## 4 Conclusions  
The optimum dosage and ratio of additives for amending compacted clay were screened and determined through moisture retention and gas diffusion test. The principal reason for the enhancement of its moisture retention capacity was investigated by the liquid-plastic limit test. Subsequently, the hydraulic performance of the compacted clay amended by additives and unamended was conducted. Based on the results, the following conclusions can be drawn:  
1. The attapulgite could enhance the moisture retention performance of the clay, but its effect on gas barrier performance was limited. When its dosage was 5%, its moisture retention percent was 66% higher than that of unamended clay. The enhancement mechanism behind the phenomenon is attributed to increasing the liquid limit of clay;
2. The diatomite could effectively decrease the gas diffusion coefficient of clay, while it had a limited effect on the moisture retention performance. As its dosage was 5%, the moisture retention percent of clay only increased by 7%;
3. Considering the moisture retention capacity, gas barrier performance, and cost, the optimal dosage of dual-additives was targeted to be 5%, and the optimal ratio of attapulgite to diatomite was 4. The moisture retention percent of the dual-additives-amended compacted clay increased by 82%, and the gas diffusion coefficient decreased by 42% compared with unamended clay. The \(k\) of enhanced compacted clay amended by attapulgite and dual-additives was 6.3 \(\times\) 10\({}^{-9}\) m/s and 5.9 \(\times\) 10\({}^{-9}\) m/s respectively, decreases of 25% and 29% compared with the unamended clay;
4. The SEM and MIP analyses presented that dual-additives effectively filled the intergranular pores of the amended clay and micropore structure increased the gas migration path. The BET test results showed that dual-additives increased the amount of microporous to enhance the moisture retention performance of amended clay. Meanwhile, the dual-additives increased the specific surface area and decreased the average PSD. The TGA results demonstrated that dual-additives increased the temperature limit of loosely bound water to enhance the moisture retention performance of amended clay.  
Conceptualization, M.W., J.W. and Y.D.; methodology, M.W., Y.D. and J.W.; software, M.W.; validation, M.W. and N.J.; formal analysis, M.W. and J.W.; investigation, [PERSON], J.W. and H.Z.; resources, M.W.; data curation, M.W. and J.W.; writing--original draft preparation, M.W., J.W., H.Z. and W.X.; writing--review and editing, M.W., Y.D., H.Z. and N.J.; visualization, M.W.; supervision, M.W., Y.D. and W.X.; project administration, Y.D.; funding acquisition, Y.D. All authors have read and agreed to the published version of the manuscript.  
Financial support for this project was provided by the National Key R&D Program of China (Grant No. 2018 YFC1803100), the National Natural Science Foundation of China (Grant Nos. 41877248, 42177133, and 41902276).  
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.