## 5 Conclusions  
In this paper, a thermo-hydro-mechanical coupled approach is used to study the valley narrowing deformation in the Xiluodu project. Similar valley narrowing deformation caused by thermal contraction of bedrock has rarely been reported before. Yet the particular combination of similar hydrogeology conditions and the super-high arch dam project is not common. It should be noted that, because of the extreme complexity of fractured rock mass, the actual cause for the valley narrowing deformation of the Xiluodu arch dam remains an open question, and the research work in this paper is supposed to provide a new perspective. The main results are as follows:  
1. The proposed THM coupling model explains the valley narrowing deformation mechanism of the Xiluodu Project. According to the analysis of in situ measurement and numerical results, high ground temperature in the river valley before the impoundment is mainly induced by the deep groundwater circulation, which continuously brings heat to the bedrock at the bottom of the valley. After impoundment, the previous seepage direction at the bottom of the valley is reversed. The reservoir water with lower temperature cools the rock mass at the bottom of the valley and thermal contraction occurs. As a result, the \"U\" shaped valley narrows.
2. Seepage field variation is the main factor of the deformation in the first two years, while thermal field variation causes long-term deformation after the initial impoundment. It shall be responsible for more than 60 percent of the total deformation. The numerical results also predict that 15 mm deformation will occur in the next five years. By 2024, the annual deformation rate will be less than 0.005 mm/day.
3. Based on the result of sensitivity analysis on hydraulic conductivity, the conductivity of the bedrock is one of the key factors that influence the magnitude of valley narrowing deformation. Hence, anti-seepage methods such as high-pressure grouting are recommended to decrease the heat exchange underneath the riverbed and reduce further development of valley deformation in the feature.  
Though the converged value of deformation is relatively large, it is still controlled within the design criteria. Dam safety is guaranteed. As there are many high dam projects still under construction or in planning in China, the analysis method proposed in this article provides reference meaning for future studies on the multi-field evolution process of dam-reservoir-foundation systems, particularly thermal field variation. For future studies on the Xiluodu project and similar cases, hydro-chemical experiments and THMC coupling simulation can be very helpful to gain a more comprehensive understanding of the mechanisms. A recent study [44] on the model has taken fracture into account due to its influence on the multi-field coupling. Fracture scanning through boreholes and THM experiment on rock samples are in progress to reveal the impact of THM process on the propagation of natural fractures. The application of a new model will be made possible by obtaining these experimental data. Further research is also necessary, as a full understanding of the valley narrowing mechanism will be an important reference for dam construction and safety assessment.  
Conceptualization, Q.L. and Y.H.; data curation, T.Y.; formal analysis, T.Y. and G.L.; investigation, T.Y.; methodology, Y.H.; project administration, Q.L. and S.Y.; resources, S.Y.; software, G.L.; validation, Y.H.; writing--original draft, T.Y.; writing--review and editing, Y.H. All authors have read and agree to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 51979145 and 51839007. The authors declare no conflict of interest.