## 2 Project Overview  
The Taizhou S1 urban rail line is the first urban rail transit line in Taizhou City, Zhejiang Province, China. The Taizhou Suburban Rail Line S1 spans 52.568 km, with 17.47 km of underground, 29.511 km of elevated, and 4.812 km of mountain tunnels. The line features fifteen stations, including seven underground and eight elevated stations. Initially, it will utilize four-carriage urban D-type trains with a maximum operating speed of 140 km/h. This study selects the underground section of the Taizhou S1 Line as the research subject, based on the following key factors: the underground section of the Taizhou S1 Line features a typical bidirectional tunnel configuration, which aligns with the development trend of suburban railways in China, making it highly representative. This configuration is also relatively common worldwide, which enhances the generalizability of the research findings. The trains on this line operate at relatively high speeds (up to 140 km/h), which generates significant environmental vibrations, making it an ideal case study for high-vibration intensity research. The vibration characteristics under high-speed operating conditions provide valuable insights for the design of vibration control measures. Taizhou is located in an area with relatively stable geological conditions. The soil structure is well-defined, facilitating long-term field observations and data collection, thereby ensuring the reliability and accuracy of the data. The geological profile consists of multilayered soils with varying elastic moduli, densities, and damping properties, all of which significantly influence the propagation speed and attenuation rate of vibration waves. The underground section of the Taizhou S1 Line has a tunnel depth of 23.03 m, an external diameter of 8500 mm, an internal diameter of 7700 mm, and a center-to-center distance of approximately 30 m between the two tunnels. The track slab length is 2940 mm, and the distance between the track surface and the tunnel inner diameter is 1450 mm. These design parameters have a significant impact on vibration propagation, particularly the distance between adjacent bidirectional tunnels and the interaction between tunnel structural materials and surrounding soils, which can markedly affect the reflection, refraction, and attenuation of vibration waves. The underground section of the Taizhou S1 Line utilizes high-strength concrete materials, which possess high elastic moduli and damping characteristics, contributing to reductions in vibration wave energy loss. Additionally, the tight bonding between the tunnel walls and surrounding soil enhances the transmission efficiency of vibration waves within the tunnel structure, minimizing the scattering of vibration energy. The interaction between the tunnel materials and the surrounding soil determines the propagation paths and attenuation characteristics of vibration waves within the tunnel and the surrounding soil, thus influencing the intensity and distribution of ground vibrations. The geological profile of the Taizhou S1 Line's underground section includes multiple layers of soil, each with distinct elastic moduli, densities, and damping properties. These geological characteristics significantly affect the propagation speed and attenuation rate of vibration waves. Stiffer soil layers, such as sandy soils, accelerate the propagation speed of low-frequency vibration waves, while highly damped soil layers, such as clay, promote the rapid dissipation of vibration energy. Moreover, the reflection and refraction effects caused by the thickness and stratification of the soil layers further complicate the vibration propagation path, impacting the distribution and intensity of the ground vibrations. The interaction between the concrete materials used in the tunnel and the surrounding soil also plays a critical role in vibration propagation, enhancing the efficiency of vibration wave transmission within the tunnel structure.