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Calculated risks

Geotechnics Stonehenge tunnel

With value engineering and early involvement of specialists, could the Stonehenge tunnel be a turning point for the geotechnical community? Paul Wheeler investigates.

Early involvement of specialists has become something of a mantra among the geotechnical community over the last few years. It is, they argue, bang in line with the spirit of both Latham and Egan, and is a surefire way of achieving technical innovation and best value in construction.

The argument will be put firmly to the test on the Highways Agency's A303 road improvement scheme at Stonehenge.

The 10.5km road improvement project includes bypassing Stonehenge in a 2km tunnel. The aim is to return the area to as near as possible a Neolithic landscape, which should allow visitors to Stonehenge - a World Heritage Site - a more authentic experience.

Mott MacDonald is the Highways Agency's consultant on the project and has managed the main site investigation as well as the preparation of the outline design and the main contract - which was awarded in March this year.

Nothing particularly unusual about that, you might think. But what certainly is unusual is that at tender stage, the preferred design for the tunnel has not been specified.

The Highways Agency's intention is that the contractor will have a key role in determining the preferred solution, taking account of engineering viability, environmental and archaeological effects and cost.

To this end, the recently appointed contractor, a joint venture of Balfour Beatty and Costain, will be pioneering an 'early design and build' contract. Supported by designers Halcrow and Gifford, the joint venture will work in partnership with the Highways Agency and other stakeholders - including English Heritage and the National Trust.

The Agency believes that partnering and the early involvement of the delivery team, applying risk and value engineering procedures, and giving opportunity for innovation, will bring benefits to the project in terms of quality, cost and time.

The interesting aspect of early contractor involvement from a geotechnical perspective is the focus it has given to ground related risk.

As David Patterson, geotechnical adviser with the Agency explains: 'At Stonehenge we looked at the key consequences of not managing the geotechnical risk effectively, and the impact this could have on a range of issues such as health and safety, the environment, project quality and financial liability.

This information is incorporated in the project risk register which is kept live and under review throughout the whole life of the scheme.'

'Mott's role in the geotechnical investigations was to ensure that the new delivery team is properly armed with appropriate and timely information, ' says Patterson. 'This contract is very much about partnering.'

Mott MacDonald project director Dr John Perry, who has headed up the geotechnical investigations, adds: 'The initial ground investigations were geared towards providing information so that the contractor can focus on real issues and is equipped to identify the ground related risks with confidence.'

The ground investigation has inevitably concentrated on the ground structure over the tunnel, hydrogeology and engineering characteristics - for earthworks, foundations and tunnelling. The site-specific data has been assessed with the benefit of the new CIRIA chalk classification and linked with feedback from recent major civil engineering projects in chalk and research initiated or supported by the Highways Agency.

New techniques were included in the investigation, such as the optical tele-viewer - a downhole digital imaging tool - the use of which allowed visualisation of the chalk and analysis of its structure.

The work also identified opportunities for sustainability - which is essentially about making maximum use of materials on site, providing value for money and achieving a minimum impact on the natural resources.

'A big focus was the ground model and hydrogeology in relation to the tunnel, ' confirms Patterson. 'We're not pretending all information has been gathered for detailed design - but the construction joint venture is sufficiently armed with quality information to take the design forward in an informed way.

When it comes to measuring how well this approach is working, Patterson says benchmarking will be used as the project moves forward. And it is not just about reduced costs; measures could for example include reduced environmental impact. 'What we're looking for is the delivery of a project that meets the broad objectives of all parties involved.'

With the recent appointment of the Balfour Beatty/Costain-led delivery team, the project is currently in a transitional stage.

The next major decision is an assessment of the tunnelling options and the Highways Agency expects a decision on the method during the summer. All being well with the inevitable public inquiry, this would put the scheme on programme for start of construction in 2005.

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