## 1 Introduction  
A tunnel boring machine (TBM) can be classified by its use of a shield, method of securing reaction force, face support, cutterhead type, and excavation method [1]. As a closed-type TBM, the earth-pressure-balanced (EPB)-shield TBM is widely used in urban tunnels, where rapid excavation is required with high safety standards. An earth pressure balanced (EPB) shield is generally used on TBMs excavating soft ground. Ideally, the support pressure of the EPB shield can be controlled by the screw-conveyor rotation speed and the TBM advance rate. However, most natural ground conditions are not ideally suited to EPB drives, and soils are often conditioned by the injection of water, polymers, and foams, to allow EPB shields to perform well. In addition, the pressure of the tunnel face is balanced with minimal surface settlement, as the chamber of the machine is filled with conditioned excavated materials (soil pastes) that act as a support medium at the tunnel face. Successful excavation relies on the machine being optimized for the project conditions, which requires prediction of its excavation performance. Typical prediction methods include laboratory testing [2], analysis of field data [3; 4; 5; 6], theoretical approach [7; 8], and numerical analysis [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23]. Among them, numerical analysis using the discrete-element method (DEM) is increasingly used in this field [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23], owing to rapid advances in computing power. Rock-cutting tests of a TBM disc cutter can predict its excavation performance [2]. [PERSON] et al. [9] used the UDEC software based on the DEM, to evaluate the impact of joint orientation on rock fragmentation using a TBM disc cutter; they demonstrated the applicability of DEM to the simulation of rock cutting tests. [PERSON] et al. [11] used the same method to evaluate the effect of joint spacing on rock fragmentation by TBM cutters. [PERSON] et al. [13] used discrete-element analysis to concurrently evaluate the effects of both joint spacing and joint orientation on rock indentation and fragmentation. Recently, [PERSON] et al. [23] used a rigid body dynamics coupling program with DEM to evaluate disc cutter performance in soft strata. They analyzed the effect of penetration depth, linear velocity, and existing wear on disc cutter performance, based on the mutual interaction between the disc cutter and a weathered granite model. Most studies have used DEM to comprehensively evaluate TBM excavation performance, including modeling both TBM excavation and the ground. [PERSON] and [PERSON] [10] conducted the first discrete-element analysis evaluation of an EPB shield TBM. They then modeled a mixed-face ground composed of six different layers, and simulated its excavation by a 9.4 m-diameter TBM. They assessed the required torque, thrust force, and much discharge rate. However, their method's limitations included a large particle size in the ground model (diameter, 600 mm) and an oversimplified TBM model. [PERSON] and [PERSON] [12] simulated the flow and strength characteristics of conditioned sands by DEM as preprocessing for the EPB-drive simulation, and determined sand calibration DEM parameters by slump and direct-shear tests. Based on the parameters obtained from [PERSON] and [PERSON] [12], [PERSON] et al. [14] created the ground model and a detailed TBM model, based on a real EPB shield TBM (diameter, 6.14 m) used for line 4 of the Beijing metro. They evaluated the chamber pressure and cutter force of each disc cutter as well as the required thrust, torque, and much discharge rate. Furthermore, they verified the results via comparison with field data to evaluate the reliability of the simulation. However, the simulation was conducted using only one speed of the cutterhead rotation (revolutions per minute, RPM). It, therefore, did not allow comparison of the torque, thrust, or cutter force (which depend on the cutterhead RPM). [PERSON] et al.