Numerical Assessment of an Asymmetric Freezing Pipe Layout for an Irregular Subway Passage Adjacent to Existing Subway and Residential Structures

Artificial ground freezing (AGF) is widely used for ground reinforcement and groundwater control in urban underground construction, but its application to irregular subway station passages is challenging when the freezing zone is constrained by existing underground structures and nearby buildings. In this study, the planned X-1 passage at Gaojiayuan Station of Beijing Metro Line 14 was adopted as an engineering design scenario. The passage was still at the engineering design stage and had not yet entered construction. An asymmetric freezing pipe layout was proposed to balance frozen wall development and frost heave control under the asymmetric surrounding constraints. Freezing pipes near the existing subway structure were arranged to compensate for the prescribed thermal disturbance, whereas those near the residential buildings were shifted inward to limit unnecessary outward freezing. A three-dimensional sequential thermo-mechanical numerical model was established to evaluate temperature and displacement responses during active freezing. After 60 days, a continuous frozen wall was formed, and the volume-averaged temperature within the designed 2.0 m thick frozen wall domain reached −10.56 °C, below the prescribed limit of −5 °C. Compared with a conventional symmetric layout, the maximum ground surface uplift, building foundation displacement, and existing subway structure displacement were reduced by approximately 2.5%, 3.1%, and 8.3%, respectively. Sensitivity analysis showed that these relative displacement reductions remained stable when the equivalent frost heave coefficients were varied by ±20%. Among the investigated cases, a 200 mm inward offset provided a reasonable comparative balance between frozen wall development and deformation mitigation under the adopted model assumptions. For the investigated design-stage case, the results indicate that a constraint-oriented asymmetric freezing-pipe layout can redistribute the freezing influence while maintaining the prescribed frozen-wall requirements.