**6. Recommendations for Future Study**

This was a preliminary study that merely focused on the feasibility of a capillary barrier for the subgrade. The parameters of soil were selected from the literature to study the feasibility. In the future, specific subgrade soils will be used to conduct experiments for more in-depth research. Furthermore, the capillary barrier was used as the cover layer for natural materials without manual treatments, which can effectively block the infiltrating rainwater. It can be applied in many areas including slope stability. It can prevent rainwater infiltration so as to improve the slope's stability. However, it cannot be applied in humid climates due to the fact of its poor water storage capacity. In the future, more studies should be carried out on improving its water storage capacity by using new materials with high water storage capacity and methods so as to be applies more in engineering.

**Author Contributions:** The formal analysis, formula reasoning, and the performance of the experiment, as well as the original draft preparation were completed by X.Z.; Review and editing were completed by Y.W. and J.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Fundamental Research Funds for the Central Universities (grant numbers: B210202034, B200204036, and B200204032).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data sharing not applicable. No new data were created or analyzed in this study.

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