## 1 Introduction  
As urbanization has progressed, the shield-driven method has become the preferred choice for constructing subsurface transportation networks in urban areas, including subways, highways, and pedestrian passages [1]. However, engineers and professionals in the tunneling industry currently face multifaceted challenges. These challenges encompass uncertainties in geological conditions and the complex interplay between construction parameters during the construction process [2; 3], as well as unexpected situations such as fire and water leakage at the service stage [4; 5]. Therefore, tunneling is increasingly becoming a research hotspot. Conventional tunnel configurations always adopt circular cross-sections renowned for their advantageous mechanical properties and developed construction methodologies. However, circular tunnels have a low utilization rate for underground space. Another type of non-circular tunnel with a high space utilization rate, named Double-O-Tube(DOT) tunnel, has high difficulty controlling soil disturbances during the construction stage [6]. To utilize these advantages and avoid these weaknesses, tunnels with a new type of cross-section, named Quasi-rectangular tunnels, are proposed [7]. Compared to circular segmental tunnels, quasi-rectangular segmental tunnels (QRSTs) exhibit characteristics of high space utilization with strong applicability in high-density urban areas. The applicability of typical QRST structures (with columns) has been substantiated through a series of joint and structural experimental investigations [8; 9], leading to successful applications of QRSTs in numerous metro projects [7]. Due to the need for transportation network planning, it is necessary to set up a crossover section in a large two-way tunnel. Consequently, during the final service stage, the original columns of QRSTs must be removed to facilitate lane changes for metro vehicles, as depicted in Figure 1. To meet these engineering requirements, engineers proposed a column-free QRST structure using steel segments as components. However, the removal of columns will significantly alter the mechanical behavior of QRST structures. Furthermore, the weak parts of these new types of structures remain unclear. To ensure the safety of the new column-free QRST structures, a comprehensive series of studies on the mechanical behavior of such structures is imperative. In order to fully reveal the mechanical behavior of non-circular tunnels such as QRSTs, conducting experimental research is often the most direct and effective approach. Several prototype tests have been conducted for double-O-Tube tunnels (DOT tunnels) [10], rectangular tunnels [11], and quasi-rectangular tunnels [12; 13], revealing the structural mechanics and enabling the determination of design parameters for these new types of non-circular tunnel structures. By conducting full-scale experiments on DOT tunnel structures, the mechanical behavior of single rings and stagger-jointed triple rings under different design states were explored, providing valuable insights for the structural design [10]. For the large-section rectangular shield tunnels utilized in the Kyoto Subway, two different types of lining structures--steel-concrete composite structures and cast-iron structures--were subjected to full-scale stagger-jointed loading tests. The distribution of internal forces and deformation of the lining structure obtained from the tests matched reasonably well with the design results, supporting the design and application of these novel types of non-circular shield tunnels [14]. Additionally, by conducting prototype structural tests, weak parts and robustness of the structure under diverse unexpected circumstances were revealed, facilitating subsequent structural optimizations. A full-scale experimental investigation of a rectangular tunnel assembled by composite segments was conducted using a \"standing\" prototype loading platform. The study delved into the structural mechanics of the rectangular tunnel structure under different loading conditions.