Design and field measurement study of semi-top-down support method for large-opening deep excavation in reclaimed coastal land, Xiamen

A deep foundation pit project in Xiamen, located in the urban center within a complex geological environment of a coastal intertidal zone backfill area, faced challenges of dense surrounding buildings, intersecting utility lines, and stringent environmental protection requirements. To achieve the dual objectives of foundation pit deformation control and project cost-effectiveness, a semi-top-down support system combining dual-wall integration with large-opening structural floor slabs was proposed, supplemented by a vertical support system featuring one column and one pile. A comprehensive technical approach combining theoretical analysis, numerical simulation, and dynamic design-to-construction information management was employed to complete the design and construction process. Monitoring data revealed that the maximum deep-seated horizontal displacement of the retaining wall was only 34.2 mm, with surrounding ground surface settlement generally less than 20 mm, effectively safeguarding the safety of the surrounding environment. This study summarizes methods for determining the equivalent horizontal stiffness of large-opening floor slabs, proposes an optimization pathway for the continuous diaphragm wall trenching process in complex strata, and develops measures such as efficient formwork techniques for horizontal floor slabs and beam-column joint construction. Additionally, it proposes improvement measures addressing the causes of defects in continuous diaphragm wall structures. Results indicate: The semi-top-down method for deep foundation pit support in coastal backfilled areas with large openings is reasonably feasible. The horizontal support stiffness of large-opening floor slabs should be comprehensively determined based on slab width and thickness, with a recommended range of 30 MN/m² to 85 MN/m². Furthermore, during design, floor slab design loads can be zoned according to construction requirements, and the vertical support system design can be optimized. This research provides technical guidance for similar deep foundation pit projects in complex coastal geological environments.