Mechanism of pile-bottom leakage and grouting control in deep foundation pits of water-rich sand strata: a case study of the Xiong’an–Xinzhou high-speed railway project

Pile-bottom leakage in retaining piles of deep foundation pits within water-rich sand strata was investigated using the Xiong’an-Xinzhou High-Speed Railway Project as the engineering background. The dynamic evolution of leakage and optimization of pile-bottom post-grouting technology were examined by combining seepage-stress coupling theory with field monitoring data. The results show that leakage-channel expansion is driven by the combined action of hydrodynamic pressure and sand grain migration, while numerical simulations based on the Darcy seepage model can accurately predict the leakage rate. Orthogonal tests were used to optimize the cement slurry mix proportion, and a grouting material with an initial setting time of 8–15 min and a 28-day impermeability strength of at least P12 was developed. A quantitative relationship between grouting pressure and diffusion radius was then established in conjunction with the pile-bottom post-grouting process, resulting in a 30% increase in grouting construction speed efficiency (volume per unit time) and an 87.3% reduction in leakage. Engineering application results show that post-grouting leakage is reduced by 87.3%, pile displacement is stabilized within 2.8 mm, and treatment costs per linear meter are decreased by 26.5%. The mechanical mechanism of pile-bottom leakage is clarified, and a comprehensive technical framework of “mechanism analysis–material optimization–process control” is established, providing theoretical support and a practical reference for deep foundation pit engineering in water-rich sand strata.