Observations of tunnelling effects on a large diameter jointed concrete pipe on Crossrail Contract C310 at North Woolwich, Newham, London.

Summary

This paper presents observations and monitoring results of ground movements arising from construction of the Crossrail Contract C310 tunnels and its subsequent impact on a large diameter concrete inlet.

The Store Road Inlet, a newly constructed 2.4m diameter concrete pipe inlet to a major pumping station in the London Borough of Newham sits less than 1 tunnel diameter above the Crossrail tunnels. Slurry shield tunnel boring machines were employed to construct the tunnels which typically allow greater control of tunnel excavation and ground movements. However, the prevailing ground conditions and low clearance to the tunnels meant the Inlet was at risk of being affected by tunnelling ground movements.

The Inlet was instrumented and monitored as part of the planned mitigation measures during construction of the Crossrail tunnels. A monitoring system comprising strain gauges, Shape Accel Array (SAA) and fibre optic sensors was installed to monitor and study the Inlet during tunnel construction.

While the usage of fibre optics in construction monitoring is not novel in approach, its adoption has been limited and historically focused on specialised structural monitoring. The deployment of fibre optics alongside more industry established vibrating wire strain gauges, proved useful as a benchmarking exercise, especially highlighting its advantage in providing semi-continuous strain measurements with length.

The behaviour of buried pipes subject to tunnelling ground movements is well theorised and presented in numerous publications. However, the number of case histories for large diameter jointed pipes is limited. This case study presents monitoring data during the construction of the Crossrail C310 tunnels between January and May 2014 and a comparison of theorised behaviour against observations.