Solution AECOM chose Senceive’s FlatMesh™ Nano triaxial tilt sensor nodes as the primary technology for monitoring the tunnel lining. This was largely due to the associated operational advantages, including their compact size, robust construction and better performance in the wet tunnel environment compared to conventional tilt sensors. Over 200 Nano tilt sensors were installed on the tunnel lining to provide a flexible and high-resolution monitoring system able to capture the limited movements anticipated. The main limitation was that the sensors would measure relative, rather than absolute movements. Whilst this was sufficient to identify and quantify the anticipated modes of movement, a decision was made to provide supplementary monitoring to verify that the system and structure were performing as expected. This used tilt nodes mounted on beams. Wireless tilt chains – Nano tilt sensors mounted onto aluminium beams - offered an alternative to Shape Accel Arrays (SAAs); however, a review of gauging clearances showed that it would not be possible to install them around the full tunnel profile due to localized pinch points. Nevertheless, shortened tilt chains were installed in high risk areas where they could be safely accommodated.
In addition to monitoring the tunnel lining, track monitoring was also conducted. This used a total of 834 Senceive High-G Triaxial Tilt Sensor Nodes at 3 metre intervals throughout the main works area and 30 m beyond the zone of influence. Relative and absolute twist and cant were monitored. The absolute cant was calculated using surveyed cant readings, which was then used as a constant offset to the incoming data from these tilt sensors. There was a further need to check the behavior of the central track drain culvert, but access considerations prevented mounting instruments directly on the buried structure. A decision was made to monitor the track above to identify any deformation that could indicate failure of the drain. This was achieved using High-G Triaxial Tilt sensor nodes installed at 6 m intervals outside the main works area. Power for the gateways was not available via the usual options of solar panels or mains supply. This was solved by using 12v 110Ah batteries connected to a 12vdc to 24vdc converter. With periodic charging of the batteries this provided a reliable means of continuously powering the system and relaying data onwards to the WebMonitor data viewing platform. The system was configured to send automated alerts warning of any movement that exceeded pre-selected threshold values.
To further enhance performance, a number of Optical Displacement Sensors (ODS) were installed as a secondary means of monitoring at selected locations. The ODS nodes provided a different type of measurement (based on laser distance measurement as opposed to MEMS sensor) and were therefore considered to be a useful tool for verifying overall tunnel deformations. The ODS nodes were positioned in three modes: on opposing sidewalls; on the haunches pointing at the sleepers, and at the crown. The ODS nodes directed at the track were susceptible to interference from construction activity. The monitoring team therefore selectively paused ODS alerts when construction activity that interfered with line of sight was taking place in their immediate proximity in order to prevent false alerts being triggered.
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