Abstract: During pipeline construction, water-supply and drainage conduits inevitably intersect existing structures. Although temporary support measures are commonly adopted to mitigate the influence of trench excavation on adjacent facilities, such measures can significantly increase construction costs for long-distance pipeline projects extending tens of kilometers. Assessing the excavation-induced response under unsupported conditions is therefore of practical importance. This study examines the section of the Jiangnan Water Plant expansion project where a distribution pipeline crosses the pier foundation of the Fuyang Bridge. Numerical analyses were performed to evaluate the stability of the trench face and the deformation response of the bridge pier. The results show that the safety factor of the excavation face remains approximately 1.3, with a minimum value of 1.289. Unsupported excavation induces soil displacements of less than 3 mm around the pier, and the corresponding stress variations are below 3 kPa. Surface displacement at the inner pier is 1～2 mm smaller than that at the outer pier, and the associated stress change is likewise 1～2 kPa lower. These findings indicate that, for the studied geological and structural conditions, unsupported trench excavation produces limited deformation and stress disturbance to the bridge pier, providing a quantitative reference for similar pipeline-crossing scenarios.

Key words: Pipeline Trench, Construction Impact, Cut and Cover Method, Unsupported Excavation