction wastewater added to the tunnelling and construction cost estimates,” said Mortimore.

Further details about current proposals are expected to be announced in the Chancellor’s Autumn Budget Statement in December 2014 and TunnelTalk will continue to follow developments.