## 1 Introduction  
Compared with traditional techniques, such as drilling and the blasting methods, the full-face rock tunnel boring machine (TBM) has certain merits such as high construction quality, high efficiency, environmental friendliness, and lower disturbance to the surrounding rock, so it has been widely used in underground construction and has gradually become the first choice for tunnel construction [1, 2]. However, from the current TBM construction cases, it can be seen that the strata encountered in TBM excavations are becoming increasingly complex. Many construction projects face complex strata such as high ground stress, fault fracture zones, and rich groundwater [3]. Due to these geological conditions, a TBM incident can delay the construction schedule for months or even years. In the Jinping II hydropower station project, the in situ stress in the TBM-excavated rock mass was as high as 30 MPa, leading to a strong rock burst and serious distortion and fractures of the steel arch frame, which affected the TBM penetration efficiency and the overall construction progress [4]. In the Alborz Service Tunnel project in Iran, groundwater directly affected the normal excavation of the TBM [5]. For rock breakage, the TBM typically utilizes a disc cutter on its cutterhead, and the disc cutter is the core tool that comes directly in contact with the surrounding rock. The effect of rock breakage caused by the disc cutter directly affects the efficiency of TBM construction [6; 7; 8]. As the basis for and key to the TBM design and construction phase, the rock breakage mechanism and penetration efficiency of the disc cutter have received extensive attention from researchers [9]. To study the rock breakage mechanism and principles of the cutter, an indoor indentation test has been widely applied, as it requires only small-sized specimens and has low equipment demands. It is also simple and convenient. [PERSON] et al. [10] studied the derivation and expansion of microcracks when a circular, flat-bottomed indenter penetrated rock, clarifying the whole process of macroscopic failure. [PERSON] et al. [11] observed the crack propagation law of limestone, marble, and granite when using a flat-headed wedge cutter, and they reported on the relationship between its contact stress and the uniaxial compressive strength and tensile strength during cracking. [PERSON] and [PERSON] [12] studied the rock failure process from brittle failure to plastic failure with the increase of the confining stress through the indentation test, and they found that when the confining stress increases to a critical value, the rock failure process stops. With the help of acoustic emission technology, [PERSON] et al. [13] found that the failure process can be divided into four stages: microcrack closure, elastic deformation, crack propagation, and final fracture. Through the cutter head indentation test, [PERSON] et al. [14; 15] investigated the effect of confining stress on the granite breakage process through sequential indentation tests in a biaxial state. [PERSON] et al. [16] and [PERSON] et al. [17] used numerical simulation software to establish a two-dimensional cutter rock breakage model and studied the effect of confining stress on the indentation and rock breakage performance of the TBM cutter. They believed that as the confining stress increases, the crack tends to develop along the free surface. [PERSON] et al. [18] and [PERSON] et al. [19] studied rock failure characteristics under different joint angles through disc cutter indentation tests and found that the existing joint planes have obvious control effects on the crack propagation mode. Previous studies have shown that geological conditions are the decisive factor affecting the performance of TBM cutters. Both the cutter design and the excavation parameters depend on the geological conditions to some extent. However, groundwater is also one of the important factors affecting the stability of rock masses in underground engineering.