Vibration Propagation Analysis of Road Pavement Using Thin Layer Method for Fiber Optic Distributed Acoustic Sensing

Abstract

Distributed Fiber Optic Sensing (DFOS) has gained widespread utilization as a tool for Structural Health Monitoring (SHM) in recent years. This technology enables the measurement of strain and temperature distributions along a fiber. Among the various DFOS measurement technology, this study focuses on the use of a Distributed Acoustic Sensing (DAS) system, which primarily measures longitudinal strain caused by vibrations. The utilization of DAS for vibration propagation analysis has not been widely practiced. Thus, to confirm the authenticity of the recorded values, we constructed a mockup consisting of multiple layers including a surface layer, base layer, upper layer roadbed, lower layer roadbed, and subgrade, with DFOS installed at multiple locations in each layer. We are trying to improve our modeling by comparing the analytical road model to the DAS measurement results. When pavement is subjected to vibration, caused by either people or vehicles passing by, it can be expected that the amplitude of the vibration will decrease as the depth increases. However, when we measured the vibrations using DAS, we observed that the amplitude of the surface layer and the subgrade was larger than that of the other layers. Therefore, we applied the thin layer method, which is widely used in the field of geotechnical engineering to investigate the factors that led to the increase in amplitude of the subgrade. In the thin layer method, the ground is considered a homogeneous continuum in the horizontal direction, while the depth direction is analyzed discretely in the plane of layer partition. This method has a smaller computational load than the finite element method. We performed sensitivity analysis by using shear wave velocity, Poisson’s ratio, and density as input parameters. The sensitivity analysis revealed that changes in shear wave velocity strongly contributed to the amplitude. To validate the analytical values obtained from the thin layer method, we conducted the Falling Weight Deflectometer (FWD) test, which provided us with the actual physical properties of the road pavement, including the deflection of the surface layer under 5t excitation forces. We reflected these results into the thin layer method’s parameters and compared the analytical values with the DAS measurement results of the exciter test. Through these procedures, we confirmed the suitability of the thin layer method as an analysis method for DAS.