Assessment of railway earthworks using DAS based active MASW surveys

Clay filled railway embankments may be particularly vulnerable to seasonal moisture variations, which induce shrink-swell behaviour, reduce stiffness, and adversely affect serviceability. This study evaluates the potential of Distributed Acoustic Sensing (DAS) based multichannel analysis of surface waves (MASW) for both large-scale characterisation and monitoring of such infrastructure. An active DAS-based MASW survey was conducted along a 5.8 km section of railway in southern England using existing loose-tube telecommunication fibre housed in trackside concrete troughing. In addition, repeated active DAS-based MASW surveys were undertaken over a 13-month period at the Withy Beds embankment, a 350 m section within the same route, using a tight-buffered fibre buried along the mid-slope. Conventional geophone surveys were also used to validate the DAS derived measurements. Strong agreement is observed between DAS and geophone derived phase velocity responses and resulting shear-wave velocity (Vs) profiles, despite slightly higher noise levels in the DAS data. A comparison of different fibre-optic cable installations indicates that, although cables housed in trackside troughing exhibit reduced signal amplitudes due to poorer ground coupling, the fundamental-mode dispersion response remains clearly identifiable, enabling reliable Vs estimation using existing telecommunications infrastructure. The 5.8 km survey demonstrates the capability of DAS-based MASW to resolve spatial variations in near-surface Vs associated with earthwork type and embankment mechanical property variations. Time-lapse monitoring at the Withy Beds embankment reveals pronounced seasonal variations in near-surface Vs, with lower velocities during winter/spring and higher velocities during drier summer periods. These variations correlate with independent measurements of volumetric water content, matric suction, and soil moisture deficit, highlighting the sensitivity of Vs to hydro-mechanical processes in high-plasticity clay fills. The results demonstrate that DAS-based MASW provides a robust and scalable approach potentially informing targeted engineering intervention of railway earthworks using existing fibre-optic infrastructure.