## 1 Introduction
[16] analyzed the influence of clear distance, buried depth, and other conditions on the blasting vibration characteristics of a small clear-distance tunnel through numerous 2D and 3D numerical simulations. For a small clear-distance tunnel, [PERSON] et al. [17] performed a numerical analysis and compared the effects of different construction methods, such as step, advanced reinforcement, and grouting reinforcement. [PERSON] et al. [18] applied numerical simulation combined with a model test to examine the two transition sections from the large-span section to the small clear-distance section of a bifurcated tunnel. They showed that the vault and middle rock pillar are greatly affected by tunnel excavation. On the basis of a shield tunnel project, [PERSON] et al. [19] established a shield tunnel construction model using refined numerical simulation technology to analyze the interaction of small clear distance and shallow-buried shield construction and investigated the interaction law of shield tunnel construction under different clear distances. [PERSON] et al. [20; 21] analyzed the influence of different layouts and excavation sequences of a double-line shield tunnel on surface settlement and the surrounding environment. Using the ANSYS finite element program, [PERSON] et al. [22] studied the stress and deformation characteristics of a shallow-buried, bias-pressure, small, clear-distance tunnel under different bias angles, spacings, and buried depths. Selecting a reasonable construction method was an important factor to ensure the stability of surrounding rock and construction safety [23; 24]. At present, the commonly used construction methods for tunnels included the bench method, cross diagram (CRD) method, center diaphragm (CD) method, and double wall heading method. The main idea was to convert the large section of the whole tunnel into small section construction, reduce the span of each excavation, and ensure the safety of construction [25; 26; 27; 28]. Many scholars had explored tunnel construction methods. [PERSON] et al. [29] carried out model tests to restore the surrounding rock displacement and stress release laws of the CD method, double wall heading method, and bench method in the process of partial excavation and found that the surrounding rock deformation experienced a slow increase-sharp increase-stable state. As a common method of tunnel construction, the CD method had the advantage of resisting the inward horizontal convergence of the tunnel. Based on the CD method, [PERSON] et al. [30] put forward the vertical center diaphragm (VCD) method of axisymmetric structure, and studied the deformation law and mechanical characteristics of the VCD method through numerical calculation and field comparative tests. [PERSON] et al. [31] used the CRD method and the four-step method to study the excavation of shallow and large-diameter twin-tunnel in soft ground. [PERSON] et al. [32] established a three-dimensional calculation model to study the influence of tunnel excavation mode on stratum deformation. It was found that the double wall heading method can better control surface deformation and has higher construction efficiency. [PERSON] et al. [33] built a three-dimensional numerical model of a large section tunnel by finite difference method, analyzed the construction of the double wall heading method, and discussed the deformation characteristics and stress conditions of surrounding rock and tunnel structure. Many scholars have studied the small clear-distance tunnels. However, there are few reports on the comparison of different construction methods of small clear-distance tunnels. The stress and displacement of the ultra-small clear-distance tunnel are complex. At present, there is no unified construction specification, and a unified and persuasive theory has not been formed. In addition, the surrounding rock stress of a small clear-distance tunnel is affected by lithology and tunnel excavation mode, so the construction and support structure design of a small clear-distance tunnel face many problems.