Mechanically coupled distributed dynamic strain measurement for track systems and earthworks

Distributed optical fibre sensors are one of the few sensing technologies that could be embedded into our infrastructure to provide quantitative and mechanistic measurements of the condition of the infrastructure at network scales. This study considers the analysis of a sensing fibre deployed along 50 m of rail with a 7.5 m section buried beneath the line of the rail in the trackbed. The fibre was interrogated using a Distributed Acoustic Sensing (DAS) system based on Phase Optical Time Domain Reflectometry (ϕ-OTDR). This technique offers a short gauge length and high sample rate suited for measurement of moving loads. Data were captured as the site was trafficked by passing trains. Complementary simulation and measurements (using accelerometers and pressure sensors) were used to aid and improve the interpretation of the signal from the sensing fibre. In general, the measured signal agreed with the simulation indicating that the buried fibre provides a means of measuring horizontal strain in the ground under moving loads. Results from the different measurement systems led to a consistent interpretation of site behaviour, particularly where local variation of stiffness and load distribution were detectable at the same locations along the track. These analyses support the idea that DOFS have the potential for providing quantitative and high-resolution, and hence, high-utility sensing by interrogating an optical fibre embedded in infrastructure.