Delivering HS2’s Old Oak Common station box

The consortium digging out the box for HS2’s Old Oak Common station is operating on a grand scale

Construction professionals couldn’t hide their dismay after the government’s decision last October to cut the planned northern leg of the HS2 high-speed rail project. Mace Group chief executive and Construction Leadership Council co-chair Mark Reynolds spoke for many when he warned that a failure to fully deliver HS2 would create a “chilling effect” on the sector.

However, there is still a major construction and engineering effort to create an HS2 link between Birmingham and London, where services will temporarily terminate at the new Old Oak Common station until a final decision is made on whether to terminate at Euston, as originally planned.

On a sprawling 18-hectare site, the BBVS joint venture between Balfour Beatty, Vinci and Systra is delivering an entirely new rail interchange with six underground platforms for high-speed trains and eight overground platforms for the Elizabeth Line and Great Western Railway services. Currently there are 1,200 workers on site but Sam Clark, senior project manager for the client HS2 Ltd, says the total should exceed 2,000 next year when the mechanical, electrical and plumbing (MEP) contractors arrive and work starts on the station building itself.

The 850-metre-long underground part of the station, known as the High-Speed Station Box, is 70 metres wide and contains the 400-metre-long high-speed platforms. The average depth of the box is 17 metres. It is 15 metres at its shallowest and 20 metres at its deepest point, says Dan Williams, project manager at Balfour Beatty and section manager for BBVS.

The box is being built using a top-down construction technique. This initially entails sinking diaphragm walls plus central supporting plunge columns, and then excavating just enough material, largely London Clay, to complete a ground-level slab that can carry the weight of construction equipment.

The ground-level slab over the footprint of the box also incorporates ‘mole hole’ openings through which soil can be lifted and removed. Excavation then takes place below the ground slab, including an intermediate service level, down to a base slab level.

When construction at Old Oak Common began in June 2021, piling contractor SB3 – a joint venture between Bachy Soletanche and Balfour Beatty Ground Engineering – poured a 1.8km fibre-reinforced concrete (FRC) diaphragm wall (D-Wall) around the perimeter of the box. The concrete is reinforced with synthetic fibres to improve the concrete’s performance.

Williams says that installation of the D-Wall’s 450 panels was completed in May 2023. Building the wall required more than 10,000 tonnes of steel reinforcement. With the wall in place, Laing O’Rourke’s concrete frame and substructure subsidiary Expanded installed 55 fibre-reinforced concrete prop beams between 44 and 65 metres long, laid across the top of the box to support the wall. The prop beams are at their shortest towards the east end of the box as its shape tapers slightly.

The main prop beams for the box are cast in situ because of their size, Williams says, although some smaller beams are cast offsite by Banagher Precast.

SB3 also drove a total of 161 bearing piles, each with steel vertical plunge columns, in the box. Each column sits within a pile that is up to 57 metres deep and 2.2 metres wide. The columns are not driven to the full depth of the pile but go as deep as 30 metres, with a 15-metre section visible from the base of the station box. They were driven mainly using the continuous flight-auger method, says Williams, though some were larger rotary piles to support a heavier load.

Shifting the muck

Phased construction of the high-speed subterranean platforms is planned for completion by 2026. But first, an extensive excavation process must be completed and the base slab poured to cover a total area of 60,000 square metres.

Excavation of the station box was 73 per cent completed at the time of Construction News’ visit, with summer 2024 as the target date for completion. So far, Expanded has removed 872,561 tonnes of soil (or “muck” as Clark calls it), equivalent to half-a-million cubic metres out of its target of 680,000 cubic metres. That’s enough to fill nearly 300 Olympic-sized swimming pools, according to HS2 Ltd.

Among the various types of excavation equipment, CN sees a Hitachi Zaxis-5 ‘chameleon’ clamshell telescopic excavator reach into the box from the surface to claw some of the muck out. This is loaded into a Moxy dump truck, which is driven to the conveyor ‘muck bin’. The muck is then sent to the Willesden Euroterminal north of the site via a 1.7-mile-long conveyor belt system. Using the conveyor saved 37 tonnes of CO2-equivalent from its entry into service in November 2022 up to February this year – the same as keeping 51,332 lorry journeys off the road, says HS2 Ltd.

“At subsurface level we use more traditional excavators such as 14-tonne diggers that dig away at the clay and collect it into a pile for the chameleon to grab,” says Clark. “We’ve excavated a large portion of the west and east ends of the box already as these were a big focus in 2023.”

About 75 per cent of the excavated material is recycled. It is sent to different parts of the country for use in various construction schemes, landscaping projects and even a wetland bird sanctuary in Kent.

More to pour

Once the excavation process ends, Expanded will pour 190,000 cubic metres of reinforced concrete. This will include not just the base slab on which the platforms themselves will sit, but also the aforementioned surface-level slab that will overlay the prop beams and the 5-10 metre-wide mole holes to seal most of the box (for example, there are eight voids across the length of the box that will be used for ventilation systems).

Williams explains that the BBVS-led team has constructed the last prop beam at surface level as a milestone towards finalising the FRC structure of the box. “The central east box has only fairly recently been excavated down to the intermediate [next] level where more prop beams will be installed,” he says. The intermediate level – typically 6 metres underground – supports the diaphragm wall when excavation progresses to the base-slab level. This prevents the wall from deflecting inwards due to pressure from the soil mass behind the wall. It therefore provides structural stability while the excavation takes place down to the base-slab level.

In summer 2024, the BBVS team aims to complete work on the prop beams on the intermediate level all over the box.

Inside the box, the floor is a sea of concrete and steel rebar. The first base slab concrete pour at the bottom of the western end of the box was completed in May 2023. This meant that the BBVS team had reached the HS2 track level and could continue to work eastwards to build the foundations for high-speed operational services.

There will be 95 pours for the base slab, each involving up to 1,000 cubic metres of concrete.

“We can’t do more as we’re a bit restricted as to how much concrete we can produce,” says Williams. The Old Oak Common site has its own concrete batching plant operated by Aggregate Industries subsidiary London Concrete – next year, the plant will have to be decommissioned to make way for continued construction work.

On average it takes about two weeks to form an area of base slab, including the time it takes to install the rebar for added structural stability.

After the initial excavation reaches its desired depth, says Williams, “we build it back up again with the base slab that’s about 2 metres deepand then the platform itself is a couple of metres above that”. The platform slabs sit on top of invert walls made from reinforced concrete. This leaves a void beneath the slabs which is used to run electricity and other services down the length of the platforms.

Expanded will install 32,000 tonnes of reinforcement in the box, in addition to the 10,000 tonnes already used in the FRC diaphragm wall, using steel rebar that is supplied by F Brazil Reinforcements and formed by hand on site.

Right now about 145 steel fixers – 12 per cent of the current site workforce at Old Oak Common – are employed solely on tying and laying out the rebar for the box. “You can argue it’s archaic, but there is robust human labour involved in laying out those bars, some of them 40 millimetres in diameter,” Clark says. Each strip of rebar is up to 14 metres long and weighs up to 150 kilograms. “It takes four or five guys to lift that.”

In contrast to the low-tech rebar, Williams says BBVS uses fibre-optic monitoring throughout the reinforced concrete structure. “Essentially, we pass fibre-optic cables through the concrete – it’s integral to the concrete itself,” he explains.

Williams says this method “allows us to sense the strain in the structure during the top-down construction process. As we start to excavate down into the ground, we can pick up the strain within the concrete elements and we can calculate any deflection [movement] in the structure”.

Still to be bored is a subsurface tunnel running to the station box from the open-air Victoria Road Crossover Box, 360 metres to the west of the box, where it will connect with tunnels from Birmingham. “At some point in the next few years, our mainline civils colleagues will break through the box, tunnel into and out of the box, to create those high-speed tunnels,” says Clark.

The week before CN visited Old Oak Common, a tunnel-boring machine was launched from Victoria Road by the HS2 London tunnels contractor, Skanska Costain Strabag joint venture, underlining that there’s still plenty of life left in HS2.