# Theoretical Analysis on the Effectiveness of Pipe Roofs in Shallow Tunnels
## 6 Conclusions  
1. In this paper, a shallowly buried soft surrounding-rock tunnel is taken as the research object, and the mechanical calculation model of surrounding rock pressure on each steel pipe in the pipe shed is established by considering the application time of the pipe sheds. The distribution characteristics of the surrounding rock pressure, increasing gradually from the vault to the arch foot in the cross section of the tunnel, are discussed through a numerical example.  
Figure 12: Relation curve between circumferential spacing and formation loss rate.  
Figure 13: Relation curve between the excavation footage length and formation loss rate.  
Based on the stress and deformation calculation model of pipe sheds shallowly buried in a soft surrounding rock tunnel and the Peck formula, a theoretical analysis model of pipe sheds' advanced protection effect was constructed, and a theoretical analysis method of pipe sheds' advanced protection effect was proposed, which took the stratum loss rate, settlement trough width, and maximum surface settlement as evaluation indexes.
3. The theoretical calculations were demonstrated to be reasonable and accurate based on a comparative analysis with field-measured data of a pipe roof at the entrance of the Huapiling tunnel. On the basis of these results, we obtained the longitudinal distribution of surrounding rock deformation and surface settlement along the tunnel by theoretically calculating the control effect of surrounding rock deformation and surface settlement after completing the excavation and pre-support protection section of the Huapiling tunnel entrance.
4. Guided by the effects of pre-support, the influence of the main design, and the construction parameters for the steel pipe diameter, we analyzed the impact of circumferential spacing and excavation footage length on the pre-support effects of the pipe roof. According to the analysis, the following conclusions can be drawn. The smaller the circumferential spacing and the smaller the excavation footage length, the better the ability of the pipe roof to control the surrounding rock deformation; when the pipe diameter is small, increasing the pipe diameter can effectively improve the ability of the pipe roof to control formation deformation, and when the pipe diameter is large, the effect on controlling formation deformation by increasing the pipe diameter will no longer be obvious.
5. In this paper, the pipe sheds of the shallowly buried soft surrounding-rock tunnel are taken as the research object, and the theoretical analysis method for pipe sheds' pre-support effect and other related conclusions are put forward, which can provide a certain reference value for the design and construction of pipe sheds of similar projects.  
Conceptualization, Y.L. and S.Y.; data curation, Y.L. and W.W.; formal analysis, Y.L. and J.D.; investigation, Y.L. and J.D.; methodology, Y.L. and W.W.; validation, Y.L. and W.W.; writing--original draft preparation, Y.L.; writing--review and editing, Y.L. and S.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Chinese National Natural Science Foundation, grant number 52068044.  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.