A two-and-half-year project for London Underground came to its conclusion in the spring after a comprehensive compensation grouting system completed directly underneath King's Cross Station. Contractor Bachy Soletanche designed and delivered the project to limit the effects of tunnel-induced ground displacements below both the station and the Great Northern Hotel.
Monitoring showed that final residual settlements have been restricted to a maximum of 5mm for the hotel and 8mm for the train shed, compared with predicted maximum settlements of 70mm and 95mm respectively if compensation grouting had not been implemented.
The grouting was required because of the construction of a new underground passenger walkway, stairs, lifts and escalators to provide a direct link from King's Cross and St Pancras stations to the Northern, Piccadilly, and Victoria underground lines' platforms.
The walkway tunnel is an essential component of the multimillion pound King's Cross St Pancras Underground station development programme. This is supporting the Channel Tunnel Rail Link extension to St Pancras and will assist the expected influx of visitors to the 2012 London Olympics.
Bachy's about £6M work pre-conditioning and stabilising the ground underneath King's Cross Station was for the Piccadilly Line Access (PLA) works and below the Great Northern Hotel for the Northern Line Access (NLA) works. Installation of a complex network of horizontal reinforced grout injection pipes were completed for both structures. These have provided the means for both passive support and for active compensation grouting throughout the tunnelling works.
Bachy Soletanche project manager Clif Kettle says: "A complex of walkway tunnels have been bored between the two mainline stations. Generally as tunnelling progresses, a small percentage of ground is always lost – face loss – which if not replaced would result in surface settlement and may cause distress to existing structures.
"With foundations of the two listed buildings of King's Cross Station and the Great Northern Hotel directly above the tunnel alignment, we were engaged to ensure settlements were restricted so the effect on the surface structures was minimised."
Site workers bored 126 holes, which took up to 10 hours each, to accommodate a specially designed heavy duty injection pipe system. This then introduced grout in a controlled manner as the tunnelling works took place.
For the PLA works the design provided a mesh of pipes, typically 1.5m apart, in three parallel arrays on different levels. Each borehole overlaps the two adjacent arrays at several locations maximising interaction between them.
The pipes are equipped with injection ports at 0.75m centres that allowed precise placement of the grout across the areas of maximum settlement. Kettle says: "Drilling the holes, installing the pipes and managing the compensation grouting over the two-and-half-year period has been an extremely delicate and challenging task, but overall the works have progressed as planned.
"Drilling accuracy was the most vital consideration – if any one borehole had deviated unacceptably from its designed alignment it could have been obstructed or deflected by adjacent boreholes."
Although the same basic drilling process was used for both the PLA and NLA projects, the pipes were bored from very different areas. For the NLA works, boring took place from the Northern Ticket Hall hub shaft, and from the PLA project, holes were bored from an existing, disused railway tunnel (called Maiden Lane tunnel) in close proximity to King's Cross Station.
"The PLA works, which controlled the settlement of King's Cross Station, proved to be the most challenging of the two," says Kettle. "We were fortunate to have the facility of the old tunnel, as it gave us more freedom to distribute the boreholes for maximum effect, facilitated installation and operation of our equipment, and meant that the installation works were isolated both from Network Rail assets and the main tunnelling works.
"However, we had to bore 150mm diameter boreholes up to 60m horizontally with a high degree of accuracy and consistency to create the desired mesh beneath the targeted platform area. This is exceptional for compensation grouting at this depth, as all the boreholes had to remain parallel to avoid collision, and the upper array is just a short depth from the track bed foundations."
For the PLA, the tube à manchette grouting pipes are 114mm diameter. Each section is a steel pipe, 1.5m in length, with injection sleeves.
Kettle says: "When stage one was complete and all the hard labour of manchette pipe installation had finished, we moved on to the second more technical stage.
"This saw our engineers working with the monitoring and tunnelling contractors to co-ordinate the response to the onset of ground relaxation as tunnelling works advanced. This involved the use of some of our most advanced injection software, design processes and equipment throughout the second phase of the project."
The project's completion has maintained the two Grade-I and -II heritage listed buildings well within the specified settlement limits, while at the same time allowing the daily operations of trains and travelling passengers to continue without disruption from the construction works being undertaken.