Work is under way on a £700m scheme to transform one of London’s main transport hubs.
Project: Victoria station upgrade
Client: London Underground
Project value: £700m
Main contractor: Bam Nuttall
Main contractor: Taylor Woodrow
Engineer: Mott MacDonald
Completion date: December 2018
Like many in the capital, Victoria station is struggling under the weight of passenger traffic far beyond the levels it was originally designed to accommodate.
As one of the main transport hubs serving London, it is used by almost 80 million passengers each year; this figure is forecast to reach 100m by 2020.
Even these current levels force staff from Network Rail and Transport for London to implement significant foot traffic management schemes, particularly at peak times, to keep passengers moving safely and efficiently.
In a bid to improve the station’s overall efficiency, Transport for London’s investment arm via London Underground is ploughing £700m into refurbishing and extending Victoria underground station, which is served by the District and Circle lines as well as the Victoria line itself.
A 50/50 joint venture team set up between Taylor Woodrow and Bam Nuttall has taken the deal, which includes providing step-free access between all of the underground lines and street level as well as a new northern ticket hall and extended southern ticket hall with associated escalators and links to all levels.
A very long time in the planning
It is a project that has been long in gestation. First mooted following an investigation into the King’s Cross fire in the late 1980s, the revamp of Victoria station has been pushed forward following the upgrade of the Victoria line itself.
“It has taken a long time to get the project off the drawing board for various reasons,” says London Underground programme manager for the project Glenn Keelan. “But the Victoria line upgrade has meant we have better, bigger trains and greater capacity.
“Unfortunately the station itself is a real pinch-point on the network. This project should help relieve some of the peak flow mitigation measures we have to put in place.”
And with peak hours now extending beyond the traditional Monday-Friday rush into Saturday mornings, the need to relieve that pressure is acute.
Complex ground engineering
But building in and around Victoria station is no easy task, and building beneath it is even trickier.
As well as the existing underground train lines, service tunnels, sewers, cables and foundations coupled with heavily trafficked main roads and a major transport hub that must continue to function throughout the work, the team is also having to contend with less than favourable ground conditions.
“The crown of the District line tunnel is shallow at around 2 m below the highway”
Craig Prangley, Taylor Woodrow/Bam Nuttall
“The crown of the Victoria line tunnel is around 17.5 m below ground level in London Clay,” says Craig Prangley, senior project manager for the Taylor Woodrow/Bam Nuttall team. “The crown of the District line tunnel is shallow at around 2 m below the highway. Our link tunnels will be built between these two levels, mainly through wet, loose gravel deposits.”
These deposits are a geological hangover from the period before the expansion of London when the area was marshland, and unlike London Clay, are extremely difficult to tunnel through.
Jet grout stabilisation
As a consequence the team is using jet grouting techniques to stabilise these wet, loose gravels and help the tunnelling process.
Specialist subcontractor Keller is halfway through the process of installing some 2,196 grout columns across the site using a 1:1 cement/water mix grout injected through a 70 mm-diameter drill string at high pressure.
“The grout is injected at 400 bar pressure at the nozzle”
Craig Prangley, Taylor Woodrow/Bam Nuttall
The grout nozzle slowly rotates as it is withdrawn from the borehole, displacing spoil as it progresses and leaving a 1.6 m-diameter grout column in its place.
“The grout is injected at 400 bar pressure at the nozzle, which is extracted slowly to form the columns,” Mr Prangley explains. “We carried out extensive trials to make sure we get the diameters we need.”
And it’s not just the diameter that has been confirmed; the solidity of the grout column array and its final strength is also vital.
“In many places these are actually being installed at angles and being stitched around service ducts and cables,” explains Mott MacDonald senior project manager Jason Rodwell. “We’ve carried out lots of tests, which have given everyone confidence. They have been harder than we anticipated.”
The grout columns have been installed with a 1 m-deep socket into the layer of London Clay to help keep the ground water out of the 6 m-diameter connection tunnel.
In some areas the ground is predrilled to accommodate a 400-mm diameter smoothbore section of pipe. This encourages spoil up through the pipe before being pumped off into holding tanks or directly into waste wagons.
Tricky construction sequencing
Thanks to the complicated and congested nature of the site and the sequencing of construction work, the team has divided the site into 24 different sections in which Keller is working.
Some of these sub-sites are very small and feature angled bores squeezed in between services and the foundations of existing buildings, including the Duke of York pub and the listed Victoria Palace Theatre.
With the theatre hosting matinee shows of the hit musical Billy Elliot on Thursday afternoons, the site team has to plan its own performance around the 2:30pm curtain up.
That means restricted operations around the theatre at that time with no drilling or grouting and restricted muck-away movements.
While many of the 24 sections are big enough to accommodate the grout silo, mixer and pumps, there are others where grout is being pumped some distance from the mixing point. It hasn’t affected installation times, though: each of the three Klemm rigs on site gets through a 65-tonne silo of grout cement each day.
The site team is just over halfway through the grout column installation, with a further thousand or so ready to go in before grouting is completed at the end of 2013.
It is then a race to help London’s commuters and visitors reap the benefits of the new Victoria station before the anticipated completion date in 2018.
Northern hall foundations
Creating a new ticket hall at the northern end of the Victoria line platform and doubling the size of the existing southern ticket hall will prove an immense benefit to passengers, but the construction of the northern ticket hall is providing a few headaches for the site team.
“There is a genuine challenge here,” Mr Prangley says. “Essentially we are building a standard piled concrete box but underneath Bressenden Place, a very busy road which is vital to the area.”
There are also numerous government offices in the area and the Victoria Palace Theatre is right alongside the ticket hall box.
The theory is that the western side of the box will be built first leaving Bressenden Place to run across the ticket hall’s eastern side. A full depth temporary road will then be laid across the western side before traffic is switched over, allowing the eastern section of the ticket hall box to be built.
“We are building a standard piled concrete box but underneath Bressenden Place, a very busy road which is vital to the area”
Craig Prangley, Taylor Woodrow/Bam Nuttall
Specialist contractor Cementation Skanska has already installed the 1.8-2.1 m-diameter plunge columns to depths of up to 50 m, while the 600 mm-thick roof slab on the western section, heavily reinforced with B40, 40 mm-diameter bars at 150 mm centres, has also gone in.
This has been installed using a C50 mix in two pours, one of 340 cu m and another of 220 cu m. “It is a substantial structure – there will be a significant development and main road built over the top of it, so it has to be,” Mr Rodwell says.
Surfacing teams are busily laying the new temporary section of Bressenden Place in preparation for the traffic switch over with the northern ticket hall and entrance, which are due for completion by 2016.
Breaking into the existing tube line
A new 24 m-deep, elliptical shaft – Shaft 2 – has been driven at the northern end of the northern ticket hall. This 12 m-wide shaft narrows to 9 m so that it can squeeze between the two existing Victoria line tunnels at the northern end of the Victoria line platform.
It will provide emergency access to the platforms, a link between the two platforms and tie-in with the ticket hall itself. But where the link tunnel breaks into the existing one, the team has developed a system that will significantly lessen the impact of work on tube travellers.
Using traditional methods would involve propping the work from the platform, with the potential impact of losing half the platform space during the work.
Instead, it is using an array of jet grouting columns to stabilise the ground behind the existing tunnel lining ring. The ground is then excavated until a steel section frame can be bolted through the lining to transfer the load through to the existing tunnel.
“The existing back is exposed and then drilled through so that the steel frame transfers the load across to the adjacent tunnel sections,” Mr Prangley says.
“If we had used traditional methods there would be 15 to 20 m of platform where the width would be down to 1.5 m; that’s just not safe. This method has almost no impact on the travelling public at all.”