Breakthrough! Further progress at Victoria underground station

Monday 14 September 2015 was a significant day in the London Underground project to upgrade Victoria station. Many readers will appreciate just how overcrowded this Underground station has become and will greatly welcome an improvement in the space available to handle the numbers using it each day. These already exceed the daily headcount at Heathrow Airport, and the growth in usage is expected to continue.

The big event mentioned earlier was the breakthrough in the tunnel between the new North ticket hall and the expanded South one. This brought to an end a key part of the complex three-year project at Victoria.

London Underground has been planning the upgrade here since 2009 and, as regular readers will know from previous articles in this magazine, work on site began back in 2012 with jet grouting by Keller UK to stabilise the sands and gravels before tunnelling and excavation could begin.

Underground bridge

The work of this project has been carried out largely behind the scenes, with the station remaining fully open with very few exceptions. This has been particularly challenging, given that some tunnelling works have been undertaken less than 30cm away from the operational Victoria line platforms.

When the Taylor Woodrow/BAM Nuttall joint venture was appointed to carry out this project, London Underground admits that there were something like 70 unanswered questions about how the construction could be achieved. The project teams and their advisors have triumphed in coming up with innovative and creative solutions to each and every one.

One of the few occasions on which part of the station did have to be closed was last Christmas, when preparations for the link tunnel crossing beneath the District and Circle lines were commenced. To allow for this crossing, it was necessary to construct a bridge to carry the tracks over the new tunnel.

One of the unanswered questions was how to achieve this with minimal disruption to passengers. The solution, which involved a sophisticated temporary works scheme, was devised by the project team working with consultant Tony Gee & Partners.

The complex process involved several temporary headings, driven beneath the existing railway tunnel. One set was perpendicular to the line, with headings below the intended support bearings for the new bridge deck. These were used to allow the construction of foundations for the bridge support system, and then to permit columns to be installed on these so as to provide the support to the bridge deck.

The new bridge deck was constructed top down during the closure at Christmas, when the track was removed from inside the running tunnel and the required area was excavated down to the design soffit level for the new bridge deck.

The deck was constructed in reinforced concrete in a normal manner, with one exception. This was the inclusion of heavy transverse steel tubes in the deck slab, which required that the slab depth be rather greater than normal to accommodate them.

Side beams and needles

The reason for the tubes was that they contained tubular steel ‘needles’ within them which came into play later in the works. Each tube contained two needles, one at each end of the tube, the needles being a snug fit in their tubes.

Once the new deck was complete, supported by the grouted (permeation grout) ground, the track was reinstated and the railway could be reopened.

A second set of headings ran parallel to the tunnel, one either side of the intended bridge deck. In each of these a side beam was constructed, spanning the intended new tunnel to the side of and below the side walls of the existing running tunnel. These two side beams were therefore parallel to the sides of the new bridge deck, one on each side and some distance from it.

This is where the needles came into play. In order to support the side walls of the running tunnel, working from the side beam headings, the needles were drawn out of the tubes within the new bridge deck and connected into the new side beams. The needles were thus each supported at one end by being socketed into the tube they were originally installed in, and at the other by a side beam.

This arrangement permitted the running tunnel side walls to be supported by the needles, to stop them being undermined by the excavation of the new tunnel below. Thus the existing railway and its surrounding tunnel were fully supported, permitting the excavation of the new tunnel down below to proceed, with normal rail traffic carrying on overhead.

Props to base level were then constructed within timber headings before completion of the remaining excavation to base level within walls temporarily supported by shotcrete before the tunnel is lined and fitted out for passenger usage.

This crossing under the operating railway has allowed the tunnel to be driven to meet up with a drive from the opposite end, so as to link the two ticket halls below ground.

Propless solution

A second example of innovation has been the development of a propless tunnelling junction system. There have been a number of challenges in constructing the tunnels, caused by junctions, changes in tunnel section and changes in inclination of the tunnels (for escalator tunnels and so on).

Mott McDonald worked with the project to develop a method of constructing tunnel junctions without complex propping within the operational side of the existing tunnel linings to the excavation and formwork, and this has proven to be safe and economical in use.

The entirely-new North Ticket Hall is now fully excavated, and work is progressing with the permanent concrete linings, stairways and so on, prior to the final fitting out works. In particular, it is worth noting the new access stairs and entranceway to this ticket hall. These will give users a totally new way in and out of the station, directly to the northern end of the platforms. Modelling shows that this entrance should save many customers some five minutes by giving them more direct access and taking them under roads that they currently have to cross at street level.

The South Ticket Hall extension is not quite so far advanced, with some more excavation to complete.

Celebrations

The event that led to this article was the staging of the final breakthrough between the drives that lined the booking halls.

A formal breakthrough ceremony took place, introduced by Nick Brown, managing director of London Underground, and Craig Prangley, project director at Taylor Woodrow/ BAM Nuttall JV. After the tunnellers joined the crowd through the opening they cut through from the other heading, they were greeted warmly by their colleagues and the other spectators before David Gauke MP, Financial Secretary to the Treasury, spoke in praise of their achievements.

All the speakers emphasised the importance of the upgrading of both the Underground network in general and Victoria station in particular. They thanked the funding bodies, in particular in this instance the European Investment Bank, whose representatives were present.

The EIB has loaned £1 billion to London to fund a range of transport infrastructure work, including the Victoria station upgrade, the upgrading of Bank station which is to commence soon, and the London Cycle Superhighway Network.

Thanks were also offered to the local community, which has borne with the project throughout and with which the project team has worked very closely to minimise inconvenience and disruption, for example by enforcing quiet periods at times when the Apollo and Victoria Palace theatres have matinee performances.

Steve Lousley, London Underground project manager of the Victoria station upgrade, was delighted with the project’s progress and is looking forward to seeing the fully-completed works open on time in 2018.