## 3 Results and Analysis  
### Shear Test of Unfilled Soil-Concrete Interface  
The variations of the shear stress with the shear displacement for the specimens with different moisture contents under different normal stress conditions are shown in Figure 6. The shear stresses of all the specimens first increased with the increase in the shear displacement, and when the shear displacement reached a critical value, the shear stress stabilized. The critical value increased with the increase in the normal stress. In the shear test of different samples, the shear stress first increases and then tends to be stable with the increase in the shear displacement. Silty sand and medium-coarse sand require less shear displacement to stabilize than clay and sandstone. This is mainly due to the fact that the silty and the medium-coarse sand are composed of loose particles, which are easily redistributed during the shear process. However, clay and sandstone are flocculated structures, which are damaged during shear slip, and the shear displacement required to achieve stable shear stress is large. Figure 7 shows the results for different soil specimens with a moisture content of 15%. When the normal stress was 100 kPa, the maximum shear stresses of the different soil specimens in descending order were clay, sandstone, medium-coarse sand, and silty sand. The difference in the maximum shear stress between the clay and the sandstone was small; yet, as the normal stress increased, the difference gradually increased. This was mainly due to the formation of a thin layer of silt at the clayey soil-concrete interface under the action of the normal stress, which reduced the interfacial shear strength. In comparison, the sandstone had more evident water softening properties than the clayey soil. For the silty sand and medium-coarse sand, a water film formed at the interface due to water discharge. However, the water film had little effect on the shear stress of the interface. The shear stress of the silty sand was lower than that of the medium-coarse sand. This was because the particle size of the silty sand was small (0.25-0.5 mm), and compared with the medium-coarse sand, the silty sand particles could more easily fill the voids on the concrete surface, thereby reducing the interface friction coefficient. The particle sizes of the medium-coarse sand were evenly distributed in the range of 0.1-5 mm. The gaps between the particles and between the particles and the concrete surface resulted in an embedding effect, which improved the interfacial shear strength. The critical values of the shear stress under the conditions of the different soil types, soil moisture contents, and normal stresses were taken as the interfacial shear strength, as shown in Figure 8. With the increase in the moisture content, the shear strengths of the different soil types decreased to varying degrees. The shear strength of the medium-coarse sand and silty sand decreased by 5-22 kPa, but the overall extent of the decrease was small. For the silt and sandstone, under the same normal stress, the interface was greatly affected by the moisture content. When the moisture content was at a low level, the shear strength of the interface decreased slightly with the increase in the moisture content. However, when the moisture content was above a certain threshold, the shear strength of the interface decreased rapidly, and the maximum decrease was 96 kPa. In addition, the shear strength  
Figure 5: Examples of interface shear surfaces: (**a**) silty sand, (**b**) medium-coarse sand, (**c**) clay, and (**d**) sandstone. The relationship between the shear strength and the normal stress for the different soil-concrete interfaces was fitted (Figure 9) with the following equation:  
\[\tau=\sigma\tan\varphi+c \tag{1}\]  
where \(\tau\) is the shear strength (kPa), \(\sigma\) is the normal stress (kPa), \(\varphi\) is the internal friction angle (\({}^{\circ}\)), and \(c\) is the cohesion (kPa).