# Shear Band Characterization of Clayey Soils with Particle Image Velocimetry
## 2 Experimental Program
### Experimental Procedure and Conditions  
Each synthetic kaolinite-rich clay sample was covered with a membrane and mounted on the plane strain test apparatus for the experiments. A thin and transparent membrane was used, so that the image patterns generated for DIA could be observed. To minimize friction between the side plate and specimen, silicon grease was applied between the membrane and side plate. Similar to normal plane strain and triaxial experiments, the experiments were performed according to a saturation-consolidation-shearing process. The specimens were saturated with 100 kPa of backpressure. When the excess pore water pressure coefficient \(B\) reached 0.95 or more, the saturation was considered to be finished, and consolidation was performed, for which the specimen's bottom plate was kept in a confined state. As indicated in Table 2, the consolidation was performed at an effective vertical stress of 200 kPa and effective horizontal stress of 100 kPa under the assumption that the horizontal earth pressure \(K_{0}\) was 0.5. Then, the OC clay was unloaded at an effective vertical stress and effective horizontal stress of 50 kPa, and OCR = 4 (= 200/50) conditions were simulated. Considering that the capacity of the test apparatus cell used in this study was 300 kPa, the NC and OC clays did not have identical effective consolidation stresses. In the shearing, stage after consolidation was complete, a displacement control method was used to apply the load at a rate of 0.5%/h until failure. The vertical load was applied by raising the test apparatus cell (i.e., the specimen), while the rod attached to the load cell was fixed. To simulate the shear band formation in an actual field, the horizontal movement of the confined bottom plate, which was restrained during consolidation, was allowed to be free [6; 14]. To confirm the reproducibility, the experiments for the NC and OC clays were repeated twice, thus four sets of experiments were performed (hereinafter referred to as NC1, NC2, OC1, and OC2, respectively). Figure 3 presents the experimental flowchart. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**NC Clay**} & \multicolumn{2}{c}{**OC Clay**} & \multirow{2}{*}{\
\begin{tabular}{c} **Back Pressure** \\ **for Both Clays** \\ **(\(\sigma_{1}\)’)** \\ \end{tabular} } & \multicolumn{2}{c}{**Effective**} & \multicolumn{2}{c}{**Effective**} & \multirow{2}{*}{
\begin{tabular}{c} **Back Pressure** \\ **for Both Clays** \\ **(\(\sigma_{3}\)’)** \\ \end{tabular} } \\
**Description** & \begin{tabular}{c} **Effective** \\ **Vertical Stress** \\ **(\(\sigma_{1}\)’)** \\ \end{tabular} & \begin{tabular}{c} **Horizontal** \\ **Stress (\(\sigma_{3}\)’)** \\ \end{tabular} & \begin{tabular}{c} **Vertical Stress** \\ **(\(\sigma_{1}\)’)** \\ \end{tabular} & \begin{tabular}{c} **Horizontal** \\ **Stress (\(\sigma_{3}\)’)** \\ \end{tabular} & \
\begin{tabular}{c} **Stress (\(\sigma_{3}\)’)** \\ \end{tabular} &
\begin{tabular}{c} **(u)** \\ \end{tabular} \\ \hline Loading & 200 kPa & 100 kPa & 200 kPa & 100 kPa & 100 kPa \\ \hline Unloading & - & - & 50 kPa & 50 kPa & 100 kPa \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Loading and unloading conditions during the consolidation stage. Figure 2: Artificial texture of the surface of the clay specimen. For DIA, a 12-megapixel (\(4288\times 2848\) pixels) digital camera (Nikon D90) was used to capture images of the specimens through the transparent side plate at vertical strain increments of 0.25% in 5-min intervals. The specimens were observed and photographed through the transparent side plate of the test apparatus, which was kept perpendicular with the fixed camera's direction. To maintain a fixed light level, constant illumination was applied at an angle from both sides during the shearing process in a space where external light was blocked to obtain suitable digital images for analysis.