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Devil of a job

The A3 is being buried under the stunning geological marvel, the Devil’s Punchbowl in Surrey, so that the area can be enjoyed to the fullest by ramblers and wildlife. Ruby Kitching reports.

Dormice roaming between Tyndall Wood and Boundless Valley in Surrey have had an inkling that something was up for a while. Overnight, some found their woodland homes had disappeared, while others found new, convenient dormice-sized bridges popping up, leading them to pastures new. Badgers, deer and grass snakes also sensed that something strange was afoot in the area, which is also known as the Devil’s Punchbowl.

That something is the A3 Hindhead Improvement project, the rerouting of a dual carriageway which previously cut through this Area of Outstanding Natural Beauty and Site of Special Scientific Interest. A new 6.5km road will now skirt the Punchbowl – a natural crater in the landscape created by springs dissolving soft rock – and dive under it along a 1.83km tunnel up to 65m below ground.

Ground clearance to make way for the new road and tunnels started last year in a carefully controlled operation (so as not to disturb the wildlife too much) headed up by contractor Balfour Beatty and it s consultant Mott MacDonald for the Highways Agency.

Work has now begun this week to excavate the two 7.3m-diameter tunnels.

The long-term aim of the five-year project will be to remove the busy London to Portsmouth route and all the noise and air pollution that goes with it, bypass the village of Hindhead and create new cycle paths and structures to carry ramblers across the A3 more safely. The other modest reason for the £371M project is to improve traffic flow, which routinely comes to a standstill at the junction with the A287.

The Hindhead section of the A3 has always been considered a temporary solution to the problem of crossing the Devil’s Punchbowl. Even when the A3 was originally dualled in the late 1970s, the tunnel was tabled as the most environmentally sympathetic solution, but was considered too costly to build at the time.

Nearly 30 years later, and after a gruelling planning inquiry, funding problems and changes in health and safety legislation, the project is finally on site. It’s something Highways Agency team leader Paul Arnold, who has been working on the project for a staggering 21 years, thought might never happen.

“It was a great relief when the project got the go-ahead in October 2006. But then it was all hands to the pump to get the orders out to build the road and serve the notices to homeowners near the route,” he recalls.

The twin bore, two-lane road tunnels will be constructed using tunnel excavators, explains Balfour Beatty project director Paul Hoyland. This method was chosen instead of using tunnel boring machines (TBMs), which have to be ordered and specified “years in advance” – not conducive to a project whose start was unknown for so many years. Also, a massive 12m diameter TBM would have been required for this job and it would still cost £30M more than the excavator method.

“The excavators advance in 1m to 2m bites at the crown, the spoil is removed by conveyor and then you install roof supporting spiles and apply a 200mm thick sprayed concrete lining using a robotic spraying machine. The 40N/mm2 strength concrete contains steel fibres and an accelerator which is applied at the point of spraying.

Spiles are steel props which provide extra support during excavation since the Lower Hythe Bed is predominately weak sandstone. The water table is also high in the area, so there is only a narrow strip of ground where it is favourable for tunnelling and would not hit water.

Tunnelling begins from the north portal which has just started and will continue for 1,500m, after which tunnelling will advance from the south. Ground conditions for the first 150m from the south side differ from the rest of the project: the tunnel is shallower and will be constructed in the more sandy Upper Hythe Beds. Here, 150mm diameter, 12m long steel pipes spaced horizontally at 400mm will be used to support the ground ahead of excavation.

“The pipe canopy will control the overbreak and provide more confidence for the excavation when it is left overnight,” adds Hoyland. Geologists from Mott MacDonald will be present to check the condition of the excavation ahead of the tunnel advancing.

The tunnel is designed to be excavated in chunks, to minimise the amount of airborne particles. This is a requirement of new health and safety legislation for improved air quality and has added a whopping 20% to the project’s original cost.

“This was due to buying new machinery [instead of hiring or buying older models] and specifying the ones with the cleanest exhaust, using more electric and less diesel plant,” explains Hoyland. “The cost of ventilating the tunnel has gone up by a factor of 10 and we now have massive de-dusting equipment – we’re talking about something 18m long by 2m by 1.5m in the tunnel and getting in the way [of construction].”

The ventilation equipment will also be used during application of the secondary lining, which consists of a 45N/mm2 200mm-340mm cast insitu concrete lining with polypropylene fibres to provide fireproofing.
To speed up construction and reduce costs there is a plan to only cast the sides of the tunnels, since the formwork for the insitu lining is massive and costs £1.5M on the project. The crown would then be lined with sprayed concrete. “It crucially depends on whether the sprayed concrete will ‘stick’ to the primary lining. And if there’s a lot of water in the ground, it’ll be even more difficult,” says Hoyland. Perched water will be checked ahead of excavation and pumped out as necessary.

Hoyland adds: “The orginal £239M budget was also based on 2.5% construction inflation, when it’s actually reaching about 5%. The increase is due to the cost of labour and raw materials going up.”

Extending the length of the tunnelled section and the decision to tunnel the southern end instead of using cut and cover construction added 6% to the original figure. The National Trust wanted to retain as much land as possible, but the original tunnel design stopped short of Tyndall Wood (at the southern end of the project). Arnold recalls: “We said at the time that we could build a cut and cover tunnel here, and then Balfour Beatty engineered the vertical and horizontal alignment, so that it could be excavated from underground.”

Other changes involved making embankments near the tunnel portals steeper. Arnold recalls that early on the slopes were made steeper to reduce the land take. But this meant that additional 120mm diameter soil nails would need to be installed up to 10m deep in the ground to stabilise the near vertical slopes.

Other savings have been made by involving the contractor during the design phase, particularly when it came to firming up the alignment of
the tunnel.

To maintain a line of sight during construction for surveying purposes, the tunnel was originally designed to be wider at one end. But with careful tweaking, a constant tunnel section was achieved. Balfour Beatty also chose to build the new underpasses using precast concrete to save time during this early stage.

Tunnelling will continue for a year while construction of the road, Hindhead bypass and cycle lanes continue. The existing road where it crosses the Punchbowl will be returned back to the Devil.

TIME LINE

1993 Hindhead Improvement project announced.
1995 Project put on hold due to funding shortage.
2001-02 Government backs project and appoints consultant and contractors under early contractor involvement deal.
2004-05 Pubic inquiry (delayed due to changes in project scope).
2004 Changes in funding announced and the project loses priority status. Instead of being a project of national importance, becomes a project which must be funded regionally.
Jan 2006 South East England Regional Authority decides to pool its infrastructure funding to finance the project.
Oct 2006 Project given the go ahead by government.
Jan 2007 Land is cleared for construction of tunnels, underpasses and new roads constucted.
Jan 2008 Tunnel construction begins.
2011 Tunnel opens to traffic.GEOLOGY OF THE TUNNEL

The Hythe Beds material found around Hindhead is predominantly weak sandstone which needs to be treated with lime and cement to create a more durable sub-grade material.

Consultant Mott MacDonald is providing the geotechnical expertise and lead tunnel designer Tom Ireland explains that building the southern end of the tunnel will be the project’s biggest challenge.

“Tunnelling will be carried out with the aid of canopy tube or steel pipe umbrella ground support. This will be used at the portals Đ you’ve got sand at the south portal, and shattered rock under soft ground at the north,” says Ireland.

“There’s very little UK experience of using these kinds of temporary supports, though they’re used a lot in Italy and Austria. Ours will be self drilling, with a lost drill head on end. With daytime-only working from the south, we expect to advance only 2m per day. That’s compared with 24-hour tunnelling from the north where we should achieve 6m/day.”DEVILS PUNCH BOWL HISTORY

According to the National Trust, local legend has it that the Devil lived at the ‘Devil’s Jumps’, three small hills near to the Punch Bowl. He would often torment Thor, the God of Thunder, who lived at nearby Thor’s Lie (Thursley), by jumping from hill to hill. Thor would try to strike the Devil with thunder and lightning and once the Devil retaliated by scooping a handful of earth and hurling it at Thor. The depression that remained is the Devil’s Punch Bowl.
It is thought that it became known as a ‘Punch Bowl’ from the way the mist lies in the bowl and appears to flow over the rim as if it were boiling over.

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