## 5 Discussion  
The findings of this study are consistent with relevant research results both domestically and internationally, while also revealing some new phenomena. Consistent with the numerical simulation results of [PERSON] et al., this study also found that tunnel structure has a significant impact on vibration frequency and amplitude, particularly in the mid-to-high frequency range. This suggests that the influence of tunnel structure on vibration propagation under varying geological conditions is a widespread phenomenon. In contrast to the studies by [PERSON] et al. [25]. and [PERSON] et al. [30], this research discovered that the tunnel structure, at certain measurement points (such as Figure 5), leads to the reappearance and amplification of high-frequency vibration components, indicating that tunnel lining materials and structural design may produce different vibration propagation effects at different locations. The soil layers in the study area consist of artificial fill, clay, silt, silty clay, and highly weathered tuff. The elastic modulus, density, and damping ratio of different soil layers significantly affect the propagation speed and attenuation rate of vibration waves. Soil layers with high elastic modulus and density (such as highly weathered tuff) facilitate the rapid propagation of high-frequency vibration waves, while soil layers with low elastic modulus and density (such as silt) accelerate the attenuation of vibration energy. Soil layers with high damping ratios (such as clay (3)) effectively absorb vibration energy, reducing the propagation distance and intensity of the vibration waves. The coupling effect between the tunnel structure and the surrounding soil significantly influences the propagation path and attenuation characteristics of the vibration waves. For example, the high rigidity of tunnel lining materials (such as concrete and steel) enhances the transmission efficiency of vibration waves within the tunnel, while simultaneously altering the propagation path of the waves through reflection and refraction, resulting in amplification of the vibration waves in certain directions. In the X-direction, the high-frequency vibration components at measurement point P3 significantly increased, which is primarily attributed to the direct contact between the tunnel lining and the track slab. High-rigidity materials, such as concrete and steel, effectively transmit high-frequency vibration energy, causing the vibration waves to propagate more quickly in the horizontal direction with less attenuation. The reflection and refraction effects between the tunnels lead to the overlapping of vibration waves at P3, further amplifying the high-frequency components. This phenomenon suggests that the coupling effect between adjacent tunnels significantly influences vibration propagation, potentially causing vibration amplification in specific areas. This effect should be considered in the design and control measures. Conversely, in the Z-direction, the disappearance of high-frequency components at measurement point P5 is due to the reflection and refraction of the vibration waves caused by the tunnel structure, which reduces the propagation of high-frequency vibrations. This indicates that the tunnel lining has an absorbing and reflecting effect on vibration waves in the vertical direction, thereby reducing the vertical propagation of vibration energy. This study, through detailed vibration data analysis and statistical quantification, has validated the significant impact of adjacent bidirectional tunnel configurations on ground vibration propagation, particularly in the amplification and attenuation of mid-to-high-frequency vibration components. These findings provide a scientific basis for developing effective vibration control strategies, ensuring the structural safety of surrounding buildings and the quality of life for residents.