Lessons learnt from installation of field instrumentation

Imperial College in London, UK, as part of an Engineering and Physical Sciences Research Council (EPSRC) funded research project and in collaboration with Crossrail urban railway project, is performing field monitoring research to investigate how tunnelling affects existing tunnels. Comprehensive instrumentation was installed in Hyde Park and bordering Bayswater Road, beneath which the new Crossrail tunnels were constructed in London Clay below the existing London Underground Central Line tunnels. Surface and subsurface instruments were installed around the Crossrail tunnel alignments to monitor the ground response to the tunnel construction. Monitoring systems of sufficient resolution and accuracy were adopted to achieve high-quality data for assessing the tunnelling-induced ground response and mechanisms of movement from earth-pressure-balance machine tunnelling. The installation of surface and subsurface instrumentation took place in the summer of 2011. This paper describes and discusses the installation of rod extensometers, in-place inclinometers and multi-level vibrating-wire borehole piezometers. Selection of the appropriate cement-bentonite grout mixes for backfilling these borehole instruments is discussed, as this is critical for representative measurements of ground response. Some practical challenges arising during the installation process and how they were overcome are also described. Confidence in the instrument performance is demonstrated using example monitoring results from the piezometer and extensometer installations.