Long-term monitoring of settlements below a transition zone in a railway structure

Settlement of ballast and subgrade is a common problem in ballasted railway tracks. In particular, in transition zones between two different track forms, a dip in longitudinal level often develops on the ballasted side. Without timely maintenance, this differential settlement may lead to damage to track components and deteriorated passenger comfort. To ensure the safety of railway operation and reduce maintenance costs, it is necessary to monitor the structural health of the transition zone in an economical manner and detect any operational changes at an early stage. In recent years with the advancement of fibre-optic technologies, fibre Bragg grating (FBG) sensors have been more commonly used in structural health monitoring (SHM) due to their outstanding advantages. The wide range of FBG sensors, such as for the measurement of strain, displacement, temperature, and acceleration, can be used to acquire data over large distances with high resolution and accuracy. In this study, an FBG-based long-term monitoring arrangement, with a high temporal resolution, is developed to verify and calibrate a model for prediction of settlement in a transition zone. The system is designed to measure an operational railway track in harsh conditions in the north of Sweden. Furthermore, the instrumentation comprises four clusters, each with an optical strain gauge array in the rail web, an accelerometer on the sleeper, and a displacement transducer. Two additional accelerometers are installed far from the transition zone to measure a reference state. Combined, the data should not only provide details on the long-term settlements, but also the change in dynamic response it underpins