Regular commuters and visitors to London’s Victoria Station are all too familiar with the crowds of people that can build up at the entrance to the Tube station, especially in the rush hour.
Feeling the squeeze
At busy times, passengers are regularly held back at ground level to prevent overcrowding in the ticket hall and on the platforms below.
Victoria is the most congested station on London Underground’s (LU’s) Tube network, handling an incredible 80M passengers a year. It has two ticket halls, one for the District and Circle Lines, and the other – located just outside the rail station’s main entrance – giving access to the Victoria Line.
It is this Victoria Line ticket hall that frequently fills up at peak times, with the result that Tube staff have to manage the crowds by periodically closing the gates at ground level.
Congestion can also occur at platform level, because the escalators from the main ticket hall disgorge passengers at one end of the platforms – the south – and they tend to stay there rather than moving all the way down to the north end.
In eight years all this congestion should be no more than a distant memory for regular travellers, as an ambitious project to dramatically increase ticket hall capacity at the station has just begun. This involves extending the existing Victoria Line ticket hall by 50% and adding another that will lead passengers to the north end of the platforms. Also included within the £700M project are 400m of new tunnels connecting the new (north) and existing (south) ticket halls.
Last year, after a public enquiry, LU was granted a Transport & Works Act order, giving it the authority to go ahead with the project, and a Vinci/Bam Nuttall (VBN) joint venture was appointed to deliver the entire scheme in the summer. Construction will be phased, in part because of constraints imposed by the 2012 Olympic Games.
“The roads here are part of the Alternative Olympic Route Network,” says LU Victoria Station Upgrade programme manager Peter Lynch. “If anything goes wrong on the main route network, the alternative network will be used, so you can’t have any reduction in highway capacity from June to September .”
“The top down method is very stiff compared with bottom up and gives very little ground movement”
Rob Dickson, Mott MacDonald
Successful construction of the new north ticket hall and the extension of the south ticket hall relies very much on the ability to impose traffic management on the busy London streets above.
The north ticket hall sits directly beneath Bressenden Place, while the extension to the existing ticket hall will take it under Wilton Road. Both are extremely busy roads, used extensively by buses and taxis, and are part of a congestion charge-free route taking north-south traffic through the city centre.
LU and VBN are currently developing the construction sequence and the traffic management that will accompany each phase. In the meantime, Birse Metro has been undertaking a programme of service diversions ahead of the main contract - a programme that has meant a considerable reduction in road capacity at times.
Lynch says that, while he does not underestimate the traffic management challenge facing LU and VBN over the next few years, “the utility diversions have already proved that we can do works in this area and keep the traffic moving”.
Construction will start next year with excavation for the new north ticket hall. This is essentially a 30,000m3 cut and cover box, which reaches a depth of 14m at the northernmost end, where the escalators will descend to platform level.
It will be constructed using “top down” methods, within a 22m deep secant piled perimeter wall, with piles 1.2m in diameter. A central row of 50m deep plunge piles will provide support that will enable VBN to excavate the box in two strips to allow half of the road to stay open at all times.
“This [top down] is a very stiff system compared with bottom up, and gives very little ground movement,” explains Rob Dickson, design manager for VBN’s designer Mott MacDonald. “The principle is to minimise temporary propping as much as possible and keep the box as stiff as possible.”
A different beast
In all, three basement level slabs will be cast during the top down excavation, providing structural stability as the excavation progresses, and eliminating the need for temporary props. The excavation will take VBN through 10m of water-bearing deposits, consisting of made ground, alluvium and river terrace gravels, before reaching the underlying London Clay.
Although top down construction will also be used for the south extension, VBN project director Bob Lloyd describes this structure as “a different beast” because of the presence of the existing ticket hall, which will remain in operation throughout construction. Work on this structure will start immediately after the Olympics, and is due to be completed in 2018.
The extension is triangular in plan, and extends out from the area in front of Victoria Station known as “the Beach”, under the busy Wilton Road. VBN intends to use top down construction for the area beneath the road and a bottom up method for the Beach excavation. The new 13,000m3 box will be 11m deep, and the perimeter walls will again be formed using secant piles, reaching a maximum depth of 21m and ranging in diameter from 900mm to 1.2m.
At its lowest point, beneath a new flight of escalators, the new box will be just 4.5m above the southbound Victoria Line running tunnel. Mott MacDonald carried out an analysis to see if the excavation was likely to cause the tunnel to heave, as a result of which a series of bored tension piles up to 36m deep will be installed to hold the box down.
Care must also be taken not to disturb structures above ground during excavation of the two ticket hall boxes and the 400m of linking tunnels. A ground structures monitoring contract has been let to ITM, which earlier this year installed prisms, robotic total stations and monitoring points on all the buildings that could be affected by the works. This will give LU 12 months of valuable background monitoring data before construction begins.
A system has been set up to identify buildings that might be at risk, predict possible movement and alert the team to take mitigating action if any structures are predicted to move as a result of settlement – although Dickson says no such movement is anticipated.
Below ground there are two major sewers that run very close to the site – the shallow Kings Scholars’ Pond sewer, a culverted section of the River Tyburn built in the late 19th century, and the 20m deep Western Deep sewer, a 1990s wedge block constructed strategic route sewer. The Western Deep runs beneath a new vent shaft attached to the north ticket hall and, again, the possible effects of heave has been modelled to ensure the wedge blocks will not be dislodged by the excavation above. The model showed that the main criterion for the sewer’s structural integrity is the volume of water inside, rather than the forces on the ground around it.
Groundwater is an important issue in this area of London, which is close to the Thames, and picks up north-south flows. Existing drainage on the District and Circle Lines collects around 120 litres/second all year round. As part of the upgrade project this water will be collected in a new sump and used to cool the air passing through the Victoria Line platforms.
“There is a balance between benefits for passenger journeys versus cost and engineering issues”
Peter Lynch, London Underground
Where this groundwater could cause a problem is during excavation of the 30,000m³ of sprayed concrete lined link tunnels, which have been designed to be as shallow as possible to minimise the distance passengers have to walk.
But, as Lynch says, there is a “balance between the benefits for passenger journeys versus the cost and the engineering issues of shallow compared with deep [tunnels]”.
In this case, the engineering challenges of building shallow tunnels are considerable. Here, the tunnel invert will be in London clay, but the remainder of the structure will sit in the water-bearing superficial deposits. The last time passageways were excavated at Victoria, in 1992, permeation grouting was used to control water ingress during construction. However, this method proved inadequate in places where unexpected silt lenses were found.
To overcome this, the Victoria team will use jet grouting to stabilise the ground around the route and create a barrier to water ingress while the tunnels are excavated. Grouting will mainly be carried out by rigs set up on the ground above the line of the new tunnel.
The specialist equipment will deliver the grout in such a way as to create a 2m wide annulus around the line of the tunnel excavation, with the jet grout keyed into the London clay at the base, and each jet grout block interlocking with its neighbour. Although the rigs can be raked to get the grout into areas where direct vertical access is impossible, there will be some sections beneath buildings where the jet grouting will have to be done from within the tunnel excavation itself.
As well as creating a jet grout annulus, every 9m an entire wall of jet grout will be formed across the proposed tunnel face, acting as a bulkhead to contain the groundwater. This will effectively create a series of 9m long “inverted bathtubs”, as Lloyd describes them. Tunnel workers will dewater each 9m long section before excavating through the bulkhead.
Keller carried out a jet grouting trial for LU in 2008. This gave the client confidence to go ahead with this innovative method. “The trial enabled us to check that the required strengths and coverage of the grout could be achieved, and that the grouting process could be controlled so that it doesn’t damage existing infrastructure,” says Lynch.
Ground treatment and tunnelling are set to start in 2012. The project has an interim milestone covering the completion of the north ticket hall and the tunnels. This will give LU advanced congestion-easing benefits ahead of the project’s full completion in 2018.
VBN has a design and construct contract, although much of the design was done ahead of the JV’s appointment. “The design is substantially complete, and our input now is to bring that constructability element,” says Lloyd.
Lynch adds: “We did some value engineering previously, but now we’ve got a contractor on board we can start looking at other options and potential savings.”
Lloyd and Lynch describe the Victoria Station upgrade as a “career-changing project”, with Lynch adding: “There aren’t many better engineering challenges in the UK.”