## 5 Conclusions  
The small clear-distance tunnel of Xiamen Haicang Evacuate Channel Project was selected as background, and different excavation schemes, lithology grades, and footage lengths were employed to analyze the construction mechanical behaviour of a small clear-distance tunnel. The following conclusions were obtained:  
1. For an urban super large and complex underground interchange hub, main line, and ramps were all constructed underground. Complex adjacent tunnels form complex small clear-distance tunnels whose construction mechanical behaviour of the small clear-distance tunnels was vital for the underground engineering system. 2. When the CD method was used for the excavation of the small clear-distance tunnel, the vertical displacement of the main line and ramp tunnel vaults was the smallest. The double wall heading method could effectively control the horizontal displacement of the tunnel arch waist. The overall stress state of the tunnel lining in the CD and double sidewall pilot pit methods was reasonable, whereas the tunnel lining in the full section and step methods had a large range of tensile stress. 3. In the CD method, the vertical displacement of the main line and ramp tunnel vault of the small clear-distance tunnel was the smallest. The double wall heading method could effectively control the horizontal displacement of the tunnel hance. The overall stress of the tunnel lining in the CD and double wall heading methods was reasonable, but a large range of tensile stress in the tunnel lining was observed in the full section and step methods. 4. The vertical displacement of the surface, main line tunnel vault, and ramp vault increased with the increase in the elastic modulus of the rock mass. In the rock mass with grades III and IV, the horizontal displacement of the main line tunnel hance was smaller than that of the ramp, and the area from the hance to the arch foot was prone to large deformation when no temporary support was available in the grade V rock mass. 5. Compared with the excavation scheme and lithology grade, the excavation footage length of the small clear-distance tunnel had less influence on the surrounding rock deformation and lining stress/deformation. 6. Reducing the tunnel excavation footage was conducive to reducing the deformation of the vault and hance. Therefore, when the geological conditions are good, a large footage can be selected for rapid excavation. Meanwhile, when crossing special sections, the footage should be reduced to control deformation. Conceptualization, J.W. ; methodology, J.W. and Z.W. ; software, Z.W. ; validation, L.L., Z.L., J.W., A.C., H.L., Y.S. and X.L. ; formal analysis, J.W. and Z.W. ; investigation, J.W., X.L., A.C., H.L. and Y.S. ; resources, J.W. ; data curation, Z.W. ; writing--original draft preparation, J.W., Z.W. and A.C.; writing--review and editing, J.W. and A.C.; project administration, L.L. and Z.L. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Excavation**} & \multicolumn{3}{c}{**Excavation Methods**} & \multicolumn{3}{c}{**Lithology Grades**} & \multicolumn{3}{c}{**Footage Lengths (m)**} \\ \cline{2-10}  & **Full-Section** & **Step** & **CD** & **Double Wall** & **III** & **IV** & **V** & **3** & **4** & **5** \\ \cline{2-10}  & **Method** & **Method** & **Method** & **Heading Method** & **III** & **IV** & **V** & **3** & **4** & **5** \\ \hline \multirow{8}{*}{Stress} & \multicolumn{6}{c}{(1) The peak tensile stress of} \\  & & & & grade III was the largest, & & & & & & \\ \cline{1-1}  & & & followed by grade IV and & & & & & & \\ \cline{1-1}  & & & grade V. & & & & & & \\ \cline{1-1}  & & & grade III was the smallest, & & & & & & \\ \cline{1-1}  & & & and of grade V was & & & & & & \\ \cline{1-1}  & & & grade IV was & & & & & & \\ \cline{1-1}  & & & grade III was the largest. & & & & & & \\ \cline{1-1}  & & & grade III was the largest. & & & & & & \\ \hline \end{tabular}
\end{table}