## 4 Conclusions  
Through large-scale direct shear tests of the interface between the concrete and the various soil types (i.e., medium-coarse sand, silty sand, silt, and sandstone), the influences of the normal stress, moisture content, and interface filling materials on the shear strength of the soil-concrete interface were analyzed, and the following conclusions were obtained:  
1. The shear stress of the soil-concrete interface increased initially and then stabilized with the increase in the shear displacement. Compared with the clayey soil, the sandy soil had a smaller shear displacement when the shear stress became stable. This was mainly because the sandy soil contained loose particles, which were more prone to redistribution than the flocculent structure of the clayey soil, and thus, the shear displacement of the sandy soil was smaller.
2. With a high normal stress, the soil pore water was discharged and formed a lubricating layer in the soil-concrete interface, resulting in a decrease in the friction coefficient of the interface. The friction coefficient between the medium-coarse sand and the concrete was greater than that of the silty sand-concrete interface. This was mainly because the gaps between the large particles and between the soil particles and concrete resulted in an embedding effect, thereby improving the interfacial friction coefficient.
3. The interfacial shear strength of the clayey soil, sandy soil, and concrete interfaces decreased with the increase in the soil moisture content. Compared with the clayey soil-concrete interface, the shear strength of the sandy soil-concrete interface was less sensitive to the moisture content. The friction angle and cohesion of the sandy soil-concrete interface decreased with the increase in the moisture content; yet, the  
Figure 12: Variation of shear strength for soil–concrete interfaces with different filling materials: (**a**) shear strength of clay–concrete interface before and after filling with sandy soil and (**b**) shear strength of silty sand–concrete interface before and after filling with silt.  
decrease magnitude was small. The friction angle of the clayey soil-concrete interface decreased rapidly with the increase in the soil moisture content, whereas the cohesion first increased and then decreased, and the peak cohesion was seen when the moisture content was near the plastic limit.
4. For a given moisture content, filling the clay-concrete interface with a thin layer of sandy soil led to a reduction in the friction angle and the cohesion of the soil-concrete interface. Filling the silty sand-concrete interface with a thin layer of silt resulted in a decrease in the friction angle of the soil-concrete interface and an increase in the cohesion. However, the filling had little effect on the overall shear strength of the interface. In this study, the number of tests of the effects of interfacial fillings on the shear strength was limited. Further studies are needed to investigate the influences of the moisture content, filling material, and filling thickness on the shear parameters of different soil-concrete interfaces.  
Conceptualization, D.L. and C.S.; methodology, D.L.; validation, D.L., H.R. and B.L.; formal analysis, D.L.; investigation, C.S.; resources, D.L.; data curation, D.L.; writing--original draft preparation, D.L.; writing--review and editing, D.L.; visualization, H.R.; supervision, H.R.; project administration, D.L.; funding acquisition, C.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (Grant No. 41,831,278 and Grant No. 51679071).  
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Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
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We would also like to thank [PERSON] for editing the manuscript.  
The authors declare no conflict of interest.