HS2: Progress on a grand scale

HS2 is the biggest construction project in the UK. David Shirres’ article in Issue 207 (March/April 2024) discussed many of its key features including 32 miles of tunnels (five bored and five ‘cut and cover’), together with 50 major viaducts including 11 viaducts just for the delta junction to the north of Interchange station. With all this going on it’s easy to overlook the fact that most of these sites would be regarded individually as major construction projects.

With construction proceeding apace, HS2 is increasingly showing off progress. Your writer is not as familiar with civil engineering projects as he is with rolling stock and those he has seen tend to be on very constrained sites for projects in London. This report, about two recent visits, showed that much of HS2’s construction is on a much bigger scale, but even then, teams still have to contend with constrained sites even though the sheer scale of the work is amazing.

Thame Valley Viaduct

Whilst not the longest bridge/viaduct on HS2, it is 880 metres long and, in any other situation, would count as a major project in its own right. But here it is just one more significant construction undertaking as the line carves its way through the Chilterns and south Midlands.

As Bob Wright outlined in his article in Issue 204 (Sept/Oct 2023), this is a novel design in many ways. Unlike more traditional viaduct designs, every major element is being manufactured in a factory before being slotted together on site like a giant Lego set, cutting its carbon footprint by around a third. The main contractor is EKFB – a consortium of Eiffage, Kier, Ferrovial Construction (FC) and BAM Nuttall, which has worked with its design partner, ASC (a joint venture between Arcadis Setec and COWI) and specialist architects Moxon.

On a sunny day in early May 2024, Rail Engineer visited the site near Aylesbury Vale Parkway station to see progress at the half-way milestone. On arrival, EKFB Project Manager Emma Bolado Arroyo, and HS2 Senior Project Manager Ben Sebastian-Green outlined progress before being transferred by minibus the mile or so to the construction site itself where Noel Cooper, FC Civils Solutions project manager, explained the details as we toured the site.

The viaduct will carry the railway over a flood plain, so it was necessary for the contractors to make the land stable enough to carry the plant (up to 600-tonne cranes) and materials necessary for excavation and construction. The contractor’s office was established just south of a newly constructed roundabout on the A41 and from there a 1.5km road was built to the north end of the viaduct where a site office has been built.

The footprint of the land available for construction (defined in HS2 legislation) is around 2.5 times the width of the viaduct, and on the east side of the line of the viaduct, a two-way causeway was constructed, which employed European style right hand running for a while to keep the heaviest laden vehicles away from the edge of the causeway.

Background

As Bob Wright reported, three key elements of the construction – piers, beams, deck panels, and parapets – have been manufactured off site, although none of the latter had been delivered at the time of the visit. All, that is, except the piers at the centre of the viaduct which have been cast in situ. These are more substantial than the others and act as anchors for the beams. Radiating from these piers, the pre-tensioned beams are supported on bearings, and each is firmly connected to the next with tensioned connectors. The bearings have to accommodate expansion and contraction of the beams, a figure of up to 450mm was mentioned. Also cast on site were the abutments.

When the construction method was being discussed, the scale of the challenge became clear, especially for installing the beams. These are 25 metres long and weigh nearly 100 tonnes. There are 64 beams in total plus eight slightly shorter 20-metre beams, totalling 2 x 880 metres overall length. The beams can only be lifted when the windspeed is below 25km/h. The beams are transported from Pacadar’s Isle of Grain factory to site and lifted directly into the piers.

As the beams represent a special load that must travel with an escort and only at agreed times, the journey from the factory to site effectively becomes part of the construction process. The truck can only depart from the Isle of Grain if the following day’s weather forecast shows an acceptably low wind speed. Moreover, if the truck arrives and the wind speed had risen above the limit, the truck is stranded until the wind speed falls. If the wind speed is above limits, there can be no lift. Added to this constraint, the causeway is blocked when lifting is taking place, so material movement must be planned around beam lifts.

Rail Engineer was able to see that most of the beams have been installed and connection and tensioning of the beams has started. Many of the deck planks were in place and installation of re-bar had started, ready for pouring in situ deck concrete.

Only at the southern end is construction a little slower whilst the contractors await the electricity network operator to disconnect an overhead power cable and connect it to an underground cable that has already been installed. The finish of the factory-made piers beams was excellent, and it was obvious that a great deal of effort had been made to replicate that finish on the piers that had been cast on site.

In case you’re wondering about the height of the viaduct, it is about 3 metres. As a naïve rolling stock engineer, your writer asked whether an embankment might have been simpler, only for it to be explained that there needed to be a bridge over the river Thame anyway and there would need to be so many other bridges and culverts to accommodate footpaths and water moving around the flood plain, that it worked out simpler to build a viaduct.

Handover to the track/signals/OLE contractor is forecast to take place during 2026.

Bromford Tunnel Portal, Washwood Heath

This invitation was to see ‘the completion of the Washwood Heath portal of the 3.5-mile Bromford Tunnel’. This is also to be the site of the maintenance depot for HS2’s trains and its main control centre. Nothing prepares one for the scale of the site, though. This is an area famous for transport as it is the former site of Metro-Cammell (Tube trains, Mark 4 coaches, Pendolinos) and Leyland-DAF Van (LDV) factories. The site is 2km long and 600 metres wide at its widest, some 65 hectares (100 football pitches) in area. Images on Google Maps or in print do not do it justice, and even on a site visit, one fails to appreciate the full scale.

The contractor carrying out earthworks and constructing the portal is a consortium of Balfour Beatty VINCI (BBV). Tim Cook, its project manager described the work. This is a brownfield site and there were many buildings in situ when the contractor arrived. These were demolished and all concrete was crushed and graded for re-use on site. Due to its industrial past, there was a lot of contamination (mainly hydrocarbons). BBV worked on the principle of ‘remediate the soil to an acceptable level by fixing or reducing the current contamination level’ and this was carried out with the support and approval of the Environment Agency.

Approximately 60,000 cubic metres of soil has needed some form of remediation, but this pales into insignificance compared with the amount of soil moved on this undulating site. It is planned to excavate 1.643 million cubic metres of which nearly 1.4 million has been completed. Much of this was moved within the site (one million cubic metres out of 1.2 million) and much of the remainder is being transported along a specially built road to various sites around the delta junction.

The tunnel portal looks small in the site diagram, but when put into a human scale it becomes clear that it is a huge construction in its own right. It is a 750-metre-long cut and cover, top down constructed structure built by Bachy Soletanche and Balfour Beatty Ground Engineering (SB3) taking the track from the Bromford Tunnel portal, 22 metres below ground level up to surface level. Each side of the open top box was constructed by digging 35-metre-deep, 1.25-metre-wide trenches in 10-metre longitudinal panels filled with reinforced concrete with horizontal beams between them to react lateral loads.

Soil was removed down to a depth of approximately 5 metres and more horizontal beams were constructed before digging down to foundation level circa 3 metres below what will be track level. Here, numerous piles were sunk into the ground to anchor the box into the ground to react the upward pressure of ground water. So far, 44,000 cubic metres of concrete has been poured into the portal works and as much again is still to be poured (finishing concrete and further reinforced concrete in the base of the box to prepare for later construction of the track slab).

In the area adjacent to the portal, a 7-metre-deep sump will be dug to collect surface water that will be pumped into surface ponds that will drain into the adjacent River Tame (not to be confused with the River Thame mentioned above). The spiral access stairs give a scale of the box depth.

The Bromford tunnel is being constructed using two tunnel boring machines (TBM). The first, Mary Ann, is due to reach the portal in December 2024/early 2025 with Elizabeth following in mid to late 2025. The spacing of the lateral concrete beams at the top of the box provides for the TBMs to be dismantled and lifted out section by section.

Alvin Pedzai, HS2 project manager, described the other works being carried out near the Washwood Heath Depot site. To the west at Saltley there are two existing road bridges (Aston Church Road Overbridge and Saltley Viaduct) over the existing Birmingham-Derby line. Both structures will be replaced to provide adequate vertical clearance above the new HS2 rail lines. A significant constraint is that both these bridges are important road connections into Birmingham, so the works will be phased so that only one of these bridges will be closed to vehicle traffic at any given time during construction.

Depot

Damian Wheeler, head of Washwood Heath, HS2 Ltd, explained how the depot will be built on the now-flat areas delivered by BBV. There will be a 300-metre-long, eight track maintenance building at the east end of the site, each track able to accommodate one HS2 200-metre train.

At the west end of the site there will be 16 stabling sidings, each capable of accommodating two coupled 200-metre trains. Trains will be able to enter and leave the sidings from/to the main line under ATO control. There will be accommodation for drivers and servicing staff adjacent to the sidings, and the main Network Integrated Control Centre (NICC) will be located between the maintenance shed and the sidings. Damian said that the contract for construction will be let later in 2024. Rail Engineer described the concept for the depot in more detail in Issue 203 (July/Aug 2023).

When the depot and NICC are in operation, it is expected that 550 people will be employed on site. In addition, in the area to the south of the depot, there will be about 30 hectares of land handed over for commercial development that is estimated to create 1,000 jobs.

In conclusion

Your writer is sure there will be many more intermediate and completion events to celebrate before HS2 civil construction concludes. Some highlights to look out for will be walking tunnels, visiting the three stations (and Euston eventually) as well as the delta junction with its multiple levels and viaducts. Rail Engineer’s writers will be busy!

With thanks to HS2’s Alastair Cowan (Thame Valley Viaduct) and Pippa Whittaker (Washwood Heath) for their assistance with this article.

Lead image credit: HS2