Published20th January 2018
Knowledge TypeTechnical paper
Organisation, Crossrail Ltd
Tunnel deformations caused by compensation grouting at Crossrail Farringdon Station
A systematic compensation grouting scheme was successfully implemented to mitigate the surface settlements caused by the tunnelling works in Crossrail’s Farringdon Station. Concurrent grouting episodes were carried out simultaneously with the sequential excavation of the tunnels comprising injections both in front of and behind a predefined exclusion zone that moved as the tunnel face advanced. The front injections aimed to pre-heave the ground surface prior to the passage of the tunnel face, whereas the target of the rear injections was to reduce the settlements induced by the tunnel excavation to an acceptable level.
The focus of this paper is on the two, 300m long platform tunnels, 10.6m high by 11.4m wide, which were enlarged to platform size from the existing TBM running tunnels using sprayed concrete lining (SCL) methods. The front concurrent grouting injections had no visible effect on the tunnel lining. However, the rear injections were carried out typically 5m above the completed, fully strengthened shotcrete lining therefore inducing additional stresses. During the grouting episodes, in-tunnel monitoring was performed systematically to ensure that no excessive distortion of the shotcrete shell occurred.
After a presentation of the basic compensation grouting principles, this paper discusses the results of the in-tunnel monitoring analysis with regards to the grouting operations and the 2D and 3D finite element analyses which were performed in an attempt to back analyse the effects of the rear grouting injections on the SCL lining. The outcome was an assessment of the effective grouting pressure that yielded the additional observed lining displacements as well as of the pressure level that would result in overstressing.