Research on Signal Characteristics of Urban Geological Defects Detected by Soil Stress Waves Excited by Subway

Urban road safety is severely threatened by the presence and development of subsurface voids, posing a significant challenge. This study proposes a new approach to tackle the challenge of detecting hidden hollows in urban areas. The approach utilizes geophysical investigations using soil stress waves excited by subway tunnel vibrations to examine void defects in the soil layers above. A shaker excited concrete plates to generate the necessary stress waves required for detecting vacancy defects. The results showed that the amplitude of the stress wave signals in both the time and frequency domain significantly attenuated after passing through void defects. There were significant differences in wave impedance at the boundaries of these voids. Wave field propagation contour maps, obtained using laser Doppler vibrometers (LDVs), visually demonstrated the propagation of soil stress waves at the boundaries of void defects. Reflected waves exhibit increased strength with larger void dimensions, while diffracted waves at the soil’s end weaken. The energy distribution contour maps in the time domain provided an intuitive indication of the presence, horizontal projection position, and size of the void defects. Sensitivity coefficients ( \(\omega\) $\omega$ ), statistical feature indicators such as root-mean-square deviation coefficients ( \(RMSD\) $\mathit{RMSD}$ ), and wavelet packet energy ratio deviation coefficients ( \(E_{RVD}\) $E_{\mathit{RVD}}$ ) were established to quantitatively characterize the size of void defects from different perspectives. The preliminary findings of this study have verified the feasibility and scientific validity of utilizing soil stress waves generated by subway vibrations for the detection of void defects.