Beneath the streets of central London, a massive upgrade programme is underway to relieve congestion at some of the capital’s busiest stations.
The largest and arguably most complex project London Underground is currently managing is the massive £700M upgrade of its station at Victoria. It is one of the busiest on the network, currently handling 82M passengers a year, with forecasts estimating that number may rise to 100M by 2020.
At peak times this volume of passenger traffic is more than the station can cope with, and LU staff often have to control the number of people going onto the platforms by closing the main gates into the Tube station. “We are facing a serious issue, because passenger numbers are continuing to rise,” explains LU Victoria station upgrade (VSU) programme manager Glenn Keelan. “If we did nothing, by 2020 the station would be closed more often that it was open during the peak hours.”
“If we did nothing, by 2020 the station would be closed more often that it was open during the peak hours.”
Glenn Keelan, London Underground
Victoria’s underground station is, in effect, two separate stations - one for the shallow District and Circle Lines and the other for the Victoria Line, built much deeper during the 1960s. Each has its own entrances, and there are short tunnels linking the two. However, the ticket hall for the Victoria Line is adjacent to the mainline station, so many commuters and visitors arriving by train head straight for this entrance irrespective of which line they are travelling on, adding to the congestion.
But it is not just the number of passengers going into the station that causes congestion: significant numbers getting off the Tube at Victoria currently travel southwards towards the main exits at the Network Rail end of the station. The numbers are increasing in part due to the introduction last year of more efficient trains on the Victoria Line that can carry more passengers and with shorter headways (the time between trains). With very limited circulation, passengers going in different directions conflict with each other, making it difficult for them to get around as quickly as they would like. LU has calculated that passengers spend an average of six minutes in the station’s underground tunnels and passageways on their journey out of the station.
“There are two conflicting waves of passengers: from the mainline station to the underground, and coming off the underground and out,” explains Keelan.
LU has also discovered that between 40% and 45% of passengers who get off the Victoria Line at Victoria actually want to head north east towards Westminster. At the moment, if they want to do this they have to go out through the District and Circle Line ticket hall, or go all the way back through the underground passageways to the Victoria Line ticket hall at the south end of the station and retrace their steps once they get above ground.
All of these issues have led LU to invest in this major project to enlarge and modernise the station, create new access and improve circulation. As Keelan says: “The station, while managed well, is so far behind where it should be in terms of space provision that any of the individual improvements we are doing would have a considerable effect.” Taken together, however, the improvements will extend the life of the station by at least 75 years, and will also contribute to a plan to entirely redevelop the Victoria area.
LU’s solution to the congestion problem is, as Keelan puts it, to “divide and conquer” by providing passengers with “the means to get to their destination quickly and without conflicting with other people”. Key to this is the construction of a completely new ticket hall - the north ticket hall - that will be linked directly to the north end of the Victoria line platform, and should progressively reduce the number of passengers using the existing ticket hall and the District and Circle Line entrance by around 40%.
The existing ticket hall adjacent to the mainline station - renamed the south ticket hall - will also be enlarged and modernised to improve capacity and circulation. There will also be nine new escalators and eight new lifts to improve access to the Victoria Line platforms, and there will be a new link tunnel between the District and Circle and Victoria lines. All of which leads to a project that is “not without a few difficulties to overcome”, according to Keelan.
“The individual elements of the scheme are not in themselves complex, but the real trick with the VSU is ensuring all the pieces of the jigsaw fit together,” he says. “We are building it in a major urban area that is living and moving around us as we build, and there are major challenges both in the ground and above ground.”
These challenges include building the secant wall “box” for the new north ticket hall adjacent to a listed theatre and above a live railway; and constructing the new passenger link tunnel - known as the paid area links (PAL) - in water bearing ground above the Victoria Line and below the District & Circle Line running tunnels.
“The ground conditions have very much governed the tunnelling solution,” explains Jason Rodwell, project manager for VSU designer Mott MacDonald. “The conditions are very typical of London: made ground overlying alluvium, then water bearing river terrace deposits - silts and gravels - then London clay. When you are tunnelling in London clay, it’s a nice material to work with, but we had to come up with a solution that goes through the river terrace water bearing gravels.”
LU did look at the possibility of excavating the PAL through the clay layer, but it would have been a lot deeper, so passengers would have had to change levels and spend longer in the tunnel, which LU felt was not acceptable. Instead, the team had to devise a solution that made it possible for the contractor - a joint venture of Taylor Woodrow and Bam Nuttall (TWBN) - to dig the PAL through the water bearing material without groundwater getting into the excavation.
That solution was jet grouting - keeping the water out by creating a full face jet grout annulus for almost the entire length of the PAL as it winds beneath the streets of Victoria to link the three ticket halls.
Since 2011 subcontractor Keller has been installing the jet grout columns that will make up this annulus. It is a task that will take until the end of 2013, and will eventually result in 2,300 jet grout
columns being installed.
Tottenham Court Road
The £480M upgrade of Tottenham Court Road station is held up by LU capital programmes director David Waboso as an exemplar project: massive in scale, fiendishly complex, yet on time and on budget.
Over 150,000 people use the station every day, and passenger numbers are expected to surge to 200,000 with the opening of the Tottenham Court Road Crossrail station in 2018. The congestion relief scheme includes an upgraded and enlarged ticket hall, three new entrances, additional escalator access to the Northern Line platforms, improved circulation space, step-free access throughout and an interchange with Crossrail - including the construction of the eastern ticket hall box. A separate contract will build the Crossrail station platforms and western station box.
Critically, the project remains on schedule and within budget. And equally critically, after an eight month closure, the Northern Line platforms were returned to service on plan on 28 November 2011.
“A really big deal for us was reopening the Northern Line platforms,” stresses Waboso. “We said we’d do it in seven months, and we did.”
It was a Herculean 24/7 effort by joint venture contractor Taylor Woodrow/Bam Nuttall (TWBN). “At the time we would have had over 450 people working 24/7,” says TWBN project director Jez Haskins.
During the works, which started in April, long lengths of the platforms had to be removed and the segments that support the tunnel replaced. In total, iron segments from 195 rings in the tunnel lining have been replaced with over 1,000t of brand new, specially manufactured tunnel segments.
But that’s far from all that has been going on. In the last year alone, the list of construction work completed by TWBN is staggering. On top of the work on the Northern Line platform tunnels it has excavated two new shafts to a depth of 30m below street level for new Northern Line escalators; excavated over 130m of new station interchange tunnels for the Central Line; driven more than 800 secant piles and a host of diaphragm walls to form new underground spaces; constructed large plunge columns for Crossrail’s western station box - the largest of this type ever built in the UK; rebuilt the staircase to the station’s Exit 4; and even found time to strengthen the station’s existing escalators.
Achieving all that in what is a very challenging environment is possible through a fully integrated, co-located project team that makes full use of early warnings/risk reduction meetings to ensure that problems are resolved quickly in a non-adversarial way.
It is also a significant achievement for the project to have reached 1.5M man hours by Christmas 2011 without a lost time reportable accident.
Now that hugely complex engineering work to reshape the Northern Line platform is complete, TWBN is focused on building new access routes linking the platforms to the new ticket hall being built in front of the Centre Point tower and to the future Crossrail western ticket hall deep below what is known as Goslett Yard. The first new station entrance is due to open in 2015 with all work due to complete in 2016 before Crossrail services begin in 2018.
“Most of the columns are installed through the highways, so we can’t do it all in one hit,” explains TWBN head of engineering John Keable. “We had to work out a logical sequence of how to do it in small areas, but areas that were large enough to get the equipment in and were safe for the operatives and the public.”
Where the columns are not in the public highway, they often have to go very close to the basements or building foundations. All have to avoid the maze of utility pipes that lurk just beneath the surface.
“Because it’s complex, and some of the areas we have to go under are very precise, we are using a 3D model that can identify targeted locations for the columns,” says Keable. “Keller can place a column within 50mm of an existing foundation. Without the 3D model it would be very difficult to achieve the levels of accuracy we’re getting.”
TWBN has divided the work into four teams - north ticket hall, south ticket hall, tunnelling and jet grouting - with overarching coordination at the top level. The contracting JV is co-located with both the client and designer, which all three parties say is key to the success of the project so far.
“With Mott MacDonald being here and LU being here, if something comes up, for example if they need to change the pile design, we can make decisions very quickly,” says Keable.
In addition to the jet grouting, work has now also started on both the north and south ticket halls and sinking some of the shafts that will be needed for access to the new PAL tunnel. The north ticket hall is being built using top down construction beneath a main road, so it has been done in two halves to minimise disruption to traffic. Initially, one half of the road was closed while the 1,200mm diameter secant piles were installed for the perimeter wall of one side of the box, and then a capping beam and slab were cast at ground level for the roof of the box in a 350m3 pour just before Christmas.
“The individual elements of the scheme are not in themselves complex, but the real trick with the VSU is ensuring all the pieces of the jigsaw fit together”
Glenn Keelan London Underground
Traffic will be diverted onto this slab later this month, so that the wall piles for the other half of the box can be installed. Once both halves are done, the box will be excavated beneath the slab, with traffic running on the road above.
Also under construction at the moment is a dedicated emergency access shaft adjacent to the new north ticket hall that has to be sunk to a depth of 24m, where it sits just below the inverts of the two Victoria line running tunnels, with a gap of just 350mm in places. “The running tunnels have a unique form of lining,” explains Keable. “On the rest of the Tube these are mainly bolted cast iron segments, but here they are articulated, with a rotating hinge.”
This form of construction makes the segments more susceptible to movement, so they are being closely monitored as the shaft is sunk. In addition, the top half of the shaft is being stabilised with an annulus of jet grout columns.
Throughout the next year work will be underway on all fronts, heading for LU’s first major milestone in 2016, when it has committed to deliver the north ticket hall and its access to the Victoria Line platform. The rest of the scheme is scheduled to be completed in early 2018.
To accommodate a forecast 30% increase in passenger numbers and provide step-free access to the Jubilee and Central Lines, Bond Street is getting a new satellite ticket hall, lift shafts and escalators.
Furthermore, about 300m of new pedestrian tunnels will connect the Underground station with the Bond Street Crossrail station. Provisions for LU’s Bond Street upgrade were included within the Crossrail Act that received Royal Assent in July 2008.
After tendering the project from 2009, LU awarded a design and build contract to Costain Laing O’Rourke (CoLOR) joint venture in August 2010. Since then, LU, CoLOR and its subcontractors have been engaged in a highly complex sequence of enabling works.
This was kicked off by LU’s compulsory purchase of 354-358 Oxford Street, a six storey building of shops and flats across Oxford Street from the existing entrance to Bond Street Tube station. LU needs the land so it can sink two 24m deep lift shafts and launch its tunnelling operations.
Adding to the complexity, the new Bond Street ticket hall is being built immediately below Number 2 Stratford Place, a Grade II listed building immediately behind 354-358 Oxford Street. The ticket hall is being excavated in the space previously occupied by the building’s basement and shallow foundations. The challenge for LU, the CoLOR team and the contractor’s designer, a Halcrow-Atkins joint venture, has been figuring out how to build the ticket hall and form a large hole in the ground alongside without damaging the listed building.
Demolition of 354-358 Oxford Street involved taking the building down one floor at a time.
As this work progressed, underpinning specialist Abbey Pynford built a 1.1m deep load transfer slab below the floorboards of Stratford Place and installed 60 mini piles, 300mm in diameter and 18m deep from within the basement corridors.
Detailed structural surveys and 3D finite element modelling of Stratford Place were then repeated, accounting for the installation of the temporary support to determine a safe sequence for demolishing the internal walls and floors of the basement. Following the basement demolition, excavation work continued downwards, with the building continuously monitored for movement. As the dig progressed and demolition continued next door, further temporary and permanent support was built.
Some mini piles were converted to columns, gradually forming the reinforced concrete box that will ultimately house the new Bond Street ticket hall.
The basement raft of the support structure is now complete, 5m below ground level. The contiguous concrete and steel piled sides of the 354-358 Oxford Street excavation are currently propped against massive temporary corbels fixed to the support structure. The whole lot sits on a secant piled wall, heavily reinforced to resist high end loads and bending moments.
Much has still to be done before CoLOR can begin sinking lift shafts in July. Compensation grouting has to be carried out from grout shafts at each end of Stratford Place to mitigate ground movement and a bank vault floor slab in the basement of 354-358 Oxford Street has to be broken out with specialist concrete breaking methods.
Work is underway to claw back delays resulting from a series of unforeseen demolition issues. At one point the work was running 16 weeks late, although this has now been reduced to six. If work gets back on schedule, a two-year tunnelling operation is due to start in the autumn, followed by 12 months of fit-out and three months of commissioning before the expanded station is ready in March 2017.