Congestion and curves – challenges in constructing HS2 in the Chilterns

Not far outside London, two railway viaducts are being built for the HS2 railway, which will pass through one of the more congested patches of the Chiltern Hills.

The railway passes along a long natural valley that runs through the Chiltern Hills, a legacy of the last Ice Age when glaciers carved a path through the hills. As a very convenient route for humans to get through the hills, it’s lined with towns, roads, and railways, making it a complicated area to fit another railway into.

Many old roads need new bridges, and vast numbers of sewers, power cables, and associated infrastructure laid down over the past century and now inconveniently in the way need to be diverted to make space before the new railway can be built.

While disruptive for local residents and causes costs and delays for HS2, one of the legacies of building a new railway will be a lot of new road bridges and utilities. Old narrow brick bridges are replaced with wide modern bridges, and new utility tunnels tend to be more reliable than the old infrastructure they’re replacing.

In some places, the railway will pass through cuttings, but in others, it needs to pass over valleys on bridges and viaducts. Two of the larger ones can be found near Wendover, where the Wendover Dean and the Small Dean viaducts are currently being constructed.

Wendover Dean viaduct spans a long valley over farmland, but the Small Dean viaduct has to carry HS2 trains over the very busy A413 main road and the existing Chiltern Railways lines, while also slipping around the electricity power lines that dot the landscape.

It’s a very congested and busy area.

The main road also runs very close to the railway, making it a difficult site to build in, and one where they had to move the main road to one side to allow space for one of the piers that will support the viaduct.

The undulating landscape also presented a challenge with the timing of building works. It’s convenient that in some places, they want to dig down a bit to create a level railway, and in others, they need to fill a small gap with soil so soil from a cutting can be reused locally. That’s ideal for reducing waste being removed, but it means a lot of careful planning to ensure that cuttings are dug down at just the right time for the spoil to be needed somewhere else.

And the British weather does enjoy messing around with carefully planned timetables.

Recently, the larger Wendover Dean Viaduct was structurally completed when a long streel structure was pushed over the valley, lining up with a row of tall concrete piers. They still need to cast the railway slab on top and cover it with more decorative cladding to reduce its visual impact, but after several years of work, the core structure is complete.

The viaducts are being built by HS2’s main works contractor, EKFB—a team made up of Eiffage, Kier, Ferrovial Construction, and BAM Nuttall. Specialists at Eiffage Metal are leading the manufacture and installation of the beams.

Just down the road, though, is arguably a more interesting viaduct, at Small Dean. While physically smaller, it is actually slightly heavier due to the need to span a very different landscape.

While the Wendover Dean is passing over what was and will again be remote countryside, the Small Dean will pass right over one of the most congested patches of land in the area — with the busy A413 and the Chiltern Railway tracks below and the electricity pylons above.

Over the past few years, contractors have been diverting utilities and even the main road itself to create space for the piers supporting the HS2 railway, which are now being installed on site.

The piers are interesting.

Normally, a pier would be built by assembling a steel rebar cage on site, covering it with a temporary cladding box, pouring concrete into the box, and removing the shield to reveal the pier.

Because of the site’s constraints, they had to do something different.

The concrete “skins” are delivered from a factory in Northern Ireland as precast segments that are assembled on-site, leaving an empty shell to be filled in. They then have rebar cages assembled in Kent and delivered to the site, which are then dropped into the concrete shells. This is a fiddly task made harder by the inability to see inside the concrete shell when lining up the joins between the different components.

Having finally done that, they would usually pour concrete into the structure and push vibrators down into the wet concrete to shake it so that it filled all the gaps and got rid of air bubbles. That’s not possible here because the steel cages needed to be so strong that there was not enough space for the vibrator to fit, so they developed a self-compacting concrete mix specifically for this job.

While the piers are being assembled, the viaduct bridge is also being assembled nearby so that it will be ready to slide over the valley next year.

That work has been underway for some time, and the contractors are now putting the finishing touches to a huge 450 metre long viaduct structure, adding the strengthening fins to the sides of the long steel box, while the piers are being constructed to support it.

The long steel structure was made from segments delivered from France and are so large that there was just a couple of inches of gap to get them off the ferry that delivered them to the UK. They were then delivered to Wendover by long load lorries overnight and assembled on site.

Next year, one of the of the more dramatic moments in the project will take place.

Over a long weekend early next year, the road and railway will need to be closed so the viaduct structure can be slowly pulled over the valley. They have to close the road and railway for safety, not because of the risk of things falling down, but because drivers distracted by the sight of a moving bridge may crash their vehicles.

It’s also slightly complicated by the fact that the viaduct isn’t perfectly straight but curves and twists slightly, so they must ensure that as they pull the steel over the valley, it lines up perfectly with the curving line of viaduct piers.

There is an option to do the work over Easter, but they hope to do it sooner so that the closures don’t affect Easter travellers. So, maybe on Monday morning in February or March next year, motorists will see a large railway bridge sitting over a road where where there hadn’t been anything the previous week.

As one of the project managers observed on the way back from the site visit, it’s probably the only time in his career that he will be able to build something as big as this and see it move.

That’s for next year though.

When the builders have packed up and the landscape restored, what will remain are the HS2 railway viaducts, a newly relaid main road, and a large section of the Misbourne Greenway. This is a new walking and cycling route that’s being built in stages between Wendover and Uxbridge and will make cycling alongside the busy A413 possible for the first time, and HS2’s work outside Wendover is delivering this section of the cycleway.

Later, when HS2 opens and takes intercity traffic between London and Birmingham away from the existing railways — that will create the capacity for Chiltern Railways to look at increasing the number of trains using the Wendover route, so after all the construction disruption, local residents can look forward to having more seats on more trains.

Updated 28th Aug – corrected the name of the Small Dean viaduct.