Abstract: In this study, a visual medium-scale direct shear test is carried out on the sliding zone soil with different coarse particle strengths. The spatial information of the shear band is obtained by placing a vertical aluminum wire to observe its deformation after shearing, and the spatial surface equation of the shear band is established. Particle image velocimetry (PIV) technology is used to extract and compare the 2D shear band information at the visible surface with the boundary extrapolation value of the space surface equation obtained from the test, demonstrating that the spatial surface equation and PIV technology can describe the characteristics of shear band. Then, PIV technology is used to analyze the evolution rule of shear band under different total and specific displacements. Finally, the influence of prefabricated damage and coarse particle strength on shear band characteristics was analyzed. Results show that the thickness of shear band presents a distribution pattern of narrow ends and wide middle, and its shape can be fitted by Gaussian surface equation. The shear band undergoes four stages during its development: compaction, free damage, damage development, and penetration. Damage causes early development of shear bands at various stages. Furthermore, coarse particle strength exerts a greater effect on the deformation of local shear bands and a smaller effect on the overall shear band. These findings hold significant implications for elucidating the formation and evolution of landslide shear bands and designing a rational slope control plan,

Key words: coarse-grained sliding zone soil, shear band, medium-scale direct shear test, visualization, PIV technology