## 1 Introduction
DEM represents a straightforward approach to modeling ground behavior and, therefore, could be applied to collecting real-time TBM data concerning the dynamic interaction of the soil and structure during shield tunnelling [22; 23; 24; 25]. However, the computational cost increases considerably as the number of discretized elements increases. Moreover, to apply macroscopic parameters (e.g., Druker-Prager, Mohr-Coulomb, and modified Cam-Clay parameters), microscopic parameters (i.e., contact parameters) must be calibrated before DEM modeling. Namely, the contact model and contact parameters must be determined via preliminary numerical analyses. Geotechnical engineering commonly uses direct shear [26], uniaxial [27; 28], and triaxial [29] tests to calibrate the contact parameters. However, these preliminary analyses might magnify numerical errors and make the analysis time consuming. The computational load caused by numerous elements and the additional calibration process are critical barriers to applying DEM in practical engineering problems. Therefore, to mitigate the limitation, DEM is frequently coupled with other numerical methods such as a boundary element method (BEM) [30; 31], computational fluid dynamics (CFD) [32], FDM [33], and FEM [34; 35]. The coupling of DEM and FDM has received notable attention for simulating shield tunnelling, because both methods adopt the same explicit time-stepping, which facilitates data exchange at each time step. Using DEM-FDM coupling, [PERSON] et al. [36] examined the progressive failure of a tunnel face during shield tunnelling. To assess the effect of particle shape on the tunnel face failure region, settlement and face supporting pressure were studied. [PERSON] et al. [37] also adopted coupled DEM-FDM to simulate scaled EPB shield tunnelling in sandy ground. The analysis applied a conditioning technique with particles having different friction coefficients. The thrust force, cutter head torque, and conditioned muck distribution in the machine were investigated with varying soil conditioning. In accordance with the current demand for, and progress in, the real-time numerical simulating of EPB shield tunnelling, this study couples DEM and FDM to establish a numerical model for EPB shield tunnelling. The numerical code is established using commercial numerical programs: Particle Flow Code in three dimensions (PFC3D, Itasca Consulting Group, Inc.) for DEM [38] and Fast Lagrangian Analysis of Continua in three dimensions (FLAC3D, Itasca Consulting Group, Inc.) for FDM [39]. Virtual ground for excavation during tunnelling is formulated using both DEM and FDM. Generation of the excavation area by DEM uses numerical calibration to determine the contact parameters of particles. The rest of the ground domain (outside the excavation area) consists of FDM grids to alleviate the computation load. A spoke-type TBM (diameter, 6.14 m) comprising a cutter head with cutting tools, working chamber, screw conveyor, and shield is then imported into the model. Within the model, data for the TBM (e.g., torque, thrust force, chamber pressure, soil discharge, and settlement) are monitored as it advances. To verify the applicability of the proposed model, the analysis is conducted by controlling the TBM operational conditions; the simulated TBM data are presented here and discussed.