## 1 Introduction  
The safe disposal of high-level radioactive waste (HLW) is a significant topic and a challenging task for all countries utilizing nuclear energy. Deep geological disposal has been proposed as an appropriate method for dealing with HLW internationally. Its fundamental strategy is to deposit HLW in deep disposal pits after solidification and scanning until its danger is reduced by radioactive decay to the point where there is no significant risk to humans or the environment [1]. Because of its high construction quality and low disturbance to the surrounding rock mass, mechanical excavation was commonly used to construct the disposal pit [2, 3]. Due to the limited diameter of the shaft pit, the thrust per cutter provided by the excavation equipment is much lower than that of the usual tunnel boring machines (TBMs). Compared to disc cutters with the same diameter, button cutters that required less thrust for rock cutting are widely used as cutting tools for disposal pit excavation. It begins with the indentation of carbide buttons in the rock and progresses to the interaction between adjacent buttons [4, 5], as shown in Figure 1. Thus, the structural parameters and arrangement of carbide buttons have a great influence on the rock breaking efficiency. Up to now, the influence of geometrical parameters of carbide buttons on rock breaking mechanisms is still an unresolved topic. Laboratory tests are regarded as the most reliable and accurate approach to studying the effects of indenter structural parameters on rock breakage [6; 7; 8; 9]. The influences of indenter structural parameters are mainly reflected in the indentation force, characteristics of chippings, and failure patterns. [PERSON] et al. [10] summarized the failure modes of quasi-brittle rocks under blunt indenters and divided the rocks into three zones: (1) the core region of crushed rock, (2) the plastic zone, and (3) the elastic zone of intact rock. [PERSON] [6] investigated the crack propagation patterns of three different indenters, including hemispherical, truncated, and cylindrical indenters, penetrating into granite. The comparison showed that the length and direction of the main crack were influenced by the contact area between the indenter and the rock, and the peak forces were also different. [PERSON] et al. [11] studied the adaptability of cutters with different blade widths under hard or extremely hard rock conditions, and found that the influence of blade width on normal force was greater than that of rolling force.Zou et al. [12] proposed a thrust prediction model using a newly designed indentation-testing apparatus. In order to better understand the rock failure mechanism, some monitoring techniques were used to observe the initiation and propagation of cracks during indentation experiments [13; 14; 15; 16]. [PERSON] et al. [17] utilized the combination of digital image correlation (DIC) and ultra-high-speed photography to monitor crack initiation, propagation, and coalescence, and found that the fracture time, fracture toughness, and crack growth velocity all depended on the loading rate. [PERSON] et al. [18] investigated the effect of the confining pressure on crack initiation and propagation modes using an acoustic emission monitoring approach. [PERSON] et al. [19] analyzed the effect of loading rate on failure characteristics of asphalt mixtures using an acoustic emission technique. According to damage degree, the rock damaged zone could be divided into crushed, dense crack, and sparse zones [20]. Although the experimental method is reliable and accurate, it is time-consuming, costly, and unable to dynamically observe the failure process inside rocks. As an alternative, numerical simulation has been widely employed to examine rock breaking mechanisms due to its low cost, visibility, and repeatability. The most commonly used numerical methods for rock breaking are the finite element method (FEM) and the discrete element method (DEM).