Space: a subterranean transformation

January 5th 2010

Work has been going on at King's Cross St Pancras ever since the tragic fire that devastated the tube station in 1987. The inquiry recommended, amongst other things, that the capacity of the station's facilities needed to be increased radically from that built by our Victorian and Edwardian forebears.

Since '87, there have been developments that would have seemed incredible only 20 years ago. The Channel Tunnel has been built with a new terminal at St Pancras International, just up the road. Passenger numbers have soared on both the main line network and Underground system. A new Javelin high speed service has started from the south-east - again landing at St Pancras - and, just to add the icing to the cake, King's Cross St Pancras will take a fair chunk of the traffic generated by the 2012 Olympics and, thereafter, to the regenerated East End. Indeed, a quantum leap in capacity has been needed.

The 29th November saw the public opening of phase II of the station's redevelopment, two days after Boris Johnson and Tessa Jowell cut the official ribbon. The work, for client London Underground, was split into two stages, both of which were project managed by the joint venture of Atkins and Balfour Beatty Management (ABB). Phase I of the capacity upgrade was completed in 2006 with the addition of a new ticket hall to the west of the original Underground station, with access to the Metropolitan and Circle Lines. Phase II saw the addition of a new Northern Ticket Hall offering direct access to the Piccadilly, Victoria and Northern lines via access tunnels and escalators. The redesign also offers full step-free access with the construction of five new lifts. The final lift, for the Northern Line, will be commissioned as planned in 2010.

Construction timetable

The enabling works for Phase II started on site in 2002 with piling, but a reassessment of government funding caused them to be suspended in 2004. Over 500 contiguous bored piles were installed before 2006 to form the perimeter of the ticket hall. The JV managed the design procured from Arup. Elements of the detailed design were given to the contractor - Arup was assisted by the architectural practice Allies and Morrison.

Tunnelling got underway in February 2006. Work on the ticket hall box started three months later. Chris Pateman, ABB's Project Delivery Manager, arrived in August 2005 just before the completion of Phase I. "We had to make sure that both the Northern Ticket Hall box construction (carried out by Balfour Beatty Civil Engineering) and tunnels construction (undertaken by Morgan Bemo JV) could progress at the same time without conflicts of access. This was challenging as we shared just one entrance to site. That entrance was immediately adjacent to Pancras Road which runs between King's Cross Station and St Pancras. At the time, the work to St Pancras was nearing completion. We had to maintain deliveries to St Pancras and make sure that buses and taxis could keep running, as well as maintaining the flow of pedestrians. The logistics of managing materials delivery to site and excavated materials from the tunnels and the main box structure away from site was a significant challenge!"

"Camden council were very good in terms of giving us Section 61 consent. We had approvals to excavate the main box structure through to 8pm with permission to load muck vehicles before 8am. Tunnelling work was allowed to go ahead 24 hours a day. The site area is bounded by two main line stations and commercial properties rather than residential development. The main concern was excessive movements on routes away from the site through residential areas."

Tunnelling challenges

There were of course interface issues. New adit openings had to be formed in the existing station tunnels. Management of these had to be carefully thought through and works agreed with the LU asset and operation teams. The station manager teams were very proactive and helpful. "The assurance processes were very strict and rightly so, but we had very good communication between the construction team's designer and the LU assurance teams. It was a key element of the work to ensure that the stations continued to operate safely and without interruption."

The escalator barrels are 8.25-8.65m internal diameter. The passenger tunnels are ~5m internal diameter, opening out slightly in key interchange areas. They are noticeably very spacious. In total, about 300m of new tunnels have been bored predominantly in London Clay. One significant feature was the construction of a temporary tunnel in order to construct the main Piccadilly line tunnel. This runs from the bottom of the new ticket hall escalators and the top of the new Northern Line escalators and continues for around 100m towards the existing Thameslink tunnel. "Because we were connecting into an existing tunnel, we had to divert passengers to a temporary tunnel. This gives an idea of the scale of the project - we had to construct a temp tunnel of longer than 50m before we could start the main permanent construction."

To add to the complications, there were interfaces with Network Rail as the team tunnelled beneath its stations. Approvals had to be obtained. These were for work mainly below but also adjacent to the western side of the Grade I listed King's Cross Station. The box structure for the Northern Ticket Hall was also very close. Oversail licences from Network Rail for the erection and use of tower cranes were needed.

Integrating systems

Under the PFI contract, EDF Power Link brought in 11kV cables to their transformers which had to be installed in April 2008. During this process, just 10m away, the excavation was going on for the escalators yet to be powered by the switchgear. This was typical of the programme in that they had to look far enough ahead so that the building services works would be ready in time for when the main structural works were completed.

The Electronic Service Update Boards were all new but one of the key points occurred in the summer at the height of the commissioning period. "We had to integrate the new communication and fire systems with the existing ones. That was one of the key risks because, clearly, once we'd completed the integration it was live in every respect and had to be maintained as a real-time system. When we connected the new to the existing there was obviously a risk of a potential period of instability but, with sound planning and control, all went well and the public did not notice the changeover."

So, the project is in its final stage of close down. The ABB joint venture is putting in a final lift shaft to give access to the Northern Line, providing step-free access to all the deep level tubes. This final shaft will be completed by mid-2010. The remaining construction complex is to be handed over to Network Rail for its ongoing works to redevelop King's Cross main line station; ABB will eventually demobilise and move out.

Vital information flows

Chris looks forward to life after King's Cross St Pancras but reflects that "It has been a fantastic project to work on with a constantly changing environment - the different phases, the tunnelling, the heavy excavation, temporary works, heavy concreting, the fit-out works and E&M installation. In the final stages, this became ever more complex. For example, the escalator installation had to be carefully coordinated with the permanent lighting and CCTV installation directly above."

"The project management and contracting organisations have integrated exceptionally well. There was an excellent flow of information from the site back to the design teams. Final snagging items have to be completed by 31st March but we're aiming to be out before then!"

So this was the story behind the impressive bright new infrastructure. The commuter may have no notion of what went on behind the hoardings but that is a true mark of success.

Fulfilling the last recommendation of the inquiry into the 1987 fire, the redevelopment of King's Cross St Pancras tube station was included in the Channel Tunnel Rail Link Act (1996), with the objective of improving safety and relieving passenger congestion. A fire-fighting lift was incorporated into an existing ventilation shaft and the station now provides step-free access to all platforms from street level.

The project was designed by an Arup-led team in collaboration with architects Allies and Morrison, working with the contractor, a Costain Taylor Woodrow joint venture.

Material selection

Phase I, opened in May 2006, was an enlarged and refurbished Tube Ticket Hall beneath the forecourt of King's Cross mainline station and a new Western Ticket Hall, located under St Pancras' forecourt. The works for the Tube Ticket Hall were in an area of major utilities and, rather than implement piecemeal diversions, a disused Victorian tunnel - the Hotel Curve - was fitted out to divert 900mm gas mains and 600mm water mains clear of the excavations. Also in Phase I, to minimise disruption and keep four lanes of traffic in Euston Road, a temporary bridge was constructed over the works.

Phase II, the Northern Ticket Hall, provides direct connections to the deep tubes, with a subway under Pancras Road. Future connections will be made into the new King's Cross station concourse. In a major interchange, wayfinding is important and 3D visualisation was used to test routes and confirm signage. The design also maximises the use of natural light. Human factors were employed to design the ticket offices and control rooms from first principles, setting standards for use elsewhere.

LUL required a visionary design that would look as good in 50 years as at the time of opening, complementing the surrounding Grade I and II listed buildings. Those aspirations were achieved by the careful selection of materials - they had to be robust for heavy usage, easy to clean and maintain, and have suitable fire-resistant properties. Public transport meets general sustainability objectives and this was augmented by the longevity of materials and 120 year design life of the structures.

Project collaboration

Key considerations in the design and construction were to avoid damage to the listed buildings, adjacent utilities and to keep the railway operational. Construction techniques set out to minimise ground movements and, for the Northern Ticket Hall, this was achieved using top-down methods developed with the contractor, Balfour Beatty Civil Engineering Ltd. The tunnelling works were not only under the listed buildings but there were large diameter escalator shafts directly beneath the overground lines in King's Cross Station.

Working with Morgan Bemo, compensation grouting was employed to offset predicted settlements, with incremental tunnelling in sprayed concrete, controlled by a comprehensive monitoring regime. Interventions into operational tunnels were limited to planned possessions, all handed back on time with no delays to services.

The project was a collaboration of all parties and the designers worked closely with London Underground, its project managers, Atkins and Balfour Beatty Management, and the contractors to meet the opening dates and budget.

Article courtesy of the rail engineer magazine. Photo courtesy of Thomas Graham/ARUP.

---

Subscribe or spread the word: