Small Dean viaduct: The launch
By Richard Byford, with Murray Cooke
On 31 January, the single most visual aspect of HS2’s work in the Wendover area will appear across the road just outside the town. When all other works are completed and the contractors have left, this conspicuous reminder will be evident for all to see for centuries to come.
Many people remember what it used to be like to drive out of Wendover on the London Road, in the direction of Great Missenden. Not far after passing the church on the left, the road would be joined by traffic coming out of Hale Road – which was was closed off some time ago. Further on, the road would be lined on both sides by trees and shrubs. A break in the vegetation on the left allowed a glimpse of a spinney in a small dip (which was marked “rifle range” on the old maps). Passing the gated boulevard leading to Boswells house and farm, you would see Road Barn Farmhouse, which had been built to look as if it had turned its back on the traffic. Apart from the addition in 1998 of the concrete-walled roundabout at the end of the bypass, most changes have been gradual. Not so now.

As long as a cruise liner…
In the last two days of January and the start of February, EKFB, the main contractor for HS2, will ‘launch’ a huge steel and concrete viaduct deck structure at an angle across the road and railway. If it were a passenger ship, it would be the 14th longest in the world, at 345 metres.
If the structure were placed along the combined length of Wendover High Street and Pound Street, it would reach from the Clock Tower to almost as far as the bridge over the existing railway – and stand nearly as high as the Red Lion hotel. Even if it were hypothetically possible to place such a massive structure in the middle of the town, then the 14 metre-wide flared top would have ripped out the top frontages of many of the shops and offices that line the street. At almost 4,500 tonnes, it weighs about the same as all the combined family cars in Wendover put together.
The end of the viaduct currently sits, mostly hidden from sight, behind a tall (permanent) embankment on the left hand side of the road as you drive South. The sliding move to its permanent position will take about 50 hours, spread over a four-day period.

The story of the viaduct
When the HS2 line proposals were detailed in the 2009 “Route Engineering Report” it identified that if the Wendover option was selected, then a 600m long viaduct would be needed to cross the A413 and the Chiltern Line at Small Dean, and the track would be about 10m above the A413.
Subsequently, there were many proposed changes to the route through the Missbourne Valley and Area of Outstanding Natural Beauty. Some went ahead. For example extending the tunnel from inside the M25 right through to Great MIssenden, eliminating the need for a green tunnel between Little Missenden and South Heath. As late as 2014, the idea of boring a ‘long tunnel’ all the way past Wendover was investigated and championed by a consortium of local authorities and other organisations. In the end the requirement for a viaduct at Small Dean was endorsed in the 2017 High Speed Rail Act, and the ‘bored tunnel’ dropped.
Consultations and development of the design
As a designated HS2 Key Design Element, the viaduct design came out for Public Consultation in September 2018. While the Department for Transport positioned the design as a “thing of beauty in the Chilterns Area of Outstanding Natural Beauty,” when Wendover HS2 Mitigation Group surveyed the local community, 94% of the respondents thought that the design was not acceptable in the AONB; and that quality of life would suffer due to the noise impact from the trains. However, detailed requirements had been developed by the HS2 Chilterns AONB Review Group (involving Bucks Council, the local District Councils, Natural England and the Chilterns Conservation Board) so the eventual design minimised the visual intrusion.
Design changes included changing the number of viaduct spans from six to seven, repositioning the piers to allow for future widening of the A413 during its 100 year design life, and realignment of Small Dean Lane to the north of the viaduct.
Many local people were surprised by the visual impact of the eventual design that has massive girders made of “weathering steel” which will look rusty and very visible to motorists with its limited clearance of only 6m above the road. (To illustrate: most delivery vans are under 3m. Articulated lorries are mostly under 4m and the highest double decker buses are usually less than 4.5m.)
A spokesperson for HS2 Ltd said: “We recognise the importance of good design and each of these structures has been developed with consideration for the technical and structural requirements, as well as the impact on their surroundings. We will continue to engage with local people and organisations on these designs.”
In November 2018, the viaduct design was awarded by the HS2 Independent Design Panel, who said: “The panel warmly welcomes the designs being developed by Eiffage Kier with Moxon Architects – finding much to applaud in how the design team has creatively embraced this sensitive context while meeting the demanding requirements of high speed rail.” Residents of Wendover – some of whom will drive under it daily – were muted in their applause.
Full details about the viaduct are available in the “Design and Access Statement” available in the Bucks Council planning applications web portal with reference 23/00052/HS2.

The other kind of tunnel
It was not just HS2 that was working on plans for tunnels. In early 2021 a small band of protestors moved into land between the railway and the A413 London Road and set up the ‘Wendover Active Resistance’ (WAR) camp. The site of the camp is approximately where one of the upright piers has been constructed to carry the viaduct. Through most of that year the camp grew and more and more placards went up calling for HS2 to be scrapped. With their eviction expected to take place from the middle of the summer onwards, the protesters took to their spades and picks and created a defensible complex of underground tunnels.
In early October of that year, a force of about 120 HS2 security personnel moved into the camp and the last occupants, Dan ‘Swampy’ Hooper and his friend Satchel, were removed from the site after five weeks of difficult work because of health and safety issues.
For some people in Wendover, the protesters were celebrated as heroes. Others complained about the scruffy and unhygienic condition of the camp and insisted the protests would almost certainly be ineffective in getting HS2 cancelled, given the amount of money and effort that had already been sunk into HS2. One thing that cannot be disputed is the fact that they drew attention to the environmental damage that had already started to happen, and the need for the HS2’s contractors to pay attention to all the environmental conditions contained in the 2017 High Speed Rail Act.
Technical
The Design Award reflects wider considerations than just the visual impact, including the viaduct’s ability to handle future generations of trains running in both directions at speeds of up to 250 mph (400kph) on a bend. To gain the required rigidity, the piled foundations for the viaduct piers go down 49 metres and the viaduct deck is a sophisticated “dual composite” approach. This was pioneered by Eiffage for viaducts on the French TGV network and uses remarkably little concrete (compared to the HS2 Colne Valley Viaduct), minimising the amount of carbon involved in building the structure.

Enabling works included the demolition of Road Barn Farm, which was in the path of the viaduct; and creation of a new temporary access to Boswells Farm, as the tree-lined drive to the estate would be blocked for construction work at the junction with the A413.
Pile-driving of thick steel sheets into the ground was undertaken in 2021 to protect the Chiltern Line before the sites for the adjacent viaduct piers were excavated in 2022. With six months’ notice being needed for “possession” of the railway needed, the works were timed for Christmas Bank Holiday weekend, and local residents were surprised to hear the hammering of the sheet piles start at 04:00 on Christmas morning. Two realignments of the A413 were also needed to create a safe working environment around the pier worksites beside the road.
The first pier was eventually completed in February 2024, with the last one in November. Construction of the piers was optimised by the use of concrete shells produced in a factory in Northern Ireland, to achieve a high quality exterior finish. The “hammerhead” shells weighing 50 tonnes were delivered by road. Having been lifted into place, the shells were filled with concrete produced at the batching plant beside the Wendover Bypass. The cantilevered hammerhead top of the piles needs to be very strong, with a complex computer-aided design of the reinforcement bars. Cages of the bars were also produced in a factory in Kent to ensure overall quality and brought to site by road.
Murray Cooke, spokesperson for Wendover HS2 Mitigation Action Group explained some of the problems caused by the viaduct construction that still remain unresolved:
“Building the viaduct was always going to require closures of the A413 at Small Dean to allow safe working. Traffic sensors were installed at Chartridge and on the Rignall Road to monitor the number of vehicles using the “hilltop routes” to avoid the works. In practice, we discovered that many motorists ignore the yellow warning signs about the closures, probably related to the length of the official diversion path through Penn, High Wycombe, and Princes Risborough. This was the shortest route compatible with Heavy Goods Vehicles.
Motorists heading back from London discovering that the road really was closed were often tempted to use Dunsmore Lane to get to Aylesbury. This was tricky as the road is “single lane with passing places” and the volume of two-way traffic resulted in damage to the historic “hollow-way” verges, as well as causing disturbance in Dunsmore village at all hours of the night. Some traffic from Chesham also ended up using Hale Lane into Wendover, with similar impact on the verges. Wendover HS2 Mitigation Group eventually persuaded the contractor to install illuminated “Variable Message” signs to attract driver’s attention, and to position them at points where knowledgeable local drivers could exploit unofficial diversion routes.
Another community issue was the decision to allow the closure of Small Dean Lane during the construction of the viaduct. This was a popular section of the Chilterns Cycleway route, allowing safe passage between Wendover and Dunsmore. We secured a formal undertaking as part of the HS2 Act to keep a “non-motorised” path available for cyclists, pedestrians and horses, and a temporary bridleway was created around the Small Dean Compound worksite. Regrettably this had to be closed to allow the creation of a bridge at the end of Small Dean Lane, with the result that cyclists now need to risk the traffic on the A413 or consider the long diversion through Wendover, Butler’s Cross and Chequers. Eventually a new cycleway will become available under the Small Dean Viaduct, forming part of the overall Buckinghamshire Greenway project. This is unlikely to be available before 2026.
Finally, we at WHS2 Mitigation Group were disappointed that our appeal for additional noise mitigation on the Small Dean Viaduct was rejected. The current design uses the 1.85m parapets to act as noise barriers, and special measures were identified to minimise the impact at St Mary’s Church and the Chiltern Way Academy school. Higher barriers are planned on the embankments to the north and south of the viaduct, but the residents at South Street and London Road will still experience peak night time noise from trains in excess of the World Health Organisation target level.”

Pre-cast concrete slabs were produced in Derbyshire, and installed to form the upper and lower layers of the deck structure. The resulting deck is expected to weigh 4,350 tonnes and will be slid north into place, at a rate of 8 metres per hour over four days. Although it will appear that it is being pushed over the road and railway, the actual motive power is provided by a very powerful winch pulling it using an attachment at the rear which will be repositioned in stages.
The A413 will be closed for one day to avoid motorists being distracted by the passage of the viaduct 6m overhead, and the Chiltern Line will then be halted for the following two days at the weekend, as the deck passes over railway tracks. An animation of the whole construction sequence is available at https://tinyurl.com/sdv2023video.
Once the deck is in place we anticipate future night time closures of the A413 throughout 2025 to allow for the road to be realigned onto its final track, and for the installation of the pre-cast concrete viaduct parapets above the road.

The launch ‘event’
You don’t have to be a supporter of HS2 to be interested in the viaduct being launched over the road. Most people recognise that HS2 is going ahead (at least the bit near Wendover), so the viaduct is going to be built regardless. The launch will be spectacular and historic. It will also be educational – even if you have no interest in construction or aspirations to be an engineer. HS2 has been made aware that many people in Wendover and the surrounding area will want to see what is going on, and has made arrangements for the activity to be observed from as close a position as it can make safe for public access. HS2’s local engagement manager, Haroun Khan, met with Wendover News to provide some of the details.
He explained that areas from which you can view the viaduct will be limited by the terrain: the structure is only likely to move at a rate of about 8m/hr, so unless you stand still for quite a while, you won’t notice much action. It will still be interesting, but you can have too much of a good thing: once you have seen it moving, you have seen it moving. Think of it as an interesting 30 minute visit – not as a day out.
From Wendover, your best bet is to walk along the London Road or Hale Lane until you get to the roundabout at the end of the Wendover Bypass. On Friday 31 January, the Bypass will be closed and it will be OK to enter the A413 on foot, as far as the safety barriers across the road.
A similar arrangement will be in place from the Wendover Dean side, but you will need to make a substantial detour to get there from the Wendover side. EKFB is intending to close the A413 to traffic as far south as the Great Missenden roundabout. The road to the A413 from Dunsmore will also be closed as well as other roads over on the east side. We walked the obvious footpaths that might give you a view of the scene, but it was not possible to find one that was not obscured by trees or undulations in the terrain. The tree-lined drive to the A413 from Boswells is a private road and access will be strictly forbidden to safeguard livestock in the fields through which it runs.
Stay safe!
On Saturday and Sunday 1-2 February, the A413 will be open once again, but the Chiltern Line Railway will then be closed. The viaduct will still be moving, but safe viewing areas will be limited. Vehicles will be transiting the roundabout, as they do normally, and it will be best to stay well back from the moving traffic. If you choose to go on those days, be particularly careful of passing vehicles – particularly as drivers are likely to be distracted. Children should be kept under close supervision at all times.