Inside HS2: the innovations at Old Oak Common

The Leeds leg of HS2 may have been pulled but, in west London, construction is powering ahead. Paul Thompson reports on the techniques employed at the megaproject to aid delivery of two major schemes in one

Project Old Oak Common Station Box
Contract value GBP1.67bn
Client HS2 Ltd
Main contractor BBVS Joint Venture – made up of Balfour Beatty, Vinci and Systra
Civils work for box Expanded
Piling subcontractor Bachy Soletanche / Balfour Beatty Ground Engineering JV

Project Victoria Road Crossover Box – part of the London Tunnels contract
Contract value London Tunnels: GBP3.3bn
Client HS2 Ltd
Main contractor SCS Joint Venture – made up of Skanska, Costain and Strabag
Piling subcontractor Design House JV – made up of Arup, Typsa and Strabag

Despite massive growth in passenger numbers and freight demand in the decades leading up to 2020, there has been little expansion of British railways in the 60 years since Beeching sharpened his axe.

Part of the rationale behind the introduction of HS2 is that fast inter-city services will be transferred onto the new dedicated high-speed line. This will open up the existing lines for more local stopping services and an increase in freight tonnage, reducing the amount of freight carried on our highway network by diesel-belching HGVs.

As HS2 offers operating speeds of up to 360km/h, it is hoped it will offer serious competition to airlines, making flights between major cities in the north of the country and London far less attractive.

HS2 was initially conceived as a Y-shaped network with the southern terminal at London Euston; Birmingham Curzon Street at the intersection and Manchester and Leeds at the northern extremes. Construction of phase one between Euston and Birmingham began in 2020 and phase 2a between Birmingham and Manchester is due to follow.

Phase 2b Birmingham to Leeds has now been controversially terminated in favour of an existing route upgrade (see feature, page 34).

As phase one trundles out of Euston, it will call at a new station at Old Oak Common, near Acton in west London, before streaking toward Birmingham, arriving just 38 minutes later.

It is here that BBVS, a joint venture between Balfour Beatty, Vinci and Systra, is working on the delivery of what will become the UK’s largest sub-surface rail station and the largest ever delivered as a single project.

With 14 platforms, the new station will provide direct passenger access between six subterranean HS2 platforms and eight conventional rail platforms with services to Heathrow and central London via the Elizabeth line; and trains to Wales and the West of England.

The 850-metre station box

Unsurprisingly, to accommodate all those new platforms, the site set aside for Old Oak Common station is vast. And the BBVS team has the tricky task of filling it with a station that will provide a connection between HS2 and the adjacent conventional train station through a covered concourse.

Initial focus for the team is the construction of the huge 850-metre-long station box that will accommodate the 450-metre platforms.

Ceredig Thomas, senior package manager for the BBVS team, says: “The site has long been railway land. Other than a metre of poor ground, we are straight into London Clay.

There were one or two pockets of asbestos, but they have been dealt with.”

Old Oak Common: concrete slabs being poured at the site’s batching area

The team is busy installing a 1.8km-long diaphragm wall around the site, which will form the 20-metre-deep reinforced concrete station box. It has been split into three sections – west, central and east – for construction purposes.

Six grabs are installing the 275 diaphragm wall panels that will line the box, with a thickness of 1-1.2 metres. They are working through synthetic polymer-based slurry, which is pumped into the excavation as it is dug and supports the sides until the panel has been fully excavated.

As the grabs continue their work, strands of polymer stretch from their buckets, like tendrils of snot from a windblown toddler. They will continue to dig to 30 metres below existing ground level before the panels are cast using sitebatched concrete.

Within the box, a forest of 161 large diameter rotary bored plunge column piles is being installed. With diameters varying between 1.8-2.4 metres, these piles will support the base slab once the box is fully excavated.

Horizontal concrete beams 44-65 metres long will span the top of the box to support the walls, measuring 3 by 2.5 metres in section and set 13.5 metres apart. With the beams in place, the team will begin the excavation of the estimated 700,000 cubic metres of London Clay set to be removed to create the station void.

Victoria Road

Barely a mile or so across London from the Old Oak Common site, a team from the Skanska Costain Strabag joint venture (SCS) is working on an equally challenging project, albeit one that is unlikely to be quite as high in the public consciousness. Under its GBP3.3bn London Tunnels contract, SCS is building the Victoria Road Crossover Box, an essential step in the engineering of the HS2 line.

Without it, the sequencing of trains in and out of Old Oak Common would be compromised, as would the management of the environment within the station itself.

SCS construction manager Michael Lewis explains: “The box allows deaccelerating trains running into Old Oak Common to cool. It also ensures that they can cross to other lines and use all of the platforms at Old Oak Common.”

London-bound trains will slow from a running speed of well over 200km/h down to 140km/h on the approach to Victoria Road and then to zero at Old Oak Common. Trains braking to slow their speed create heat as does the air friction that develops as they run through narrow tunnels.

The box not only enables trains to cross lines, but also acts to vent that build-up of heat. Without it, the designers could be looking at temperatures as much as 8 degrees C warmer at the station.

Improving the design

The box requires construction of a space 125 metres long and 25 metres deep. The team will excavate its 130,000 cubic metre volume and tie over 3,700 tonnes of steel reinforcement while casting its walls and base slab.

The team will also build a 25-metre diameter, 30-metre-deep ancillary shaft to provide ventilation and emergency access to the 13.5km Northolt Tunnel, to the west of the crossover box.

Inside HS2: the innovations at Old Oak Common

The central conveyor bridge being lifted into place | Credit: John Zammit @ Absolute Photography

The final box layout made great savings over initial plans for a 215-metre space. Kate Hall, director of design partner Design House, a JV made up of Arup, Typsa and Strabag, says: “We looked at minimising all the structures. This is a much more efficient design [than the original plan].

We think it is now around 25 per cent cheaper with a reduced excavation, less concrete and offering time savings.”

As well as chopping 90 metres from its length, the design team has reduced the number of reinforced concrete props for the 1.5-metre-thick diaphragm walls to just four at ground and intermediate levels. An array of 77 continuous flight auger piles will be installed 20 metres into the London Clay, supporting the 4-metre-thick reinforced concrete slab.

The ancillary ventilation shaft has been dug to 30 metres. The top half of the 25-metre-wide shaft has been constructed using 11 rings of precast concrete to form the inner lining.

Below this, the team has used sprayed concrete to form the lower shaft lining.

Across the Victoria Road site, the team has taken various steps to reduce its environmental impact, including use of low-carbon concrete. The cement-free geopolymer concrete offers a saving in embodied carbon compared to standard concrete mixes, according to supplier Capital Concrete, helping reduce the project’s overall carbon footprint.

“There are areas where we are looking to reduce our emissions. Our machines use hydrotreated vegetable oil and we are also working with Geo Pura to provide zero-emission hydrogen fuels using renewable energy produced hydrogen,” Lewis says.

Conveyor innovation

While the completed projects at Old Oak Common and Victoria Road will offer huge levels of convenience for passengers once they are operational, it would be naive to suggest that residents around the sites are not facing high levels of disruption during their construction.

The volumes of muck involved in the excavation of the crossover box and station box are staggering, as are the arisings from various tunnel drives.

The combined total is eyewatering.

During the Old Oak Common station build, BBVS will move 750,000 cubic metres of excavated material from site while SCS has to move almost four million tonnes of arisings from the crossover box and tunnelling work.

Those volumes will be loaded onto trains at the former Euroterminal site at Willesden and transported away from the capital to Barrington in Cambridgeshire, Cliffe in Kent and Rugby in Warwickshire, where they will be used as inert fill on redevelopment and housing projects. At its peak, six trains a day will make the journeys.

Conveyor network benefit

In order to alleviate the potential impact of its massive excavation clearance works on local roads, temporary works include the construction of a conveyor system linking the sites to a train terminal at Willesden.

Capable of taking as much as 800 tonnes per hour from the BBVS station site, the Old Oak Common leg of the conveyor is estimated to reduce the number of daily lorry movements by 425. The SCS conveyor which will take material from the Victoria Road Crossover Box site and tunnel boring arisings will save over 195,000 lorry loads during its operational period.

The conveyor crossing over Victoria Road and Network Rail tracks accommodates conveyors from both Old Oak Common and Victoria Road sites and measures 6.3-metres-wide.

The 77-metre-long crossing sits 5.5 metres above the railway lines and 10 metres above Victoria Road itself. It was lifted into position in two parts using a 1,000-tonne mobile crane. The conveyor will be fully operational in the spring.

Carrying those volumes across an already congested road network would require an enormous fleet working day and night to clear stockpiles of arisings.

The impact on the local environment and traffic flows could be catastrophic.

In a bid to mitigate that risk, the HS2 team is installing a 1.2km covered conveyor system that will transfer material between the Old Oak Common and Victoria Road sites and on to the Willesden terminal (see box, left).

The conveyor network will be run at ground level for some lengths but will also feature high-level gantries where it crosses roads and the Grand Union Canal. There will be 10 transfer points where the system turns as it moves excavated materials to the Willesden site.

“The conveyor construction is part of the temporary works contract and will be a vital tool in reducing the impact of our work,” says BBVS’s Ceredig Thomas.

Standard gantries will measure 1.8 x 3.8 metres and be fully enclosed and acoustically clad at transfer points, helping reduce noise to ambient level. Water sprays will reduce dust.

With the station not due to be fully operational until 2030, these are early days indeed for the BBVS and SCS teams and there is much still to build before anyone might jump on the next train and head to Birmingham for lunch.

But the innovations developed so far are highly impressive.