Study on the Vibration Isolation Effect of a Collaborative System Composed of Widened Trench and Diaphragm Wall in Foundation Pits on Subway-Induced Vibrationss

With the continuous increase in urban rail transit network density, environmental vibrations induced by subway operations have become a significant concern affecting the structural safety of adjacent buildings and the quality of life of residents. Taking a frame structure building adjacent to a subway tunnel in Zhengzhou as the engineering background, this study establishes a three-dimensional vehicle–track–tunnel–soil–building-coupled dynamic analysis model based on field vibration tests and ABAQUS finite element numerical simulation, with model accuracy validated through measured data. The foundation pit widened-trench and diaphragm wall are considered as a collaborative vibration isolation system. The effects of backfill material type, widened-trench width, diaphragm wall embedded depth, wall thickness, and wall position on the vibration response of building floors are systematically investigated. The results indicate that the difference in wave impedance between the backfill material and the surrounding soil is the core factor determining isolation performance. Rubber particles, characterized by low dynamic stiffness and high damping, exhibit the best isolation effect, reducing the Z vibration level by 5.1–6.6 dB across all floors compared with conventional plain soil backfill, followed by graded sand and gravel, while fluidized stabilized soil shows limited effectiveness. The benefit of increasing the trench width tends to saturate beyond a certain value. The embedded depth of the diaphragm wall exerts the most significant influence, with isolation effectiveness improving by 6.5–7.8 dB when the depth is increased from 11 m to 20 m. Increasing wall thickness yields only marginal improvement. Adjusting the horizontal distance between the concrete wall and the subway tunnel can effectively enhance isolation performance. A comparative analysis among the widened-trench/diaphragm wall collaborative system, open trench, hollow concrete wall, and rigid-flexible composite wall reveals that the collaborative system outperforms the hollow wall and conventional concrete walls, is comparable to the open trench, and exceeds the rigid-flexible composite wall. Considering isolation efficiency, construction cost, and long-term maintenance convenience, the widened-trench/diaphragm wall collaborative system leverages existing structural components to achieve satisfactory isolation benefits with minimal additional engineering investment, and is therefore recommended for similar projects adjacent to subway lines. The findings provide quantitative references for the vibration isolation design of sensitive buildings along urban rail transit corridors.