Sustainable thinking underlies the A3 Hindhead scheme in Surrey, which includes a tunnel under the Devil’s Punch Bowl. Max Soudain reports on progress of the long-awaited bypass.
Legend has it that the Devil’s Punch Bowl near Hindhead in Surrey was formed by the devil scooping a handful of earth from one of the Surrey Hills to throw at Thor, god of thunder.
The devil was tormenting the god, who lived nearby at Thor’s lie (now Thursley), by jumping from hill to hill. Thor hurled thunder and lightning at the devil, who retaliated with his earth missile. The ‘punch bowl’ name is thought to derive from the way mist lies in the large depression and appears to flow over its rim, as if boiling.
In prosaic reality the Punch Bowl was formed by springs cutting down through the soft sands and sandstones of the Hythe Beds, a process that continues. Now a site of special scientific interest (SSSI) for its flora and fauna, the hill above Hindhead is a beauty spot of heath, small streams and woodland.
At Hindhead, south west of Guildford, traffic on the busy dual carriageway A3 between London and Portsmouth is funnelled into a single carriageway through the village. The resulting congestion is miserable for residents and motorists, and pushes traffic on to roads in the surrounding countryside.
A bypass has been proposed for decades and its latest incarnation, including a tunnel under the Punch Bowl to protect the SSSI, was first put forward in 1993. Funding problems meant plans were shelved at the end of 1995.
The scheme was given a new lease of life in 2001 when it was added to the government’s programme of improvements for motorways and trunk roads.The Highways Agency, with the assistance of Atkins, awarded the design and build contract to Balfour Beatty Major Projects in October 2002 (GE November 2002). It is now awaiting a public inquiry.
The 6.8km bypass will start from Bramshott Common on the Surrey-Hampshire border, running north past Hindhead and into 1.9km of twin bored tunnels under the Punch Bowl before rejoining the A3 at Boundless Road near Thursley.
The £165M scheme was let as an Early Contractor Involvement design and build contract, which meant Balfour Beatty helped in design, assisted with statutory processes and was involved in assessing environmental impact at an early stage.
Mott MacDonald, working for Balfour Beatty, is responsible for all design aspects: geotechnical, civil, structural, mechanical, electrical, fire and safety systems engineering. The Highways Agency has also retained Atkins as employer’s representative on the team.
‘The project is now in Phase 1B, leading up to the public inquiry in September, ’ says Mott MacDonald principal engineering geologist Andrew Davis. ‘This phase has involved looking at alternatives to the design. If the scheme gets the all clear. then it moves into Phase 2, with detailed design finished in early 2005 and construction starting later that year.’ It is estimated construction will take five years.
Davis says there has been a big push for environmental measures on the project. The area around Hindhead is designated an area of outstanding natural beauty and the Devil’s Punch Bowl is not only an SSSI but also a special protection area because of its wild birds.The National Trust owns large tracts of the area and both it and English Nature have been involved throughout the project to ensure the heathland is not damaged.
Several site investigations have been carried out over the years. The most recent was by Norwest Holst in 2003, which was ‘quite thorough and exhaustive’ Davis says, focusing on earthworks, deep cuttings, structures and the revised tunnel alignment. It cost £500,000 and included an angled borehole along the alignment and rotary coring up to 70m deep (GE January) as well as geomorphological mapping to identify slopes at risk from failure for further investigation.
‘The idea in an early contractor involvement scheme is to get investigations done early. This gives the contractor more information to start with, without having to delay the project, ‘Davis says.
Investigations were carried out under strict environmental control to prevent damage to the surrounding area and to protect the Hythe Beds, the area’s major aquifer. This meant having to tank in water for drilling and careful reinstatement.
The route passes through the Cretaceous sequence from Weald Clay to Bargate Beds, although it runs primarily in the interbedded sand and sandstone units of the Hythe Beds.
These are gently folded and lie at the crest of the Wealden anticline.
Travelling north, the bypass leaves the line of the A3 at Bramshott, where residents will be shielded from the new road by noise protection including bunds and fences.
Next comes the 800m long Hazel Grove cutting, which deepens to the north to a maximum of 20m.’Extensive soil nailing will be used here to allow steep slopes and to minimise land take, ’ Davis says. Composite slopes - with different angles within one slope - will also be used.
‘The real issues start when the road approaches National Trust Land, ’ Davis explains.
One is the Miss James footpath, named after a wheelchair-bound elderly woman, now dead, who regularly used the route through National Trust property.This link must be maintained and the current solution is to build a ‘green footbridge’ using planting and careful shaping of the bridge deck to give walkers the illusion they are still in the countryside and not over a major road.
From this point the road crosses the Nutcombe Valley on a 20m high embankment before entering the tunnel’s south portal. Davis says this will be landscaped into the valley side.
Four tunnel services buildings, a pair at each end, will also be built, this time in grassed bunds with local stone walls to help them blend in.
The 1.9km long tunnel consists of twin, 10m diameter bores, with regular pedestrian cross passages, planned at the moment at 100m intervals.There is about 30m of cut and cover at each end for the portals. The bores will run almost exclusively within the weak sandstone of the Hythe Beds and up to 65m below ground level, the deepest point beneath Gibbet Hill.
Excavation will be carried out using the sprayed concrete lining method. Hydraulic excavators will dig out the sand and sandstone in a range of heading lengths and benches, depending on ground conditions. Concrete will then be sprayed on to exposed areas.
There will be probing ahead of excavation and some spiling. Grouting is to be minimised because of the aquifer. The final lining will be cast insitu fibre reinforced concrete.
‘We have changed the vertical alignment to encounter better ground conditions, ’ Davis explains.’We are trying to maximise excavation in the most favourable material. It’s a balance of vertical alignment with tunnelling ease.’
The behaviour and condition of the Hythe Beds is so important to the success of the scheme that Mott is using a project-specific stratigraphical classification of the formation.
Groundwater generally sits above the Atherfield Clay, which for much of the route is at significant depth (5m to 10m below the invert).
Because the project must not interfere with the groundwater, its level was a key factor in determining the vertical tunnel alignment, to minimise environmental concerns.
Boundless Copse, where the tunnel emerges, is ‘a very environmentally sensitive area’ Davis says.’The wetland area is a wooded, acidic environment formed by a spring line at the outcrop of the Atherfield Clay, which has to be considered.’
The acidic environment is caused by water sitting on a thin (300mm to 400mm) peat layer in the valley. Monitoring of surface water has shown the level of acidity can be seasonal and is related to flow - the longer water sits on the organic ground, the more acidic it becomes.
The road will cross the copse on a 10m to 15m high embankment with 6m high bunds on one side to reduce visual intrusion in the picturesque valley. All of the material used for this will be won elsewhere on site - from the tunnels and the deep cuttings.
This, Davis says, is an example of the sustainable thinking underlying the scheme.’It is a balanced cut and fill - there is no offsite dumping.’
Spoil unsuitable for earth structures or for pavement sub-base will be improved with lime.
However, using lime (alkali) in Boundless Copse’s acidic environment needs careful controls if the sensitive ecology is not to be damaged.
‘TRL helped look into the possible effects and problems with lime improvement. Planned prevention measures include pre-construction drainage; stringent construction control; placing of a drainage blanket beneath the embankment to prevent mixing of leachate with local water;
and inert fill placed on the embankment shoulders and above the lime stabilised material to prevent run-off.’
As elsewhere on the bypass, the embankment will have green slope finishes.Where possible, local species will be planted, Davis says.
‘There will a programme of seed collection before construction starts.’
The final stretch of the bypass is a cut and fill section following the contour of the valley before meeting the A3 at Thursley. Reinforced soil embankments will again be used, minimising land take and protecting woodland next to the road.
Davis believes the success of the project to date is down to team co-operation. They have also worked hard to take on board public comments gathered during regular meetings and exhibitions in the Hindhead area.
Until recently Atkins, Balfour Beatty, Mott and the HA shared accommodation in Atkins’ offices in Epsom, but the team has now moved to Grayshott to be closer to site in readiness for the public inquiry.
If all goes well, quiet will be restored to the streets of Hindhead when the tunnel opens in 2009. Unless the devil gets up to his old tricks again.