Field investigation of tunnel construction effects on an ancient pagoda in deep soft soil

With rapid urbanisation in China, tunnelling near historical buildings on soft soil has grown common, yet its dynamic impact on vulnerable structures remains insufficiently investigated. In this study, a field investigation was performed into the influence of a large-diameter tunnel excavation on a 300 year old pagoda only 8.3 m away. Ground settlement, excess pore water pressure (EPWP) and deep horizontal displacement were monitored during tunnelling and for 200 days afterward. Field data show that the surrounding soft soil was dynamically disturbed, primarily due to excavation activities. Specifically, slurry pressure and synchronous grouting altered the stress state within the foundation, mobilising the deformation potential of the soft soil. The synchronous grouting, a volume twice that of the over-excavated soil, raised EPWP by up to 80 kPa, causing 9 mm horizontal displacement near the tunnel bottom and 10 mm ground heave. Meanwhile, the pagoda experienced uniform uplift without significant tilting. Within 30 days after the passage of the tunnel-boring machine, however, EPWP dissipation and soil consolidation induced rapid settlement. Non-uniform EPWP distribution led to differential settlement, making the pagoda incline toward the tunnel up to 0.18°. The results highlight the non-negligible delayed effects of tunnelling on historic structures, and that soft soil foundation disturbance should be minimised for protection.