## 1 Introduction  
The major expressways in the Seoul metropolitan area of South Korea have exceeded their originally planned capacity, preventing them from fully functioning as highways. However, conventional expansion methods to increase road capacity are virtually impossible due to the urbanization surrounding these expressways. In this context, underground expressway projects could serve as an effective solution. The underground expressway projects for the section of the Gyeongbu Expressway between Giheung IC and Yangjae IC, and the section of the Gyeongin Expressway between South Cheongna IC and Sinwol IC (Figure 1), have passed preliminary feasibility evaluations and are scheduled to begin construction in 2027. The Gyeongbu and Gyeongin underground expressways are planned as large cross-section, deep tunnels with four to six lanes, extending 26.1 km and 15.3 km in length, respectively. To ensure the successful implementation of these projects, it is essential to employ an appropriate combination of the conventional drill-and-blast method and mechanical excavation methods, along with effective responses to geological and geotechnical variability. Mechanical excavation machines can be categorized into full-face machines, such as tunnel boring machines (TBMs), and partial-face machines, such as roadheaders. TBMs typically employ disc cutters to excavate the tunnel face, whereas roadheaders use drag picks to break the rock. The performance of these machines is significantly influenced by geological and geotechnical conditions. Among these, joints are considered one of the key factors. Accordingly, many empirical models developed to predict TBM performance have incorporated joint-related parameters such as rock quality designation (RQD), joint spacing, and volumetric joint count [1; 2; 3; 4; 5; 6]. Furthermore, various studies have been conducted to investigate the effects of joints on rock fragmentation. Numerical simulations have been widely employed to investigate the influence of joints on TBM performance. [PERSON] et al. [7] and [PERSON] et al. [8] investigated the influence of joint spacing and joint orientation, which is defined by the angle between the tunnel axis and the joint planes, on rock fragmentation by disc. They identified two distinct fragmentation modes in jointed rock masses: one caused by disc cutter indentation, and the other induced by tensile cracks initiated at the joint planes. [PERSON] and [PERSON] [9] conducted numerical simulations and showed that cutting efficiency decreases as joint spacing increases and reaches a maximum when the joint orientation is 60\({}^{\circ}\). [PERSON] and [PERSON] [10] and [PERSON] et al. [11] simulated the cutting process of disc cutters, rather than simple indentation, to better understand the influence of joints on TBM performance. [PERSON] et al. [12] further advanced this line of research by modeling a full-scale TBM disc cutter to simulate TBM excavation in jointed rock masses. Figure 1: Underground Expressway Projects That Passed Preliminary Feasibility Evaluation in South Korea. Experimental methods have also been employed as an effective approach to investigate the impact of joints on TBM performance. [PERSON] et al. [13] employed a full-scale linear cutting machine (LCM) test to investigate the effect of joint spacing on rock fragmentation. On the other hand, [PERSON] et al. [14] and [PERSON] et al. [15] analyzed the effects of joint orientation and spacing on the cracking behavior induced by disc cutters through rolling indentation abrasion tests (RIATs). They reported that cutting performance was optimized when joint spacing was smaller and the joint orientation ranged between 45\({}^{\circ}\) and 60\({}^{\circ}\). [PERSON] et al. [16], [PERSON] et al. [17], and [PERSON] et al. [18] conducted double disc cutter indentation tests combined with digital image correlation (DIC) to analyze the effects of joints on crack propagation and rock fragmentation.