Features of the Morphology of the Shear Failure Surfaces of Coarse-grained Slip Zone Soils

Analyzing the change in the roughness of the shear plane under different particle composition is important to reveal the mesoscopic mechanism of strength change of coarse-grained slip zone soil. Slip zone soils were grouped according to different particle gradations and coarse particle contents for experiments. To collect shear plane coordinate data points, the customized shear plane measurement die was examined in the early stage to measure the shear plane. Then, the measured 3D data points are drawn into a 3D surface map by using the Surfer drawing software to show the actual situation of the shear plane. The shear planes were analyzed from different angles by using box plots and ice crystal flower plots. Finally, the roughness of the shear plane was quantified by the root mean square of the relative undulation (Z2). Findings indicate that the shear plane undulation characteristics of coarse-grained slip zone soil are obviously correlated with the proportion of particles with a size between 5–20 mm. Moreover, whether the gradation is good or poor has a significant effect on the shear plane characteristics. The well-graded shear plane is coarser and more undulated, whereas the poorly graded shear plane is the opposite. As the normal force increases, the undulation and roughness of the shear plane decrease. As the content of coarse particles increases, the roughness of the shear plane increases, and Z2 has a highly positive linear correlation with the coarse particle content.