# Rock Fragmentation Characteristics by TBM Cutting and Efficiency under Bi-Lateral Confinement
## 1 Introduction  
In recent years, a number of studies have been undertaken on the breaking process and characteristics of the rock fractures between tunnel boring machine (TBM) disc cutters. Based on two rock-breaking mechanisms [1; 2], involving shear and tensile stress, some scholars demonstrated that, on the one hand, the horizontal shear stress between adjacent disc cutters is the main reason for the rock-breaking phenomenon. On the other hand, other researchers suggested that the breaking occurs due to the interpenetration of tension cracks caused by neighboring disc cutters. The content of a study regarding the characteristics of rock fragments can be also divided into two subsections: the first concentrates on the interior of the rock (e.g., uniaxial compressive strength, regarding joint, moisture content, and other efficient factors [3; 4; 5; 6; 7; 8]), and the second focuses on external factors including tool size, shape, and machine operating parameters [9; 10; 11; 12; 13; 14; 15; 16]. In addition, geo-stress increases in excavation of deep tunnels. An on-site investigation was performed by [PERSON] et al. [17], and the findings showed that high ground stress could keep rock from breaking and did not produce plate cracking. When a local stress reaches a threshold, cracks are generated in the target rock mass. These cracks amplify the breaking of the rock under TBM disc cutters, leading to a reduction in the excavation index of the rock characteristics, and an improvement in the efficiency of rock breaking. Micro-fractures were observed in the rock on tunnel heading under high ground stress in on-site investigations, which were conducted by [PERSON] et al. [18] and [PERSON] and [PERSON] [19]. Some scholars have recently studied the influence of confining pressure on the rock-breaking process and characteristics by using indoor experimental tests and numerical simulations. [PERSON] et al. [20] studied the cutting efficiency of a boring machine tool under different confining pressures using a fracture analysis code (Franc2D) numerical method and an indoor test with a micro-rock cutting machine for rock. They reported that the penetration force gradually increased with the confining pressure. [PERSON] et al. [21] utilized TBM disc cutters for a penetration test, in which the uniaxial compressive strength of a rock specimen reached 234 MPa, and the major confining pressure was 10 MPa. The experimental results showed that the increase of penetration force does not significantly increase confining forces in order to break the rock. [PERSON] et al. [22] employed fast Lagrangian analysis of continua (FLAC) software to numerically simulate and analyze the effect of confining pressure on the crack development and the initiation of tension as well as fracture of the tool. The results demonstrated that a small increase of confining pressure caused the position of the maximum tensile stress to move significantly from the vertical thrust line and increased the crack vertical angle. [PERSON] et al. [23], [PERSON] et al. [24], and [PERSON] et al. [25] reported that cracks tend to develop to the free surface once the confining pressure increases. It is worth mentioning that field investigations, laboratory tests, and numerical simulations can promote perceiving the mechanism of rock breaking of TBM in the presence of various confining pressures. However, most previous studies were undertaken as two-dimensional (2D) [23; 24; 25], imposing barriers to truly representing the breaking mechanism in three-dimensional (3D) spaces. The development of a true triaxial test platform facilitates the accurate simulation of the rock-breaking process in different broken rock conditions. [PERSON] et al. [25] studied the impact of different cutter spacings on the excavation's efficiency using a TBM cutter, in which a series of rock cutting tests were undertaken using five sets of cutter spacing. [PERSON] [26] and [PERSON] et al.