Abstract: The subway section between Yingmen Tan Station and Matan Station on Lanzhou Metro Line1 crosses the Yellow River at an upstream distance of 38~49 m from the Yintan Yellow River Bridge, with Matan Station located immediately adjacent to the riverbank. Influenced by upstream river turbulence and long-term sand-and-gravel excavation near the banks, several bored cast-in-place piles at the small-mileage end of the station were required to be constructed within deep artificial fills. Conventional bored-pile construction methods in these strata exhibited significant problems, including drilling-slurry loss, borehole instability, and related defects. This paper presents a case study of the enclosure-structure construction at the small-mileage end of Matan Station. The challenges associated with bored-pile installation in deep fills are summarized, and the underlying reasons for the difficulty in achieving stable pile formation in this soil layer are analyzed. Improvement trials were conducted to enhance pile-forming performance by adjusting slurry density, replacing the original slurry materials, and optimizing the drilling procedure. Results indicate that successful bored-pile installation in deep fills depends primarily on improving the self-stability of the fill layer while minimizing hydraulic connectivity between the borehole and surrounding ground. Finally, a soldier-pile installation scheme incorporating ground sleeve valve-pipe grouting reinforcement was successfully implemented. The engineering measures and practical experience obtained from this project provide valuable reference for similar underground works constructed under complex geological conditions.

Key words: Lanzhou Metro, Deep Fills, Bored Cast-In-Place Concrete Pile, Construction Techniques