# Shear Band Characterization of Clayey Soils with Particle Image Velocimetry
## 4 Experimental Results and Discussion
### Localized Displacement Increment Field  
To analyze the spatial distribution and evolution of deformation for the NC clay and OC clay in each stage, PIV was performed at vertical strain intervals of 0.25%. As shown by the red bars in Figure 7, vertical strain intervals that showed typical deformation behavior were selected for each stage of the stress-strain behavior. The vertical and horizontal contours for each strain interval were diagrammed based on the displacement vectors from the PIV analysis. The positive (+) values are in blue and indicate downward and rightward displacement for the vertical and horizontal displacement contours, respectively. The negative (-) values are in red and indicate upward and leftward displacement for the vertical and horizontal displacement contours, respectively. As aforementioned, the experiments were repeated twice to verify reproducibility (NC1, NC2, OC1, and OC2); the trends were similar, thus only representative figures evaluated in one test for the same conditions are presented in this paper (NC1 and OC1). PIV was performed on the 0-0.25% strain interval in the initial stage immediately after axial stress was applied. Figure 8 shows the internal displacements in the NC and OC clays. As the vertical load was applied by raising the specimen, a large overall upward displacement occurred following the vertical upward movement of the specimen during shearing for both clays. However, no distinct deformation characteristics were identified with the vertical and horizontal contour lines. The initial stage in NC and OC clay showed a similar tendency with that of granular soils [17]. Figure 9 shows the displacement contours for the peak stage with 1.65-1.9% strain, which had the maximum deviatoric stress for both clays. In the case of the NC clay, the horizontal displacement contours indicate that the upper part of the specimen moved left and the lower part moved right centered around a point about one-third from the top of the specimen (Figure 9a). The vertical displacement contour lines indicate that the bottom part of the specimen showed a greater upward displacement than the top part centered around a point about one-third from the top. The top and bottom parts showed opposite displacement behaviors at the boundary of the potential failure surface (Figure 9b). For the OC clay, the top part of the specimen showed leftward movement and the bottom part showed rightward movement centered around a point one-third from the bottom of the specimen (Figure 9c). The specimen's bottom part showed greater upward displacement than the top part (Figure 9d). However, there was no clear difference in displacement at each location in the OC clay specimen compared to the NC clay in the peak stage. This confirmed that the development of the potential failure surface during shearing was delayed in the OC clay compared to the NC clay. In addition, in the case of granular soils [17], shear planes were clearly formed at the peak stage. Figure 8: Displacement contours for the initial stage of stress–strain behavior (0–0.25% strain): (**a**) NC1 horizontal; (**b**) NC1 vertical; (**c**) OC1 horizontal; and (**d**) OC1 vertical. Figure 9: Displacement contours for the peak stage of stress–strain behavior (1.65–1.9% strain): (**a**) NC1 horizontal; (**b**) NC1 vertical; (**c**) OC1 horizontal; and (**d**) OC1 vertical. Comparing the results between the sandy and clayey soils, it was found that shear surface formation was delayed in the clay. In the softening stage, the NC and OC clays were both divided into a shear band zone where the shear deformation was clearly concentrated and upper and lower sliding wedges occurred only where rigid-body movement occurred. Opposite behaviors occurred in the top and bottom parts centered on the shear band zone (Figure 10). The NC clay showed a maximum leftward horizontal displacement of 0.12 mm in the specimen's top part but almost no vertical displacement (Figure 10a).