## 2 Prototype Experiment  
### Project Background and Test Specim  
As intended, column-free QRSTs are located in the section used to connect two metro stations. Most of the tunnels in this section adopt typical QRST structures, while the part of the tunnels in this section near the metro station, as shown in Figure 2, adopt column-free QRST structures. The tunneling construction utilizes a newly developed 11.83 m \(\times\) 7.27 m quasi-rectangular EPB shield machine, with the typical QRST linings constructed first, followed by the column-free QRSTs. The specific soil parameters are shown in Table 1. The column-free QRSTs are situated predominantly within the (2)2b-silty clay layer. In the area close to the metro station, the column-free QRST structures reach a maximum buried depth of approximately 13 m.  
The proposed column-free QRST has not been used before, let alone the relatively high buried depth. Considering its large span (11.5 m) and limited segment thickness, whether this new type of structure has sufficient suitability subject to the given engineering conditions is unclear. The deformation characteristics of the structure under the given joint distribution pattern are also not clear as the weak parts of the column-free QRST structure, the load-bearing capacity, and the deformation performance of joints have also not been fully explored. In addition, the mechanical behavior of this large-span column-free tunnel structure is relatively complex. The appropriate numerical models need to be determined to simulate the structural behavior. Since there is no central column support, the sensitivity of this new structure to changes in complex external geological conditions also needs to be studied. Therefore, a series of studies based on experiments and numerical simulations are very necessary. The specimen utilized in this experimental investigation is a column-free QRST structure, the schematic diagram of which is shown in Figure 3. The outside dimension of the structure is 11,500 mm \(\times\) 6937 mm. The structure comprises two large arcs at the top and bottom and two small arcs at the left and right waists. The centers of these two small arcs are located 2550 mm left and right of the center of the structure, respectively; the demarcation between the large and small arcs is also indicated in Figure 3. The structure comprises 10 steel blocks (segments), including two T blocks (T1 and T2), three C blocks (C1, C2, and C3), three B blocks (B1, B2, and B3), an L block, and an F block (key block). The thickness and width of the segments are 450 mm and 1200 mm, respectively. The thickness of the internal and external steel plates of the segments is 30 mm (Q345). There are ten longitudinal joints, designated as Joint 1-Joint 10, as illustrated in Figure 3.  
Inspired by research findings indicating that bolt movement can effectively enhance the mechanical behavior of longitudinal joints and tunnel linings [8; 9], corresponding measures are adopted in the design of the joints of the column-free QRST structure. Bolts of positive moment joints (intrados tension) have been inwardly repositioned, whereas bolts of negative moment joints have been outwardly repositioned, as shown in Figure 4. The distance between the M48 bolt position and the external arc surface is 325 mm at positive moment joints and 180 mm at negative moment joints. Furthermore, to enhance the bending stiffness and load-bearing capacity of joints, a row of four M30 bolts has been added at positive moment joints in addition to using a row of six M48 bolts. Joint contact \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Soil Layer**} & **Layer Thickness** & **Water Content** & **Unit Weight** & **Internal Friction** & **Cohesion** \\  & **(m)** & **(\%)** & **(kN/m3)** & **Angle (\({}^{\circ}\))** & **(kPa)** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistics of geotechnical parameters of the geology in the background project. Figure 2: Background project for column-free QRST: (**a**) Top view; (**b**) Side view.