Objective  The underground interval tunnel project between Xiata Rd. Station and Fumin Rd. Station (hereinafter referred to as the 'Xia-Fu Interval') of Qingdao metro line under construction passes beneath elevated highway bridge piles and rivers. The construction area has complex geological conditions, and some intervals need to pass through long strata featuring upper-soft and lower-hard layers. To ensure the smooth implementation of the interval project and the safety of residential buildings in adjacent areas, it is necessary to study the design scheme and interval construction scheme for this line.

Method The planning status conditions and horizontal alignment of the Xia-Fu Interval are introduced. The horizontal and vertical alignment schemes of the line are refined in design to maximize the distance from risk sources as much as possible. Measures such as MJS (multi-directional high-pressure jet grouting) enclosure and grouting reinforcement are adopted to isolate the interval from residential buildings. Key construction schemes, including tunnel alignment design for under-passing elevated highway bridges, the arrangement of interval drainage pump stations and cross passages, and construction in short-spacing intervals with upper-soft and lower-hard strata, are discussed in detail. Considering the complex interval engineering conditions, multiple construction control measures are proposed.

Result & Conclusion  The tunnel design scheme for the Xia-Fu Interval is reasonable and effective. When the line passes beneath elevated bridge piles, the transfer piles are avoided, ensuring that the vertical clearance between the elevated bridge pile structure bottom and the metro interval tunnel is not less than one tunnel diameter. After optimization of the design scheme for the interval drainage pump stations and cross passages, structurally fractured zones are effectively avoided, and the interval length passing through upper-soft and lower-hard strata is reduced. After reinforcement by MJS pile enclosure, the risk of over-excavation in the interval is reduced, thus minimizing the impact on adjacent residential communities.