Taking the strain, coping with pressure or shooting targets? An objective comparison between monitoring solutions within sprayed concrete lined tunnels at Crossrail Farringdon Station, London
Summary
Methods of monitoring the structural behaviour of tunnels during construction traditionally rely on in-tunnel displacement monitoring, with designers occasionally specifying the use of pressure cells in critical areas. The advent of fibre optic strain technology (using Fibre Bragg Gratings) offers the tunnelling industry an additional method of monitoring tunnel behaviour. Experiences at Crossrail Farringdon Station have offered a unique insight into the comparative benefits between conventional in-tunnel absolute displacement monitoring, pressure cells and fibre optic monitoring methods, where all three systems were deployed within the same area.
This study continues the pioneering work by BFK JV and Monitor Optics Systems at Bond Street Station tunnel CH3 where the viability of installing fibre optic cables within sprayed concrete lined (SCL) tunnels was demonstrated. The parallel arrays of all three monitoring methods at Farringdon Station were installed in SCL tunnel RTE2, in an area immediately adjacent to the buried TBM, the Eastern Ticket Hall (ETH) and the CH2 enlargement and escalator incline (ES2) within challenging geotechnical conditions.
The paper discusses the relative performance of each system in relation to ease, safety and speed of installation, robustness of the system, user-friendly outputs and accuracy of the data.
This wealth of data has provided the project team with additional real-time monitoring of SCL lining performance, offering an additional level of assurance that the tunnels were behaving in line with predictions in this critical area. This has also enabled the designers to validate their complex 3D finite element model, and provided a better understanding of soil-structure interaction.
This study offers the industry with a unique and objective benchmark for the benefit of future projects in relation to monitoring and validating the behaviour of complex tunnel configurations.