## 5 Conclusions  
In order to study the mechanical properties of sliding shear of different soil and concrete interfaces under different loading modes of variable normal stress, this paper adopted the self-developed large-scale interface shear equipment to carry out the interface shear test and mechanism research under the single loading of constant normal stress, repeated loading and unloading of variable normal stress and continuous loading of variable normal stress, and draws the following conclusions:  
(1) With the increase of cement soil curing time, the shear mechanical properties of cement soil concrete interface are gradually enhanced, the interface friction coefficient is gradually increased and the normal pressure drop is gradually weakened. (2) When the interface shear test satisfies the Mohr-Coulomb criterion, the minimum friction coefficient formula proposed in this paper can be used to determine the friction coefficient of the interface. At the same time, from the point of view of engineering practicality and safety, the initial normal stress can be used to calculate the interface friction coefficient. (3) For the shear test of the cement soil concrete interface, the repeated loading and unloading of a single specimen can be approximately equivalent to the loading of multiple specimens separately. Therefore, when the number of samples is insufficient, the interface friction coefficient can be determined by repeated loading and unloading. (4) The overall shear performance of the cement soil concrete interface under continuous normal pressure loading is weaker than that under repeated loading and unloading. Therefore, the actual normal stress loading path should be considered in the project to avoid overestimating the shear performance of the cement soil concrete interface. (5) Under the condition of continuous variation of variable normal stress, the greater the cohesion, the greater the variation of the interfacial friction coefficient of the material and therefore better applicability to the minimum interfacial friction coefficient. (6) In the case of continuous change of normal stress, the rapid increase of normal stress will lead to accelerated entry into the ultimate shear state, resulting in plastic failure of the shear plane as a whole. Although the shear test under variable normal stress has some limitations in the application range of shear materials, the minimum interfacial friction coefficient and the shear interface damage mechanism under variable normal stress proposed in this paper have a wide range of application prospects. It is a good reference for evaluating the horizontal slip resistance of the suspension bridge beam anchorage foundation and accurately analyzing and calculating the additional bending moment effect generated by the vertical frictional force on the pile side. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number** & **T\({}_{\text{C28d}}\)** & **T\({}_{\text{S-C}}\)** & **T\({}_{\text{CLC-C}}\)** \\ \hline First stage displacement/mm & 1.3 & 6 & 5 \\ Second stage displacement/mm & 0.2 & 2 & 0.5 \\ decline/\% & \(-\)84.61\% & \(-\)66.67\% & \(-\)90.0\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Change of the critical displacement. **Author Contributions:** Conceptualization, M.Z., H.L., X.L., Q.Y., J.L. and C.L. ; software, H.L., M.Z. and G.D.; validation, H.L. and M.Z. ; investigation, H.L. ; resources, H.L. and Q.Y. ; data curation, H.L. ; writing--original draft preparation, H.L. and M.Z. ; supervision, M.Z. and X.L. ; project administration, M.Z., X.L. and G.D.; funding acquisition, M.Z., X.L. and H.L. All authors have read and agreed to the published version of the manuscript. This research is supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province, Project No. : SJCX21_1794; the National Natural Science Foundation of China, Project No. : 52201324; the youth fund of the National Natural Science Foundation of China, Project No.