# Shear Band Characterization of Clayey Soils with Particle Image Velocimetry
## 5 Conclusions  
This study evaluated the failure behavior of NC and OC clays based on their deformation and shear band characteristics as vertical stress was applied. Compressive tests were performed on reconstituted kaolinite under plane strain conditions, and digital images of the specimens were captured during the shearing process for analysis. The deformation behavior of the specimens was evaluated according to the vertical and horizontal displacements and the strain increment contour lines. The shear bands were characterized according to their slope and thickness. The conclusions are as follows:  
1. Both clays showed no clear shear deformation in the initial stage (\(\epsilon_{a}\) = 0-0.25%). In the peak stage (\(\epsilon_{a}\) = 0.25-2.00%), the shear deformation started to become concentrated, but a clear failure surface still had not appeared. The development of the failure surface tended to be more delayed in the OC clay. 2. In the softening stage (\(\epsilon_{\rm a}\) = 2.00-5.00%), the specimens were divided into a shear band where the shear deformation was clearly concentrated as well as top and bottom parts where only rigid body movement occurred without shear deformation. In the steady-state stage (\(\epsilon_{\rm a}\) > 5.0%), the development of the shear band was completed, and the excess pore pressure remained at a fixed level. 3. The slopes of the shear bands that formed in the NC and OC clays were compared to those calculated from existing equations. In the softening stage, the shear band slopes of both clays were similar to that of Roscoe [35]. In the steady-state stage, the slope of the final shear band was between those of [PERSON] and [PERSON] et al. [36] for the NC clay and similar to that of [PERSON] et al. [36] for the OC clay. This indicates that the dilatation angle mainly influences the softening stage when the shear band is continuously developing and that the internal friction angle influences the steady-state stage when the shear band slope is gradually increasing. 4. For both clays, the shear band thickness tended to gradually increase as the shear band developed. In the softening stage, the thicknesses of the NC and OC clays were 4.42, 5.20, 3.84, and 4.66 mm, and they gradually increased up to 7.56, 6.34, 6.56, and 7.01 mm, respectively. The thickness of shear band of clayey soil did not show any particular tendency between NC and OC clays, but further research work with the experiments should be performed under the same effective confining stress of NC and OC clays. The results of this study show the characteristics of shear bands that occur in clay and can be used to predict failure behavior and guide reinforcement at actual sites with soft ground. In addition, these experimental results can be extended to modeling the failure behavior of clayey ground. However, this study used one type of clay (commercial-grade EPK kaolinite) that has a low plasticity and low compressibility. Caution should be exercised when predicting the failure behavior in different soil (e.g., highly compressible or collapsible soil), whose shearing characteristics may considerably differ from those of the tested soil. The applicability of the results of this study should, thus, be verified for other types of clayey soil that exhibit significantly different plasticity or compressibility. Conceptualization, T.-Y.K. and C.-K.C. ; methodology, J.K.; software, T.-Y.K. ; validation, K.-H.P., C.-K.C., and S.-H.B. ; formal analysis, T.-Y.K. and S.-H.B. ; investigation, T.-Y.K. ; resources, C.-K.C. ; data curation, T.-Y.K. ; writing--original draft preparation, T.-Y.K. and K.-H.P. ; writing--review and editing, S.-H.B. ; visualization, T.-Y.K. and J.K.; and supervision, C.-K.C. All authors have read and agreed to the published version of the manuscript. This research was supported by a grant (code: 19 SCIP-C151438-01) from Construction Technologies Program funded by Ministry of Land, Infrastructure and Transport of Korean government.