**5. Conclusions**

Taking the typical mountain excavation and city construction project fill slope in Yan'an as the research background, through the physical model test of filling slope considering interface effect under rainfall, the following conclusions are drawn:


The interface and its effect in the geological body is a very complex, difficult and meaningful research topic. As for the simulation of the original slope in the indoor model test of the excavated and filled slope of the City-building gully, since the original slope is formed for a long time and has its own structure in practical engineering, and it is impossible to simulate the original slope in the laboratory through simple ramming. Therefore, how to simulate the original slope needs further discussion and verification. In addition, the interface simulation is difficult in the model test. The slope is formed by tamping; therefore, the original slope and filling are simulated by plastic window gauze, and the test effect is not ideal. How to simulate the interface and reflect the interface effect in the model test needs further discussion and verification.

**Author Contributions:** Writing—original draft preparation, W.T.; conceptualization, Q.H.; writing—review and editing, Q.H. and X.C.; visualization, W.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by The National key research and development program of China (NO.2021YFC3000404), The State Key Research Development Program of China (Grant No. 2017YFD0800501) and Major projects of the National Natural Science Foundation of China (41790443) are gratefully acknowledged.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
