Numerical Simulation of Trapdoor Experiments for Estimating the Loosen Load Around a NATM Tunnel

In NATM tunnels, the concrete lining serves as a secondary support structure playing subordinate roles. However, when the primary support deteriorates or the ground conditions are poor, the lining may be subjected to unexpected earth pressure. To prevent damage caused by this pressure, it is necessary to identify the loosen load, allowing the lining to perform its mechanical function. In this study, a numerical model was developed to simulate the trapdoor experiment and to propose a method for calculating the relaxed load. The discrete element method (DEM) was adopted in the model to represent the characteristics of the ground surrounding the tunnel. The numerical model simulated a series of experimental trapdoor steps and analyzed the ground loosening load. The contact properties of the soil were calibrated by the analysis of the triaxial compression test, and the reliability of the developed numerical model was ensured by comparing the results with laboratory test results. The model calculated the contact force applied to the trapdoor plate and the settlement of the soil particles. By controlling soil characteristics, ground height, and trapdoor plate width, the developed model can analyze the design of tunnel cross-section size and stability under various ground conditions.