## 1 Introduction  
In South Korea, the main expressways in the Seoul metropolitan area have lost their intended functionality as they operate beyond their designed capacity. In most sections, severe traffic congestion makes it take nearly an hour to travel a short distance of about 30 km. However, conventional methods of expanding road capacity have become practically unfeasible because of extensive urbanization surrounding these expressways. Consequently, plans for the undergrounding of existing expressways have been under consideration. Currently, the South Korean government is promoting a total of four underground expressway projects, as illustrated in Figure 1. Among these, the project to underground the Gyeongbu Expressway (Hwaseong-Seoul) passed its preliminary feasibility study in the latter half of 2024 and is expected to commence construction in 2027. The Hwaseong-Seoul Underground Expressway Project involves constructing a four- to six-lane road tunnel at a deep level across a 26.1 km stretch from Giheung IC to Yangjae IC, which will be an unprecedentedly large cross-section and ultra-long tunnel project. To successfully carry out this project, it will be essential to employ both conventional drill-and-blast methods and mechanical excavation in an appropriate manner, as well as to effectively respond to geological and geotechnical variations. Joints are geological and geotechnical variables that influence the performance of mechanical excavators such as tunnel boring machines (TBMs) and roadheaders. In this context, various empirical prediction models for TBMs have considered parameters that represent joint density or frequency, such as rock quality designation (RQD), joint spacing, and volumetric joint count [1; 2; 3; 4; 5; 6]. Among the empirical models for predicting roadheader performance, some have also included RQD as a parameter to reflect the influence of joints [7; 8; 9]. However, [PERSON] et al. [10] emphasized that only RQD is not sufficient to describe the properties of jointed rock masses, which could lead to inaccurate results from these models. To overcome this, they proposed a model that considers additional factors such as the strike and dip of joints and joint aperture. Numerous researchers have investigated the effects of joint orientation, which is defined as the angle between the tunnel axis and the joint plane, and spacing on TBM performance using numerical simulations. [PERSON] et al. [11] and [PERSON] et al. [12] numerically investigated crack propagation and chip formation in jointed rock masses with varying joint orientations and spacings. They found that crack propagation and rock fragmentation in jointed rock occur in two modes. [PERSON] and [PERSON] [13] conducted numerical simulations considering both joint spacing and orientation. The results showed that cutting efficiency improves as joint spacing decreases and is maximized when the joint orientation is 60\({}^{\circ}\). To better understand the interaction between disc cutters and jointed rock masses, [PERSON] and [PERSON] [14] and [PERSON] et al. [15] conducted numerical simulations of the rock cutting process. Furthermore, [PERSON] et al. [16] simulated TBM cutterhead excavation in jointed rock masses and analyzed the influence of joint spacing and orientation. Laboratory test methods have also been used to investigate the impact of joints on the cuttability of TBM. [PERSON] et al. [17] studied the impact of joint spacing on rock fragmentation using full-size LCM tests, considering only cases where the joint plane was perpendicular  
Figure 1: Planned underground expressway projects in South Korea. to the direction of penetration. The results showed that smaller joint spacing significantly reduces the required normal force compared to intact rock. When the spacing between the cutting surface and the joint plane is smaller than a critical value, the rock fragmentation is dominated by the joint. Rolling indentation abrasion tests (RIATs) have also been employed to analyze the effects of joints on TBM performance.