Expanding the existing hall while keeping the passengers moving is the biggest challenge in the Tube Ticket Hall. Margo Cole reports.
There is one factor that makes the Tube Ticket hall project very different: people. At present, 55,000 people pass through King's Cross tube station during the morning three hour peak, making it one of the busiest stations on the Underground network.
That figure is growing all the time, and is projected to reach 70,000 - plus another 12,000 a day once the CTRL opens.
Over the next three years, the existing Tube Ticket Hall must be extended significantly without causing danger or major disruption to those people.
Travellers through King's Cross station will be only too aware that there is a major congestion problem in the ticket hall. Passengers entering, leaving and interchanging between five different underground lines, two main line and one suburban line create near mayhem.
King's Cross also has the dubious honour of being home to the busiest staircase on the Underground system - linking the ticket hall to the Metropolitan and Circle lines , emerging into the pedestrian subway under the Euston Road dubbed the Khyber Pass.
Congestion there is symptomatic of the whole problem at King's Cross, where lines have been added piecemeal over the 140 year history of the station.
Lack of direct links between lines means passengers sometimes pass through ticket machines as they swap between fipaidfl and fiunpaidfl areas.
Understandably, this was of particular concern in the aftermath of the 1987 fire. It was picked up by the Fennell report into events surrounding the fire, with its recommendation: fiLondon Underground shall build a direct subway link between the deep tube lines and the Metropolitan and Circle Lines at King's Cross or provide alternative satisfactory means of relieving the serious congestion. fl Construction of the new Western and Northern Ticket Halls will eventually relieve some of the congestion, but the main solution is enlargement of the existing ticket hall and the building of a completely new pedestrian subway west of the Khyber Pass.
The original ticket hall will be extended by 50% by demolishing existing subsurface structures to the west, most of which date back to the early 1900s and are now converted to fiback of housefl functions. Demolition will also create space to improve facilities for London Underground's operating staff.
fiOne of the problems the Underground has to deal with is sometimes poor conditions for staff, many of whom are involved in front line duties with the public, fl explains Arup associate John Cripps. fiSo a lot of space in the Tube Ticket Hall is staff accommodation, such as welfare facilities, as well as control rooms and operating space. fl Cripps describes the Tube Ticket Hall as the fibrainfl of the station system. It already contains the Station Operation Room that must be kept operating throughout the works (see page XIV), and the station's life critical systems, including sprinkler control chambers and water supply. This will be expanded to house all the operating equipment and control rooms.
Key to the new plan is improving headroom in the ticket hall, creating more circulation space and providing logical, clear, uncongested routes between the different parts of the station.
Technically, the structure of the extended ticket hall is not complicated - steel columns and composite roof linking into the existing steel frame - but building it presents major challenges in maintaining a safe operating environment throughout the three years of construction.
Arup, with SSL and contractor Costain Taylor Woodrow (CTW), has developed a construction sequence that involves 10 phases - known as fiinterim stationsfl.
Because each phase may be in place for long periods of time - several months - each has to meet LUL's standards for an operational station.
fiFrom a constructor's point of view we want to take the biggest space we can in each stage, fl explains CTW's Tube Ticket Hall section manager Fred Garner, fibut operationally you want to lose as little space as possible. fl Setting up an interim station involves ensuring everything from fire equipment and emergency access to handrails and lighting complies with LUL's requirements. An Arup/SSL/ CTW taskforce manages this process, with subgroups handling the individual disciplines.
Station One - the first of the interim stations - is in place, allowing CTW to install a bored pile wall around the western edge of the site. This will support the adjacent Pancras Road during the Pancras sewer diversion and demolition of the old back of house areas. Just before Christmas the contractor erected the first of two tower cranes, sitting on four piles squeezed in between the Metropolitan and Circle and Thameslink tunnels.
The second will be in place this spring.
During the Station Two phase, with the entire fiback of housefl demolition complete, CTW will cast a large raft slab and erect the structural steelwork in this area. The station's ticket machines will be relocated on the concourse outside King's Cross main line station to enable the contractor to hoard off the area where they currently stand, provide temporary support for the existing structural steel frame and concrete roof slab, and demolish this section of the roof.
This work is essential for linking the new structure into the old and for creating more headroom in the ticket hall. It will also facilitate the widening of the firing passagefl pedestrian walkway around the west side of the ticket hall that takes passengers from the Metropolitan and Circle lines to the main line station.
Subsequent interim stations will see that layout change as the contractor installs new lifts for mobility impaired passengers, knocks down existing walls to widen the ring passage further, and installs a new roof.
By the time Station Five comes into operation, roof demolition will have started. fiSome areas will require partial or total roof removal, which could lead to puddles forming and slippery floors, fl says Cripps. fiThe contractor will have to provide weather proof tents of some sort - as much for safety as comfort. fl Garner says the weatherproofing will also affect construction methods. fiWe have to be able to work underneath, so we have to decide if we use small tents that can be moved, or erect large tents and work with mobile cranes underneath. fl Alongside the demolition and reconstruction, CTW also has to undertake a short section of shaft construction and tunnelling 11m below ground. The new tunnel will provide an emergency link between a lift, being installed to take mobility impaired passengers to the deep tube tunnels, and a new staircase for fire and rescue personnel inside an old vent in the back of house area. The link will join at midpoint of the lift shaft.
Also being built in parallel with the main station works is a new subway under Euston Road, squeezed between the road and the shallow Metropolitan and Circle line tunnel (see pVI). A phased form of top-down construction is being used .
All work on the Tube Ticket Hall is due for completion in April 2005, giving six months for testing and training before it is fully handed over in October.