Deformation behavior and control of transmission tower foundation cap excavations in soft soils under adjacent asymmetric loading

Excavation of transmission tower foundation cap pits in soft-soil regions is often significantly affected by adjacent asymmetric loading, such as surcharge from nearby high-rise buildings and heavy traffic loading. Based on typical soft-soil conditions in Zhejiang Province, this study employs a three-dimensional finite element parametric analysis to systematically investigate the evolution of excavation-induced deformation under different foundation types, namely single-pile, 2 × 2 pile-group, and 3 × 3 pile-group foundations, as well as under adjacent building bias loading and traffic surcharge loading. Deformation control strategies for extreme loading conditions are further proposed. The results show that large pile-group foundations are more sensitive to excavation-induced unloading disturbance. Building bias loading induces pronounced asymmetric deformation, characterized by distance-dependent attenuation and height-dependent amplification, and is governed by stress transfer over a relatively deep zone. By contrast, traffic surcharge loading exhibits stronger near-field sensitivity and a more pronounced shallow concentration effect. To quantitatively evaluate deformation risk, a dimensionless influence zoning method was established based on normalized spatial distance and loading intensity parameters, by which the response was divided into hazard, transition, and safe zones. Support analysis under the most unfavorable conditions shows that steel sheet piles can markedly reduce ground settlement, lateral wall displacement, and pit-bottom heave, with the major deformation indices reduced by 50–95%. At the same time, increasing support depth exhibits a clear trend of diminishing marginal benefit: 4 m steel sheet piles already provide effective control, 6 m steel sheet piles further improve the deformation response and may be regarded as the preferred support scheme, whereas increasing the depth to 8 m yields only limited additional benefit. The findings provide a useful reference for the support design and risk control of transmission tower foundation cap excavations in soft-soil regions. A 3D FEM is developed for transmission tower foundation cap excavations under adjacent asymmetric loading. Building bias loading and traffic surcharge loading amplify deformation through deep stress transfer and near-field concentration, respectively. Steel sheet piles markedly reduce settlement, wall displacement, and bottom heave, with 6 m embedment identified as optimal.