Five teams to brainstorm Stonehenge tunnel project

From: Ground Engineering, November 2001, p.4.

FIVE CONSORTIA have been shortlisted to design and build the proposed Stonehenge tunnel in Wiltshire.

The 2km long cut and cover tunnel is part of the upgrading of 10.8km of the A303 near the 4,000- year-old monument.

Amec/Alfred McAlpine, Balfour Beatty/Costain, Mowlem/ Morgan Est, Sir Robert McAlpine/ Bouygues and Skanksa are working with the Highways Agency to develop solutions for the project.

The winning contractor will work with the scheme's consultant Mott MacDonald on value engineering design solutions before a planning inquiry. Mott MacDonald was appointed in March last year (GE August 2000) and has since carried out two phases of site investigation.

The main investigation, which focused on the properties and variability of the materials to establish general design parameters for the earthworks and tunnel excavation, was completed in summer 2001. The interpretative report is now being prepared.

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

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

Site topography means that as the tunnel runs through the dip it will be above the 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.

The tunnel will be a shallow cut and cover construction, with numerous options up for consideration. 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 good enough it may be possible to construct a box in deep-sided cuttings and backfill, or simply build a roof over a steep-sided cutting.

One of the main geotechnical issues for fill materials is classification of the chalk. Also of key interest is its insitu quality in terms of tunnel and cutting stability. Understanding the groundwater and any changes in the groundwater regime that may result from the construction is also important.

Groundwater along the route could have a significant impact on construction. Chalk is essentially a good material to work with if it is handled correctly and enough is known of its properties and behaviour on a particular site.

This will be the first major project to proceed using the Highways Agency's accelerated planning procedure scheme which gets contractors involved earlier in the planning process. The initiative is expected to cut the time it takes to build a road scheme by up to two years.

It will also involve the Agency taking on greater financial risk by paying contractors for work on their proposals before the scheme gets planning consent.