Abstract
To address the practical engineering challenges of compaction difficulty and low efficiency associated with traditional backfilling methods for narrow and confined urban foundation trenches, this study proposes an eco-friendly backfill material: sodium silicate-activated fly ash-cement composite fluidized stabilized soil, using in-situ loess from the Xiong'an New Area as raw material. Material components were optimized through curing agent ratio determination and single-factor tests. The development laws of mechanical properties, the micro-scale solidification mechanism, and the flow-filling characteristics within narrow trenches were systematically elucidated by integrating scanning electron microscopy and computational fluid dynamics (CFD) simulations. The results indicate that a fly ash-to-cement mass ratio of 1:1 serves as the critical benchmark for optimizing the composite curing agent proportion. This ratio effectively inhibits excessively rapid early water evaporation and provides a stable hydration environment for the later pozzolanic reaction of fly ash. The optimal mix proportion achieves a balance between high fluidity and high strength. A water content of 51.25% ensures sufficient cementitious reactions. A sodium silicate content of 2.0% strikes a balance between activating fly ash and avoiding excessive slurry viscosity. A curing agent content of 35% facilitates the formation of a continuous and dense cementitious network. Under the optimal proportion, the material exhibits a spread flow of 175 mm and a 14-day compressive strength of 4.53 MPa, meeting the requirements for pumping construction and strength. The compressive strength of fluidized stabilized soil results from the synergistic interaction of fluidity, molding compactness, and water loss behavior. This is manifested microscopically by the generation of cementitious products, pore filling, and particle cementation, and macroscopically reflected in the coupled effects of physical water migration and chemical water consumption. Adopting a single-pour length covering three utility tunnel Sect. (9 m) and a pouring speed of 5 m/s combined with a terminal speed reduction process can significantly enhance the compactness and construction quality of trench backfilling. This approach facilitates the formation of a stable flow field and reduces air bubble retention in corners. Field application demonstrates that this process can meet the dual requirements of construction efficiency and quality. The research findings provide a theoretical foundation and key technical guidance for the construction of underground comprehensive utility tunnels in the Xiong'an New Area.