Anyone who has visited Tottenham Court Road in the vicinity of Centre Point recently couldn’t fail to have noticed the huge construction site adjacent to the tower block. It is all related to a comprehensive upgrade of Tottenham Court Road underground station and preparations for Crossrail which will have an exchange station on the same site.
To find out what is going on, both behind the hoardings and underground, the rail engineer joined a recent visit by the Railway Civil Engineers’ Association.
The existing station is composed of two originally separate facilities, one serving the Northern line and the other the Central. These two were combined in 1908 to form one station allowing passengers to transfer between the two lines. This history has left a facility which is far from ideal for its users and its managers. Today, roughly 70 London Underground staff have to manage over 150,000 customers each day and the station often has to be closed due to overcrowding.
Forecasts of future usage predict more than 200,000 passengers/day once the new Crossrail route opens. In addition, the facilities available to users in the immediate surroundings of the station above ground are not adequate for the numbers of pedestrians coming and going to or through the station and surrounding streets. The road junction of Tottenham Court Road and Oxford Street is routinely one of the top two junctions in the capital for pedestrian accidents each year.
The advent of the new Crossrail route, traversing the site and needing an interlinking station facility, necessitated a total reappraisal. It was essential to ensure that the resultant overall station provision was appropriate for the needs of present and future users of the three lines and those of the external surroundings of the station.
The station upgrade includes a full refurbishment of the existing facilities, provision of new ticket halls and new and improved access arrangements for the existing London Underground lines. In particular, it was seen as essential to overcome the limitations of the current access routes to the Northern line platforms. These allow passengers predominantly to enter and leave only by one end of each platform, meaning that entering and departing passenger flows conflict. The new layout provides access to the other end of each platform, permitting the institution of a one way system and the ending of the conflict.
The Crossrail factor
In addition, the project is constructing the reinforced concrete shell or ‘box’ and escalator decline for the Eastern Ticket Hall of the new Crossrail station. This will be fitted out later by Crossrail themselves.
The influence of Crossrail is not restricted to the need to accommodate its station and ticket hall facilities within the site. The London Underground project design has also had to allow for the compensation grouting pressures involved in the Crossrail tunnelling work, and fit in with the over-site development works that are to be associated with or follow on from the Crossrail scheme. Finally, it has clearly been absolutely essential to ensure that the architecture and the mechanical and electrical installations of the two projects fit together.
Of course, the project also has to deal with all the ‘usual’ London issues such as sensitive existing assets (both London Underground’s and others), noise and vibration issues and so on. As a result, the project is constrained by things like settlement criteria and the complex 3-D geometry driven by surrounding structures both above and below ground. Extensive monitoring systems are in place and take effect in real time, 24/7, providing live data and automated alerts. These cover such things as noise and vibration or ground and structural movements. It is not uncommon for project managers to have to respond immediately to an alert from one of these systems in order to avoid a complaint or incident.
The site is on the boundary between the City of Westminster and London Borough of Camden, meaning there are two local authorities to work with. There are also a very large number of conservation areas in the surrounding area.
Although the project is being enacted under the umbrella of the Crossrail Act, which gives useful powers, it also entails many binding obligations too. A great deal of effort is going into the relationship with the local community, and there are regular meetings and the frequent use of newsletters and door-to-door campaigns.
Getting it all in and out
Logistics are a major concern because of the constrained space available on site and the busy surroundings, so the project has a full-time logistics manager. There is only one access into the site and deliveries have to be made on a just-in-time basis as there is nowhere on site to store anything.
Simon Buck, senior project manager for the Taylor Woodrow/BAM Nuttall Joint Venture that is carrying out the works, explained that the quantities of materials to be removed from site or brought into it are huge. At the time of the visit, Simon said that
210,000m3 of muck had been excavated and removed whilst some 42,000m3 of concrete had been delivered to site and placed. An off-site storage facility at Erith near Dartford is being used to assist in managing the logistics issue.
The project’s people are crucial to its success. Competence is key, and the project has become a mini skills academy in its own right as there are around eight apprentices on the team at any one time. The joint venture and London Underground teams are co-located and work together on the critical planning of the project. Collaboration within the joint venture is intense, and between the partners something like 90% of the current work is being delivered ‘in- house’.
Halcrow came up with the original design for the client, London Underground. The same team was then adopted by the JV to take the design forwards into and through the delivery phase. The Crossrail designers are different, however, with Atkins responsible for the relevant part of that project. This has necessitated further careful collaboration between the two teams, so far with great success.
Simon outlined some examples of the challenges that have already been overcome by the project. The first he mentioned was that of constructing two bridges over the Central line tunnels below ground for accesses to the new Central line routeways. The platform tunnels are twin bores, with cast iron (CI) segmental linings, and at the relevant location they lie below the midlevel sewer, a 1,950mm diameter brick sewer less than two metres above the crown of the overbridge. The sewer could not be moved and is obviously a sensitive structure which could not be disturbed at all without risking leakage or worse. So a solution was developed that required a series of 340mm diameter horizontal piles to be driven from the adjacent Royal Mail parcel tunnel to form a support raft beneath the sewer. The available room in the mail tunnel was limited, but proved just sufficient.
In the original design, the overbridge tunnel was to be driven using traditional timber handworks to support the ground around a substantial rectangular steel structure. However, the JV developed an alternative design from a value engineering scheme by OTB, their temporary works designer, with input from Halcrow and Dr Sauer, the SCL (Sprayed Concrete Lining) designer. This utilised a machine-excavated shotcreted cavern spanning the platform tunnel.
After excavation to expose the outer face of the CI tunnels, concrete saddles and abutments were constructed in readiness for installation of the bridge decks. All this was undertaken during traffic hours with London Underground operating a full service in the tunnels below, something of a first. Finally, during a weekend closure, the CI segments in the crown of the platform tunnels were removed at the two overbridge locations, concrete encased beams were installed, and cheek plates erected to seal off the platform areas at the tunnel crown.
Another example of tricky work that has been undertaken by the project is the provision for the new access passages and staircases that were required for the Northern line platforms, where the tunnels for these had to run between the existing Northern line platform tunnels. This work was carried out during a nine month platform closure in 2011. Once more, the space available between the north and southbound platform tunnel lining was really tight and this time it was necessary to remove part of the side of each existing tunnel in order to increase the space between them to allow for the new stairs. Existing tunnels are sectional CI structures, and so it was necessary to remove the existing CI segments and replace them with new, specially shaped steel units that left a vertical external wall on the side nearest the intended new tunnel. Finally, new sections of concrete platform had to be cast.
The construction of the ‘New Plaza Ticket Hall’ for London Underground and Crossrail, to the south of the existing ticket hall, has been a major undertaking in its own right.
The project has many other complexities. It was necessary, in order to unify the site, to divert a section of Charing Cross Road around the other side of Centre Point and close a section of another road entirely. The complex foundation works have included hundreds of secant and contiguous bored piles, seven of the former being two metres in diameter and 55 metres deep and coming as close as 1.1 metres from the Northern Line tunnels. 44 panels of diaphragm walling were put in, each panel requiring three reinforcement cages 18 metres long. 11 two metre diameter plunge columns have been inserted, each with a 700 x 700 steel column 42 metres long down its centre. Augur bored and CFA piling methods have both been employed on the job.
BIM has been essential on the project for avoiding clashes with underground structures such as services and tunnels, for identifying working constraints, and to assist with the logistics and sequencing of the works, as well as for the design benefits more conventionally expected.
The project has made use of Voltex waterproofing below ground, a first for a London Underground project. This material is a sheet material incorporating a bentonite blanket, and is easily fixed so as to ensure a watertight structure.
Looking to the future, in late 2013 there will be a key milestone for the project as it is due to hand over certain areas of the site to the Crossrail project, particularly the Goslett Yard box in which Crossrail will construct one of the two ticket halls for their new station. In time, other areas leased temporarily from neighbouring landowners will be handed back for redevelopment. The area around Centre Point is to be redeveloped into a large piazza that will incorporate, among other things, new high- capacity glazed station access structures 17 metres tall.
The station will be completely finished for the opening of Crossrail in 2018. But that may not be all. If Crossrail 2 (otherwise known as the Chelsea to Hackney line) is built, it will also have a station at Tottenham Court Road. However, the designers have already taken account of that…