Analysis of tunneling effects on existing adjacent tunnels using 2D physical model experiment and numerical simulations

As surface land in metropolitan areas becomes saturated, underground construction has become essential for expanding urban infrastructure, but excavating new tunnels near existing ones introduces challenges of stress redistribution, ground deformation, and potential instability, especially in diagonally aligned twin tunnels with asymmetric soil–structure interactions. This study presents an integrated framework combining physical model tests, Particle Image Velocimetry (PIV), and Discrete Element Method (DEM) simulations using PFC2D. Laboratory experiments with carbon rods replicated granular soil under six tunnel spacing scenarios, with excavation modeled by a scaled Tunnel Boring Machine (TBM), while ground deformation and segmental loads were monitored and used to calibrate numerical simulations, which were then extended to field scale with rock-like properties. Results demonstrate that tunnel spacing critically influences structural response: when an existing tunnel lay within the relaxation zone of new excavation, arching effects were suppressed and inverted segment loads decreased by up to 70%, whereas outside this zone, stress redistribution was more uniform, and load variations stayed within ±10%. A segmental load ratio–based classification system is proposed to assess excavation impacts, and the validated framework offers practical guidance for spacing, reinforcement, and risk mitigation, advancing safer and more sustainable subsurface development.