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Emotional rescue

Collaboration between the client, engineer, archaeologist and landscape architect is considered critical to planning of the Stonehenge cut and cover tunnel.

Driving past Stonehenge on the A303 in Wiltshire - the extent of most people's experience of the 4000-year-old monument - is frankly disappointing. The Neolithic stone circle is sandwiched between two busy roads, intrusive fences, security guards and too many reminders of modern life for the visitor to possibly capture its spirit and significance.

But plans to 'restore'the environment in the vicinity of the stones are afoot and a traffic-free horizon could become a reality within the decade.

In March, the Highways Agency awarded Mott MacDonald the design commission for the A303 Stonehenge and adjacent Winterbourne Stoke bypass. The project involves upgrading 10.8km of the A303, including a 2km cut and cover tunnel to, as Mott MacDonald puts it, 'hide this section of the road in the most sensitive and visible area'.

The estimated £130M solution, developed with government, environmental and local interests, is designed to reduce the impact of the road and ease congestion.The A344 and the car park adjacent to the stones will also be removed and a new visitor centre built at nearby Amesbury.

Stonehenge, a Unesco World Heritage site, is one of the most important archaeological areas in the world. The sensitivity is not just about the stones: there are many important archaeological works in the ground and probably some yet to be discovered.

Such a high-profile project will undoubtedly attract controversy and objections are likely to lead to a local public inquiry to examine whether final permission should be granted.Already some environmentalists have argued for a more expensive and deeper bored tunnel, while others have questioned a contribution from heritage funds.

The then English Heritage chairman Sir Jocelyn Stevens last year commented in a letter to The Times, 'Even if a bored tunnel was affordable, it would have its own negative impacts. . . . Nobody claims there is an ideal solution to the problems facing the site. But we believe our plan is the best way forward.'

With construction work not likely to start until 2005, the project is at the very early stages, explains Mott MacDonald geotechnical manager Dr John Perry. The cut and cover tunnel will be the major structure, but there will also be a lot of earthworks with cuttings and embankments.

The route will run mostly on Upper Chalk, although at the western end of the scheme (away from the Stonehenge tunnel), the route crosses the River Till whose 250m wide flood plain may include soft alluvium and possibly peat.

Another area of interest is at Stonehenge Bottom - the dip in which the A344 presently meets the A303.This is a chalk dry valley, which may have superficial deposits and where the chalk may be more weathered and fragmented.

The topography is such that as the tunnel runs through the dip it will be above present ground level, so careful landscaping will be needed to produce a realistic and sympathetic landform and disguise what will in effect be a short section of tunnel sitting on top of an embankment.

'One of the main geotechnical issues for fill materials will the classification of the chalk and the project will be an opportunity to test new ways of thinking, ' says Perry.Also of key interest will be the insitu quality of the chalk in terms of tunnel and cutting stability.Other geotechnical issues will be understanding the groundwater and any changes in the groundwater regime that may result from the construction.

Little information is yet available on groundwater along the route, which could have a significant impact on the construction.Chalk is essentially a good material to work with if it is handled correctly and much is known of its properties and behaviour.

The tunnel will be a shallow cut and cover construction, with numerous options up for consideration. The choice will largely depend on the condition of the chalk. If it is poor grade, construction could include piled walls or diaphragm walls, but if it is sufficiently good it may be possible to construct a box in deep-sided cuttings and backfill, or simply build a roof over a steep-sided cutting.

The procurement approach is yet to be determined. Mott MacDonald project manager Stuart Bromley says: 'the option of early contractor involvement in the scheme is being given serious consideration, with a view to improving buildabilty at the planning stage.' The aim is to minimise duplication of design and get value for money.

Ground investigation will be carried out in two phases. The main investigation will be focused on the properties and variability of the materials to establish general design parameters for the earthworks and tunnel excavation.

A supplementary investigation will then concentrate on details at the position of structures.

Investigation techniques are likely to be a combination of deep (5m) trial pits, rotary coring and possibly a man entry shaft taken down to formation level.Geophysics is another possibility. In the soft ground areas of the River Till flood plain, investigation will probably include CPTs and trial pits.

'One of the most important challenges on a job of this scale is pre-planning and maintaining good communications with the interested parties, ' says Perry.

'Working with archaeologists gives a different perspective to ground investigation planning. It's very important to embody the needs of both archaeologists and geotechnical engineers. The ground investigation and of course the subsequent construction work must be sympathetic to the overall aim of the project.'

Paul Wheeler, with additional reporting Nina Lovelace Mott MacDonald's role involves developing the scheme from outline proposals and progressing it through public inquiry and construction.The firm will be working closely with archaeological consultant Wessex Archaeology and landscape architects Nicholas Pearson Associates.

The scheme is being developed with the Department of Culture, Media and Sport, the Department of the Environment, Transport and the Regions, English Heritage, the National Trust, English Nature, the Environment Agency, Wiltshire Country Council, Salisbury District Council, Amesbury Town Council, the Government Office for the South West and local community organisations.

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