## 3 Results  
### Interface Shear Test under Repeated Loading  
When the curing time reached 7 d, 14 d and 28 d, one of the cement soil test blocks was randomly selected to carry out the experimental study on the shear characteristics of the cement soil concrete interface under the mode of repeated loading and unloading of normal pressure. The normal pressure of the 7 d and 14 d tests was increased step by step. The normal pressure of the 28 d test was increased step by step and then decreased step by step. During the test, when the applied normal pressure was stable, the horizontal thrust was applied. The corresponding test results are shown in Figures 9-11. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Number** & **Material Science** & **Loading Method** & **Load Classification** &
\begin{tabular}{c} **Curing Time of** \\ **Cement Soil** \\ \end{tabular} \\ \hline T\({}_{\text{R7d}}\) & Cement soil-concrete & Repeated loading and unloading & 3 & 7 d \\ T\({}_{\text{R14d}}\) & Cement soil-concrete & Repeated loading and unloading & 3 & 14 d \\ T\({}_{\text{R28d}}\) & Cement soil-concrete & Repeated loading and unloading & 4 & 28 d \\ T\({}_{\text{S28d}}\) & Cement soil-concrete & Load separately & 1 & 28 d \\ T\({}_{\text{C14d}}\) & Cement soil-concrete & Continuous graded loading & 3 & 14 d \\ T\({}_{\text{C28d}}\) & Cement soil-concrete & Continuous graded loading & 3 & 28 d \\ T\({}_{\text{S-C}}\) & Sand-concrete & Continuous graded loading & 3 & \\ T\({}_{\text{CL-S}}\) & Clay-concrete & Continuous graded loading & 3 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of the test scheme. Figure 8: Variable normal stress continuous multi-stage loading. Figure 11: Interface shear test results of T\({}_{\text{R28d}}\) group: (**a**) Load displacement curve of the T\({}_{\text{R28d}}\) group and (**b**) Friction coefficient displacement curve of the T\({}_{\text{R28d}}\) group. Figure 10: Interface shear test results of the T\({}_{\text{R14d}}\) group: (**a**) Load displacement curve of the T\({}_{\text{R14d}}\) group and (**b**) Friction coefficient displacement curve of the T\({}_{\text{R14d}}\) group. Figure 9: Interface shear test results of T\({}_{\text{R7d}}\) group: (**a**) Load displacement curve of T\({}_{\text{R7d}}\) group and (**b**) Friction coefficient displacement curve of T\({}_{\text{R7d}}\) group. According to the test results under multi-stage repeated loading and unloading shown in Figures 9a and 11a, in the initial stage, the horizontal thrust and the relative shear displacement of the interface maintained an approximate linear relationship; when the critical displacement was reached, the horizontal thrust was basically stable and did not change with the increase of the relative horizontal displacement. It indicates that the shear plane entered the ultimate shear state, and the shear plane presented a plastic failure mode as a whole. According to the feedback from the vertical pressure sensor, the normal pressure was reduced at the initial stage of shear, and the normal pressure remained basically unchanged after entering the ultimate shear state. The ability to monitor the change of vertical pressure value in real time is a feature of this set of independently developed equipment and this test. With the increase of curing time of cement soil, the pressure drop amplitude of normal pressure in the shear process was significantly reduced, especially at 28 d, when the pressure drop amplitude of normal pressure in the shear process was 1.2%\(\sim\)3.2%. This occurred because the strength of cement soil increased with the increase of curing time, the compression deformation of cement soil under the constraint of upper shear box was significantly reduced and the pressure drop of normal pressure was effectively suppressed.