# Study on Strength of Slip Soil-Tuff Contact Surfaces in Tuff Landslide Based on Model Test
## 3 Analysis of Shear Test Results
### Effect of Roughness on Shear Strength  
The shear strength of the contact surface of slip soil and concrete under different vertical stresses with different roughness was compared, as shown in Figure 7. Overall, increased roughness of contact surface and vertical stresses resulted in more significant peak and residual strengths. As the vertical stress increased, the contact surface shear strength variation curves for each roughness followed a transition from fold linear to  
Figure 6: The stress–displacement curves for different roughness of soil–concrete surface. (**A**) For R1. (**B**) For R2. (**C**) For R3. (**D**) For R4. (**E**) For R5.  
hyperbolic. This means that the higher the roughness, the more the failure of the contact surface is biased from a surface slide failure to a failure within the slip soil.  
After comparing the shear strengths of the slip soil and contact surfaces with different roughness levels (Figure 8), the following conclusions can be drawn:  
1. When the roughness of the contact surface was low, the contact surface's peak shear strength and residual shear strength were lower than that of the slip soil itself.
2. When the roughness of the contact surface reached R3 (11.304 mm), the shear strengths of the contact surface fell near the strength of the soil itself at different vertical stresses, which can be regarded as basically equal to the strength of the slip soil.
3. When the roughness of the contact surface was more significant than 11.304 mm, the shear strength of the contact surface was higher than the strength of the soil itself.  
Such a result implies that within the test regime designed in this paper, a critical roughness R0 of the contact surface exists, which determines the shear failure mode of the contact surface, as shown in Figure 9. Figure 9 shows failure pattern diagrams for contact surfaces with different roughness levels; the figure shows a side view perpendicular to the direction of shear. When the roughness of the contact surface was higher than R0, the shear failure penetrated deep into the interior of the soil, thus forming a shear failure region, which is shown in Figure 9A. The softening of the slip soil strain was concentrated in this region. At the same time, the shear strain and normal deformation of the contact surface were also concentrated in this region, and the final shear strength was greater than the  
Figure 8: Shear strength of slip soil and contact surfaces with different roughness. (**A**) For peak intensity. (**B**) For residual intensity.  
Figure 7: The stress curves for different roughness and vertical strength of the soil–concrete surface. (**A**) For peak intensity. (**B**) For residual intensity.  
strength of the soil itself. When the roughness was equal to R0, the shear process proceeded roughly along a curved surface with a slight curvature at a very shallow surface in the slip soil, and the final strength result was close to the strength of the soil itself. When the roughness was less than R0, the shear failure was mainly concentrated at the roots of the soil columns filling the grooves of the concrete block, which is essentially the shearing of the protruding parts of these soils and the sliding between the two materials, as shown in Figure 9C. The shear strength of the contact surface was then less than the shear strength of the soil itself.