Large-scale shaking table tests on seismic mitigation effect of a novel flexible joint and buffer layer on TBM pipeline tunnels

In high-intensity earthquake zones, seismic mitigation for pipeline tunnels passing through soft-hard strata is of importance to the safe operation of pipeline systems. This study proposed a combined seismic mitigation approach integrating novel flexible joints and buffer layers for tunnel boring machine (TBM) pipeline tunnels. Large-scale (1:5) shaking table tests were conducted to simulate TBM pipeline tunnels passing through soft-hard strata, with realistic modeling of bolt-connected segmental structures and consideration of buried pipelines within tunnels. Dynamic response and failure mechanism investigation verified the efficacy of the proposed mitigation measures. Test results indicated that the buffer layer effectively reduced peak acceleration and high-frequency components in the Fourier spectrum of the tunnel, while promoting greater consistency in its dynamic responses in soft and hard rocks. The novel flexible joints demonstrated superior deformability to adapt to forced displacements induced by soft-hard strata, thereby alleviating deformation in conventional circumferential joints near the interface. The combined measures reduced the maximum bolt axial forces of the tunnel under seismic motion, with reduction rates ranging from 14.47% to 23.71% in the soft rock and 11.91% to 23.08% in the hard rock. The peak ground acceleration (PGA) threshold for tensile strain of the tunnel exceeding failure limits increased from 0.4 g to 0.8 g . Furthermore, seismic damage was substantially mitigated or eliminated, including backfill cracking, bolt hole fractures, concrete spalling, and segment dislocation. Although pipeline strains exhibited an increasing trend due to the flexible joint deformation compared to the unmitigated case, measured strains remained well below allowable limits, confirming ample safety margins for the tunnel-protected pipeline system.