King's Cross station redevelopment

A further reason for the construction project is to implement the recommendations from the Fennel Report following the King's Cross fire in 1987. Extensive development was required to improve the safety of the station in the event of a fire. The project commenced in 2001 and includes the construction of two new tickets halls, the Western Ticket Hall and the Northern Ticket Hall together with the expansion of the existing King's Cross underground station, Tube Ticket Hall.

Western Ticket Hall

This has been constructed in between the St Paneras station building and the heritage faade on Euston Road. The structure is a basic reinforced concrete and steel frame. There are three levels:

1. Gallery - at road level.

2. Concourse - the main public area, containing the ticket office and access through to the Tube Ticket Hall and deep underground lines (Piccadilly, Victoria and Northern).

3. Concourse for access to the subsurface Metropolitan and Circle Lines.

There were two main areas of construction, each with its own major issues. In the main construction area, the heritage faades had to be retained and protected during construction. The Metropolitan and Circle Line (MCL) access had to be constructed in a very limited area between Euston road and the MCL underground tunnel.

Metropolitan and Circle Line underground tunnel decapitation

A major section of civils work was the decapitation of the MCL underground tunnel. This involved demolishing the existing arched roof above the tube line and replacing it with a flat concrete roof at a lower level. This was necessary to achieve a greater distance between the top of the tunnel and the ground level, allowing the access box to be constructed in between. There would have been insufficient height if the tunnel roof had not been lowered.

The decapitation of the tunnel roof was undertaken over 13 weekend (52-hour) possessions. The initial design scheme was to pour the concrete roof in-situ. However, to minimise the construction time Costain Taylor Woodrow proposed the use of precast slabs. This was accepted as a preferred alternative and removed the need for a continuous closure of the tunnel. The basic methodology used was as follows:

* excavate down from ground level to expose the tunnel roof

* construct a crash deck system within the tunnel

* break down the brick arch to the correct level

* trim the sides of the exposed roof arch

* lift in the precast bridge deck units and pour concrete on top

* waterproof and make good

* remove crash deck system and carry out safety checks in time for reopening of the underground line.

Each possession was managed successfully. The construction of the access box then followed directly above the underground line.

Tube Ticket Hall

The Tube Ticket Hall construction involves a complex extension to the existing Ticket Hall while keeping passengers flowing through at all times. The approach was to construct the base and roof slabs in a sequence of interim station layouts that allowed the passenger flows to be directed between worksites separated by fire-rated and crowd-loaded hoardings. The construction, similar to the Western Ticket Hall, is a steel frame with 900mm reinforced raft slab below and 500mm-thick roof. The roof construction was made simpler by the use of Omnia precast planks to form permanent formwork for the slab, topped off with C40 concrete with Lytag aggregate to reduce slab weights once the reinforcement had been fixed.

Construction also required a major, Christmas 2002, decapitation of the Network Rail Thameslink tunnel which passes directly below the new main entrance, and Bazalgette Fleet sewer which required a flat roof to support the link passage to the Northern Ticket Hall passing through its previous roof line.

Eastern pedestrian subway

To comply with the recommendations of the Fennell Report a new subway beneath Euston Road was proposed. Sandwiched between the four-lane carriageway above and the Metropolitan and Circle underground lines, running tunnels and platforms below the subway had to be carefully constructed in four sections, each as a top down island site in the middle of the road. This led to logistical challenges in terms of manoeuvring concrete wagons but was completed successfully.

Constraints during construction

The station had to remain operational throughout the project, with the only closures being in official station possessions, arranged several months in advance. Over 78,000 people pass through the station at peak time so public safety and limiting any disruption to the travelling public were major factors when planning the works. Other considerations were:

* maintaining access for the public around the station

* conforming to pedestrian flow routes and fire safety measures

* limiting inconvenience for the main line rail passengers using King's Cross, Thameslink and St Paneras stations

* maintaining traffic flow along Euston Road, directly south of the site, with at least two lanes flowing east and west.

Protecting third party assets was also a major consideration during the construction process and among these were:

* the Thameslink rail tunnel running directly under the Tube Ticket Hall site

* the St Paneras faade and station building along Euston road which is a Grade 1 listed building of historical interest

* two live sewers running under the site

* various utilities, including gas, electric and water mains

* two disused tunnels.

Quality control

Strict quality control for the concrete pours was implemented during construction. Each element of work had a detailed Inspection and Test Plan that included quality checks on materials and workmanship. At each stage of the works a check sheet was filled out and agreed with the client before commencement of the next phase. This resulted in high-quality concrete pours and limited remedial works.

Concluding remarks

Scanmoor was the main subcontractor for the Western Ticket Hall and Gleeson was the main subcontractor for the Tube Ticket Hall. Concrete was supplied from Hanson's batching plant at nearby King's Cross Lands.