Skanska Costain STRABAG Joint Venture (SCS JV) is now just weeks away from completing the earthworks for a series of tunnels and embankments it is building for HS2 just outside London that will help to propel the rail line across 134 miles of country between the capital city and the new Curzon Street Station in Birmingham.
SCS JV senior project manager Raymond Moloney is on site with bridge builder Dominique at HS2’s Copthall Tunnel project which will eventually connect with Gatemead embankment and the West Ruislip portal where his colleague Dan Townsend is also working.
Copthall Tunnel and a network of embankments and bridges in the London Borough of Hillingdon, connect HS2’s London tunnels to the Colne Valley Viaduct. This 880-metre-long tunnel, one of five “green” tunnels on the HS2 route, is designed to blend into the landscape with a layer of trees, plants, and shrubs on top.
Ahead of installing permanent drainage systems for the embankments and before the landscaping phase gets underway, however, the men explained the complex construction engineering process of designing and delivering an active tunnel system capable of withstanding and dispelling immense volumes of air pressure delivered at high speed by a commuter train, via a series of vent shafts and “transition zones”.
“Where we’re stood now on the West Ruislip retained embankment, we’re about six metres above where we were a couple of years ago,” said Townsend, who is an agent for SCS JV. “At the foundations, we improved the natural ground with a series of (about 7,500) stone columns across the site.
“The [embankments], a total of 800 metres long, comprising of 12 different types of material to build those. A unique feature about these high-speed rail embankments is ‘transition zones’. Essentially, what they do is reduce the force of the train as it goes from the stiff structures either side of us to the softer embankments.
“Not only does it improve passenger comfort, it also reduces the stresses on the track, the structures, the embankment, and improve overall performance and safety of that embankment and the trains.”
Different surfaces and environments occurring because the train is in motion result in its constantly varying relationship to external forces, including the air. These forces must be carefully managed and in certain situations redirected as part of the design and construction process, not just to improve journey quality but to mitigate pressure on surrounding structures, creating more efficient and safer transport infrastructure built to last.
Moloney explained: “We’ve completed two more vent shafts (at Copthall Tunnel) and we are well on our way with our fourth and starting to move through on the roofs, past our fifth vent stack.
“At the back of the eastern portal, Dominique has installed successfully 1,000 segments and has completed constructing the Colne Valley Viaduct. As you enter […] into the central section (at Copthall Tunnel), which is 550 metres long, we have five vent stacks which are at 100-metre intervals.
“The rationale behind the vent stacks is to allow the train to continue at line speed and not build any pressure up on the front of the train […] these vents shafts allow the pressure […] to go up and be released into the atmosphere.”
He added: “Since you’ve last been here, you can see our derailment walls are now being built, we have our walkway walls on either side for the down line and the up line, which are progressing through the 880-metre-long tunnel.
“We are on target to complete the central section of Copthall Tunnel by the end of 2024 – meaning we can focus on the eastern end of the tunnel through early 2025 to work towards completion, eventually connecting with Gatemead embankment and completing the trace to the West Ruislip portal.”
With the completion of the embankment earthworks now just weeks away after moving close to 350,000 tonnes of aggregate, SCS JV will soon connect all sites together and move onto drainage and then landscaping.
It follows Align JV, a team consisting of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, reaching the end of a more than 10-year planning, design, and construction process with the completion of HS2’s 3.4 km-long Colne Valley viaduct, now officially the longest rail bridge in Britain.
Major construction work on the wider HS2 project is at its peak, with more than 30,000 people employed, and will shortly begin awarding railway and track contracts.
“We’re now in our final stages of building these embankments,” said Townsend. “So, over the coming weeks […] we’ll finally be able to see the trace of the line and exactly where the train is going to go, connecting all the bridges, structures, and tunnels together.
“Once the earthworks are complete […] we’ll be moving into installing permanent drainage for the embankments themselves and the surrounding areas, the landscaping […] we will also be installing noise barriers along the full route of these embankments to connect to the tunnels.”
To watch the video interview, click below:
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