Discrete Element Method-Based Mesoscopic Insight into Effect of Soil Mechanical Properties on Shield Tunnel Deformation

Excessive deformations and associated structural defects in urban metro shield tunnels are often observed because of the surcharge load and other disturbances in soft soil areas, threatening the serviceability and even safety of metro systems. However, the quantitative effect of soil mechanical properties on shield tunnel deformation remains unclear. This paper aims to characterize the development pattern of tunnel deformation under increasing surcharge load, considering the effect of random migration and the mechanical properties of the tunnel-surrounding soil. Therefore, a discrete element model is developed to simulate the macro and mesomechanical behaviors of the soil–tunnel interaction system. The numerical simulation results show that tunnel transversal convergence develops with a nonlinear growth trend with the increase in surcharge load. This trend is accompanied by a certain degree of dispersion because of the random movement of granular soil. A 20% reduction in soil elastic modulus increases tunnel convergence by 24% under 0.14 MPa surcharge, while a 20% increase in soil internal friction coefficient reduces convergence variability by 28%. Based on the simulation results, a mathematical model for the quantitative prediction of tunnel transversal convergence is proposed, which incorporates the effects of surcharge magnitude and soil properties. The results from the mathematical model are compared with real data and show the promising possibilities of the model to predict tunnel deformation under varying load conditions.