**4. Conclusions**

A series of microscopic tests, water retention tests, and shear tests were carried out to explore the effect and mechanism of the strength characteristics of silt treated with xanthan gum. The main conclusions are as follows.

The drying and wetting curves of the specimens all shifted to higher water retention capacity with increasing xanthan gum content, and the hysteretic circle area between the drying and wetting curves also gradually increased. Microscopic analysis showed that the pores of the specimens gradually filled with increasing xanthan gum content, and xanthan gum itself has strong water-retaining properties. The water retention capacity of the treated silt therefore increased.

The peak strength and residual strength of the specimens gradually increase with increasing vertical stress. The shear strength, cohesion, and internal friction angle of the pure and treated silt specimens significantly increased with decreasing water content. During the wetting process, the strength of the pure and treated silt specimens all significantly weakened with increasing water content, but the strength of the treated silt was notably higher than that without treatment. The microscopic analysis showed that xanthan gum fills the uneven pores in silt during the wetting process, which makes the pore distribution more uniform and the pore structure more stable. Xanthan gum forms hydrogel bonded with the silt particles, and the cementation between the soil particles was notable, thus significantly increasing the strength of the silt treated by xanthan gum.

As an eco-friendly and efficient material, xanthan gum can improve the strength and stability of silt after rainfall infiltration. Therefore, xanthan gum can be used to improve the engineering properties of soil during many projects, such as deep foundation treatment, slope treatment, highway subgrade treatment, etc. The shear strength prediction model based on experimental data needs to be further studied.

**Author Contributions:** Conceptualization, J.Z. (Junran Zhangand), Z.M. and T.J.; methodology, J.Z. (Junran Zhangand), Z.M. and J.Z. (Jindi Zhao); data curation, J.Z. (Junran Zhangand), J.Z. (Jindi Zhao), X.Z. and Z.M.; writing—original draft preparation, Z.M.; writing—review and editing, J.Z. and S.W.; visualization, Z.M.; supervision, J.Z. (Junran Zhangand); project administration, J.Z. (Junran Zhangand) and T.J.; funding acquisition, J.Z. (Junran Zhangand) and T.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study is financially supported by the National Natural Science Foundation of China (Grant No. 41602295), the Foundation for University Key Teacher by the Ministry of Education of Henan Province (Grant No. 2020GGJS-094), the Key Scientific Research Projects of Colleges and Universities in Henan Province (Grant No. 21A410002), and the Doctoral Student Innovation Foundation of NCWU.

**Conflicts of Interest:** 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.
