# Shear Band Characterization of Clayey Soils with Particle Image Velocimetry
## 4 Experimental Results and Discussion  
### Global Stress-Strain Relationship during Shearing  
Figure 7 shows the stress-strain behavior of the NC and OC clays during the plane strain test. The stress-strain behavior can be divided into four stages according to the strain: (1) initial (0-0.25%); (2) peak (0.25-2.00%); (3) softening (2.00-5.00%); and (4) steady state (>5.00%). The stages are divided by dotted lines in Figure 7, and DIA was performed in the intervals shown as red bars.  
For the NC clay (both NC1 and NC2), the vertical load was applied at a deviatoric stress of 100 kPa (confining stress of 100 kPa) immediately after anisotropic consolidation (see Table 2). In the initial stage of NC1 and NC2, the deviatoric stress increased to 120 and 117 kPa, and the excess pore pressure increased to 18 and 11 kPa during the period with a low deformation of 0.25% (around 30 min), respectively. In the peak stage, the deviatoric stress slowly increased within the interval of 0.25%-2.00% strain, and the maximum deviatoric stress of NC1 was 125.9 kPa at 1.66% strain, while it reached 124.3 kPa at 1.30% strain in NC2. The excess pore pressure of NC1 and NC2 in the peak stage continuously increased to 50 and 54 kPa as the shearing progressed, after which the rate of increase gradually slowed. At the interval of 2.00%-5.00% strain, softening behavior was observed, in which the deviatoric stress rapidly decreased, and the excess pore pressure gradually increased to 63 kPa for NC1 (pressure increment of 4.9 kPa/1% strain) and 64 kPa for NC2 (pressure increment of 4.1 kPa/1%  
Figure 6: Selected pixel subsets and center points for the PIV technique.  
Figure 7: Stress–strain behavior for normally consolidated (NC) and over-consolidated (OC) clays.  
strain) before stabilizing. In the steady-state stage, the deviatoric stress and excess pore pressure both remained stable without large changes. This confirmed that the failure was complete.  
For the OC clay (both OC1 and OC2), isotropic consolidation was performed in the consolidation stage. Vertical loading was performed at a deviatoric stress of 0 kPa after loading-unloading of the consolidation (confining stress of 50 kPa) (see Table 2). In the initial stage (0%-0.25% strain) of OC1 and OC2, the deviatoric stress increased to 77 and 86 kPa, and the excess pore pressure increased to 15 and 14 kPa, respectively. The deviatoric stress increased continuously up to 0.99% strain and reached around 100 kPa, but the rate of increase in the stress then gradually slowed down. Up to 1.75% strain, peak deviatoric stresses of 105.6 and 104.1 kPa were observed in OC1 and OC2, respectively. The peak deviatoric stress was less than that of the NC clay because the shearing process took place at half the confining pressure. The commonly observed trend in which the OC clay has a greater peak deviatoric stress than the NC clay would most likely be observed in experiments in which the confining pressures for the two clays were the same. The excess pore pressure decreased until the peak deviatoric stress was reached. In the softening stage, the deviatoric stress and excess pore pressure tended to decrease at fixed rates. Similar to the trend for the excess pore pressure in this study, [PERSON] et al. [33] reported that OC clay with an OCR of 4 (i.e., light to moderate OC clay) showed a decrease in the excess pore pressure in the peak stage, which then increased again due to localization in the softening stage (the excess pore pressure was less than that of the NC clay). In the steady-state stage, the deviatoric stress and excess pore pressure both remained stable without great changes, as was the case with the NC clay.