Tunneling beneath the pile-raft foundations of high-speed railways: Progressive arching deformation and pile settlement behavior

Due to the unclear mechanisms behind tunneling-induced deformation of pile-raft foundations, there are strict global restrictions on tunneling beneath embankments of high-speed railways. This study conducted a series of two-dimensional tunneling model tests to investigate the tunneling-induced deformation characteristics and mechanisms of pile-raft foundations. Soil displacement field and pile settlement were measured using particle image velocimetry and displacement transducers. The changes in soil displacement and the flexure of the pile-raft foundation in response to varying tunnel-pile distances, ground surface loads, and tunnel volume loss were analyzed. The results indicate that the tunneling-disturbed zone can be categorized into a loosened zone and an arch zone as identified by the propagation and separation of shear bands, with significant soil settlement occurring in the loosened zone. The maximum settlement of piles in a pile-raft foundation is greater than that in greenfield due to the larger loosened zone. However, the settlement width at the ground surface in pile-raft foundations is reduced due to the blocking effect of the piles. According to the relative position between the piles and the formed arch structure, three patterns of tunneling-ground-pile systems can be identified. As the tunnel-pile distance increases, the maximum settlement of the piles decreases. Increasing surface loads hardly affects the maximum settlement value of the pile, while the tunneling-induced arch zone expands significantly. This study provides a fundamental understanding of pile settlement behavior for tunneling beneath the pile-raft foundations of high-speed railways.