## 1 Introduction  
In recent years, with the increasing demand for underground transportation projects, the development of highway tunnels has been rapid. The construction of four-lane or larger cross-section highway tunnels has become a trend. Based on the characteristics and purposes of such projects, four-lane or larger cross-section highway tunnels can be divided into three major categories [1]: conventional new four-lane highway tunnels, in situ expansion of four-lane highway tunnels, and four-lane or larger bifurcated tunnels. Among these, bifurcated tunnels are tunnel projects where two lines intersect or separate, often occurring at the junction of bridges or tunnels and underground interchange ramps. They are often used as transition sections along with large cross-section tunnels and tunnels with small clearances. Therefore, bifurcated tunnels have the characteristics of large cross-sections and complex structures. According to statistics, from 2005 to 2020, there were 12 four-lane or larger bifurcated tunnels completed and under construction in China [2]. Typical projects include the Qingdao Jiaozhou Bay Subsea Tunnel [3], Xiamen Lu'ao Road Bifurcated Tunnel [4], and Yunnan Huitaoxia Bifurcated Tunnel [5]. Because of rapid development and application, the research focuses and popular topics have shifted to optimizing construction methods for large cross-section tunnels [6; 7; 8], the dynamic construction mechanics of surrounding rock [9; 10; 11], and the determination of support structure parameters [12; 13]. A bifurcated tunnel with a super-large section consists of three segments: the large cross-section construction segment, the small-clearance tunnel construction segment, and the variable cross-section transition construction segment. Of these, the variable cross-section transition construction segment involves excavating tunnels with at least three cross-sections. During construction, one must consider the stability of the surrounding rock in the large cross-section area and the presence of rock in the small-clearance section. Simultaneously, ensuring the rationality of the tunnel cross-section transition construction method and construction safety is of utmost importance. This segment is the most complex and hazardous of these three segments. Hence, we focus on the study of variable cross-section transition construction in this paper. The transition segment of the variable cross-section involves two cases: construction from a small cross-section to a large cross-section, and construction from a large cross-section to a small cross-section. It is evident that excavating from a large to a small cross-section is simpler. However, for some complex tunnel structures, it is necessary to transition from a small to a large cross-section. During this process, the transition of the arch height and area differences must be managed, and the rational sequence of operations will determine the safety of the transition segment. [PERSON] [14] and [PERSON] [15] proposed bidirectional construction techniques for large-span small-clearance tunnels in the Hangzhou Zizhi Bifurcated Tunnel. They used auxiliary construction adds for small cross-section tunnel construction, climbed to the top within the large cross-section tunnel, and completed the transition from a small to a large cross-section. [PERSON] et al. [16] introduced the reverse excavation method in the Yang'an Bifurcated Tunnel, entering the large cross-section tunnel through inclined shafts to achieve sequence alignment, and evaluated the feasibility using numerical simulation. Based on the Shantou Bay subsea shield tunnel, [PERSON] et al. [17] used a refined lining model considering the details of lining components, and adopted XFEM and CDP (Concrete Damage Plasticity) constitutive models in part of the model for stress analysis and damage development. [PERSON] et al. [18] proposed an improved Kriging filtering algorithm in the NATM tunnel, utilizing 3D laser scanning and ranging point cloud data to estimate the full-space deformation field of tunnel excavation.