Monitoring of Mining Induced Movements Between Bridge Components with Fibre Bragg Grating Sensors

This paper outlines a novel use of Monitor Optics Systems’ (MOS) Fibre Bragg Grating (FBG) sensor cables to monitor relative movements between bridge components. Twin bridges that form part of the M1 Motorway in Helensburgh were to be subjected to ground movements caused by subsidence from planned underground Longwall coal mining parallel to the bridges. An engineering assessment was carried out to determine the risk of movements of bridge pier and abutment footings relative to the bridges’ headstocks, abutments and deck. It showed that relative movements of 5mm or more between headstocks and abutments could cause cracking in areas that cannot be visually inspected and that are difficult to repair, and relative movements of more than 16mm could lead to structural failure. A technical committee formed to manage subsidence effects on RMS infrastructure agreed that a surveying accuracy of 2mm was the minimum requirement to determine trends in movements between bridge components. As conventional relative 3D survey has a tolerance of +/- 2.5mm, it was agreed that a more accurate monitoring solution was required.

MOS initially proposed using their FBG sensor cables between bridge footings to measure relative movements, but it was deemed impractical and expensive due to the difficulties associated with installing sensor cables underground. MOS then proposed using their sensor cables held in tension between headstocks and abutments to measure relative horizontal movements. The solution would provide measurements of relative horizontal orthogonal shifts at the corner of each abutment and headstock at an accuracy of approximately 0.02mm. Sensor cables could not be installed between headstocks and the bottom of piers due to the risk of vandalism, so tilt meters were proposed to measure in-plane transverse tilts of the outer piers instead, and traditional 3D surveying for all other measurements. The technical committee concluded that the proposed solution would be adequate for their requirements.

A clamping system was developed for the sensor cables and the monitoring system was installed in 2016. The sensors are monitored at pre-determined times with demodulation equipment that is battery powered. Data is automatically sent to key personnel which is post processed by engineers to determine stresses in key bridge components.