FDTD analysis of ballast fouling status using PFC with discrete random medium model

Abstract

Numerical simulation techniques for ground penetrating radar (GPR) railway ballast inspection offer significant advantages, including the avoidance of extensive field surveys and excavation work. This helps minimize construction challenges and costs while providing crucial technical support and insights for railway maintenance. Nevertheless, the intricate nature of ballast particles and bed structures, combined with the challenges in discerning their patterns, present formidable obstacles to achieving high-precision modeling. This paper employs the Particle Flow Code (PFC2D) to extract and project 2D natural ballast particles from laser scanning, generating a clean ballast physical model considering mechanical interactions. As fouling arises from fine particles smaller than 25 mm, the discrete random medium theory is applied to validate the heavy ballast fouling. This involves filling the voids in the clean ballast to simulate and analyze the electromagnetic properties of the ballast fouling. The generated ballast physical model is converted into HDF5 files and simulated using a 2.0 GHz Rayleigh wave excitation through the Finite Difference Time Domain (FDTD) method. Through S-transform and Hilbert energy results, it becomes feasible to accurately differentiate the ballast fouling. The study reveals that highly fouling ballast predominantly exhibits frequency energy concentrated within the 1.0–3.0 GHz range. As depth increases, the energy experiences faster attenuation, and the distribution of Hilbert energy becomes denser and stronger. Field tests conducted on a specific railway line in southern China validate the method's effectiveness, making it a valuable tool for guiding GPR-based ballast fouling detection projects and providing a scientific basis for railway infrastructure maintenance.