Analysis of Metro Station Ultra-deep Diaphragm Wall Trench Wall Stability in Soft Soil Stratum

［Objective］ Under complex geological and ultra-deep excavation conditions, maintaining trench wall stability during diaphragm wall construction remains a critical challenge in engineering research. To identify the causes of trench wall collapse during the construction of the 76 m ultra-deep diaphragm wall trench section (WE-15) on the east side of Ningci Road Station of Ningbo Metro Line 7, it is necessary to carry out targeted analysis and propose corresponding solutions. ［Method］ The PLAXIS 3D finite element software is used to simulate the diaphragm wall trenching process, investigating the stability characteristics of trench walls in soft soil stratum and their key influencing factors. The accuracy of the model is validated by comparing the simulation results with on-site ultrasonic detection data. ［Result & Conclusion］ The horizontal displacement of the trench wall primarily occurs in the early stage of trenching, specifically when excavating the ④2b silty clay layer. The reason is that the fourth soil layer in Ningbo region contains a high silt content, and repeated excavation with a trenching grab disturbs the slurry, leading to uploading and impact effects on the trench wall. This process creates adsorption pores in the slurry wall, allowing groundwater to easily flow into the trench and carry local silt deposits, thereby increasing the risk of localized collapse. At the end of trenching, the maximum horizontal displacement of the trench wall is 18.18 mm, occurring at a depth of 25 m below ground level, closely aligning with the actual collapse location. The stability of the trench wall is significantly influenced by slurry density and reinforcement measures, and both increasing slurry density and reinforcing the trench wall effectively control horizontal displacement.