An observational approach to railway tunnel voids grouting

Water and fine sand ingressed into a section of running tunnels on the Jubilee line of the UK's London Underground railway in 2010. Concern arose that voids around the tunnel would lead to tunnel instability due to a reduction of lateral confining pressure from the soil. Ground investigation works identified that the running tunnel passed through a very large buried sand channel in the Lambeth Group. Initial attempts to arrest the ingress using an expanding grout were unsuccessful, and manual monitoring showed a slow but continuing deformation of the tunnel. A detailed tunnel investigation revealed that intermittent voiding was present over a wide area. A more comprehensive tunnel investigation and grouting solution was procured and carried out successfully by a group of specialists working together with London Underground. An important aspect of the design was the use of an observational approach, a key component of which was an innovative wireless monitoring system with trigger levels linked to the grouting injection system.