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Back to the Stone Age

TUNNELLING

Cut and cover or bored, there is a long way to go before construction of the Stonehenge tunnel can start, not least the small matter of a public inquiry. Max Soudain reports on progress.

Dawn at Stonehenge will one day light up a landscape restored as nearly as possible to the Neolithic. That, at any rate, is intention behind the planned Stonehenge tunnel.

But the project team has its work cut out, as the 4,000-year-old stone circle is surrounded by roads often choked with tourist traffic. Along the northern edge is the A344, while to the south runs the A303, a trunk road notorious for its long queues of caravans heading southwest to the beaches ofDevon and Cornwall during the summer.

The 2km tunnel is part ofa larger scheme to widen 10.5km of the A303 including a new bypass around the small town of Winterbourne Stoke. Where traffic passes the historic monument it will do so below ground, freeing the area from this blight and allowing visitors to Stonehenge a more authentic experience.

'The project is driven by environmental improvement as well as traffic improvement, ' says John Perry, technical manager for Mott MacDonald. 'The aim is to return the area to a Neolithic landscape as near as possible.' Despite its obvious benefits, the scheme is controversial because Stonehenge is a Unesco world heritage site within an area ofimmense archaeological signif icance. The area around Winterbourne Stoke is also a site ofspecial scientific interest (SSSI).

Objections have already been raised over the planned shallow cut and cover construction ofthe tunnel, with some advocating a deeper, bored - and more expensive - option (GE August 2000).

So opponents ofthe scheme should be pleased to hear that a bored tunnel is now being considered as a viable alternative to the original plan.

'The tender documents included both options, ' confirms Perry.

'The tunnels will be the same length, although a bored tunnel would have longer and deeper approach cuttings.' Most ofthe rest ofthe route will run on chalk, although at the western end it will cross the River Till whose 250m wide flood plain includes soft clayey gravel alluvial deposits. This has obvious implications for foundation design.

Another challenge lies at Stonehenge Bottom, the dip where the A344 now meets the A303. 'This is a potential problem area, ' Perry says. This chalk dry valley has superficial deposits and the chalk is more weathered and fragmented here, 'but it is not unusually deep for a dry valley' he says.

Mott MacDonald is the Highways Agency's consultant on the project and has carried out two phases ofsite investigation and prepared the outline design in the tender.

Five consortia are bidding for the design and build contract:

Amec/Alfred McAlpine, Balfour Beatty/Costain, Mowlem/Morgan Est, Sir Robert McAlpine/ Bouygues and Skanska (GE November 2001).

Site investigation contractor Soil Mechanics (part ofthe Environmental Services Group) carried out the preliminary ground investigation at the end of 2000. This comprised three rotary boreholes, with downhole video cameras used to examine the condition ofthe chalk bedrock.

'We looked at a number of options for the site investigation, ' explains Mott MacDonald Metros and Civils divisional director Jim Beveridge.

'We thought about digging inspection shafts but they were safety implications, so we chose instead to use the cameras.

'The preliminary investigation also gave us a first look at the groundwater and the stratigraphy ofthe chalk, ' Beveridge explains. Findings were used to design the main investigations.

These began in spring 2001.

Planned as one phase, work had to be suspended because ofthe foot and mouth disease outbreak and investigations only restarted in the summer.

'The first phase could be carried out because it concentrated on the tunnel and was in one farm that had no livestock, ' Beveridge says. The second phase looked at earthworks that will be needed on the surface sections.

Before geotechnical investigations could start, archaeological digs were carried out. A number ofdeep, shored pits were dug in areas thought to be ofinterest.

The main thrust ofthe ground investigation, also carried out by Soil Mechanics, was to examine the groundwater regime and the impact ofany tunnel built on this, and to investigate the chalk quality for tunnel stability and earthworks, Perry says.

Beveridge emphasises: 'It was quite a technical site investigation, not a routine one.' Thirty rotary cored boreholes were put down with air mist flush to between 15m and 50m, mainly on the tunnel alignment but also at the location ofdeep cuttings.

The 50m one was the stratigraphy control, says Perry. Five cable tool boreholes were sunk for the River Till crossing to a maximum depth of20m, 54 'shallow' trial pits were dug to 3.5m and 35 'deep' ones to 5m depth.

Downhole cameras were also put down selected boreholes on the alignment. 'These were useful for finding discontinuities in the rock - vitally important in this type ofinvestigation, ' says Beveridge.

One ofthe main fndings ofthe investigation was that the Seaford Chalk underlying the area 'is slightly different to average chalk', says Perry. 'It has high intact density but weak strength, so it doesn't conform to normal relationships - we are having to examine this further.' He says the chalk also contains local phosphatic bands. 'But we planned for this when we designed the investigation and the chalk is OK for the tunnel and the earthworks.' Extensive pumping tests were carried out in some ofthe boreholes to get a better picture ofthe groundwater regime and some pressuremeter testing was undertaken. Groundwater is below the cut and cover tunnel but varies seasonally and rises more in Stonehenge Bottom, so long term monitoring and sampling is being carried out, says Perry.

Seismic techniques were considered but not used. Beveridge explains: 'There are no solution features along the route, chalk variation is small and we had a hold on groundwater so we decided not to use it in the main investigation.' After the contractor is appointed a supplementary ground investigation will be carried out at the location ofthe fundations of overbridges for local roads and this could include some geophysics, he adds.

Bids from the five consortia were due back before Christmas and Mott MacDonald will analyse them with the Highways Agency before the contractor is appointed in March.

'The winning team will be judged on quality and cost, ' says Perry. 'Then the public inquiry will begin.' The scheme will be the Highways Agency's first major project to be run under the 'Early Contractor Involvement' process.

'This is a relatively new process for the Agency, ' Perry confirms. This accelerated planning procedure aims to cut the time it takes to build a road by up to two years. It also puts more financial risk on the Agency, as it will be paying contractors for work on their proposals before planning consent is given.

This early involvement includes a buildability element which means contractors have their own designers working on the scheme. Once final design is under way, Mott MacDonald is likely to have some overview role, although this has yet to be finalised.

Construction is due to start in 2005, although, as Perry says, this depends on the outcome ofthe inquiry.

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