Displacement response of ancient marine sedimentary soils to boring construction of triaxial cement mixing piles adjacent to metro.

In foundation pit projects adjacent to Wenzhou Metro Line S1, controlling the disturbance of ancient marine sedimentary soils caused by the boring construction of triaxial cement mixing piles is crucial to safeguarding the operational safety of rail transit. This study systematically investigates the influencing mechanisms of boring parameters on surrounding soil displacements via a combination of field monitoring, numerical simulation, and parametric optimization. Based on the typical geological conditions of the coastal alluvial-lacustrine plain in Longwan District, Wenzhou-characterized by a narrow separation between the foundation pit and the metro station, and by highly sensitive marine sedimentary soft soils-this research reveals the disturbance characteristics of boring construction through monitoring the horizontal displacements of retaining structures and surface settlement. The results show that shallow soil displacements are significantly affected by time-dependent effects, deep-soil displacements remain relatively stable, and surface settlement increases as the distance from the foundation pit decreases. A finite element model integrating the Mohr-Coulomb model and Biot's consolidation theory was developed in COMSOL, where simulation results exhibited high degrees of consistency with field data (R2 = 0.92 and 0.96, respectively), thus verifying the model's reliability. Orthogonal simulation experiments were conducted to analyze the coupled effects of boring depth and radius, indicating that increasing the boring radius significantly increases deep-soil horizontal displacements but reduces near-zone surface settlements; increasing the boring depth exerts a notable influence on upper-soil horizontal displacements, whereas bottom-soil displacements undergo only minor changes. To achieve differentiated protection objectives, a "Construction Drilling Safety Evaluation Coefficient" is proposed, which comprehensively accounts for the deformation control requirements of above-ground buildings, underground structures, and near- and far-zone environments. The optimal construction scheme is identified as one adopting a smaller boring depth and a moderate radius. This research provides theoretical support and engineering guidance for deformation control during the construction of triaxial cement mixing piles in marine sedimentary soils adjacent to metro lines within coastal cities.