To investigate the impact of water chemistry on the mechanical properties of coastal soft soil, one-dimensional consolidation tests and scanning electron microscope analyses were conducted on unsaturated soil samples subjected to varying concentrations of Na₂SO₄ pore water and different salinity levels. The results show that: With the increase of salt content, the yield stress of coastal soft soil structure decreases by 9.5%, the compression index increases linearly, and the permeability coefficient increases; When the salt content is greater than 4%, the precipitated mirabilite crystals exist between the particle pores, and the soil skeleton effect is obvious, which leads to the increase of the yield stress of the soil structure and the decrease of the compression index and permeability coefficient. SEM images show that the average particle size of coastal soft soil increass with the increase of Na₂SO₄ solution concentration, and the flocculation structure soil particles increased. In addition, the structural parameters of coastal soft soil considering the concentration of salt content and consolidation pressure are established. The results show that the structural parameters decrease with the increase of consolidation pressure; When the consolidation pressure is less than the structural yield stress, the structural parameters first decrease and then increase with the increase of pore salt content, but when the consolidation pressure is greater than the structural yield stress, the structural parameters do not change significantly with the increase of salt content.

With continuous development of urbanization, the settlement of existing buildings induced by shield construction disturbances has become a core issue of concern for environmental safety assessment. For the transverse settlement, longitudinal settlement and twist deformation of the raft foundation building induced by the whole process of shield construction, FLAC^3D finite difference software is used for in-depth study, and the shield construction disturbance plan affected zone has been divided into negligible affected zone, transition zone and dangerous affected zone. The results show that the average transverse and longitudinal settlement affected zones of buildings are parabolic, the transverse overall tilt affected zone is semi-elliptical, the longitudinal overall tilt affected zone is quasi-parabolic and the twist deformation affected zone is quasi-circular. During the shield construction process, the maximum values of transverse average settlement, transverse overall tilt and longitudinal average settlement of the building occur at the completion of the shield crossing, the overall longitudinal tilt and twist deformation of the building will have reached a maximum by the time the shield reaches the central part of the building, which indicates that the assessment of the safety of existing building settlement in shield crossing construction should pay attention to the process of shield construction. The results of the study can provide a theoretical basis for assessing the environmental safety of shield construction in dense urban areas.