# Real-Time Behaviour of Dredged Slurry Treated by Air-Booster Vacuum Consolidation
## 7 Test Results
### Displacement Field of the Slurry  
From Section 7.1, it is clear that the entire air-boosting duration (10 min) can be divided into different stages, i.e., 0-7 s and 7-10 s, etc., according to the characteristics of the air pressure/flux variations. Accordingly, during the first stage (i.e., 0-7 s), a horizontal displacement field of the slurry can be constructed using the PIV technique, as shown in Figure 8a. The coloured contour shows a nearly symmetric deformation pattern; that is, the slurries on the right and left sides of the booster plate moved to the right (red colour) and left (blue), respectively. The amplitudes of both the right- and leftward displacements were the same, i.e., 1.5 mm. Since the displacement was small, no observable change happened to the slurry (see Figure 8b) at time \(t^{\star}\) = 7 s compared to the instant before air boosting (i.e., \(t^{\star}\) = 0 s). The randomly distributed cavities in the picture (indicated by the dashed circles) were due to the detachment of the soil from the glass window during the vacuum consolidation of the slurry. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Time (h)** & **Discharge of Water (g)** \\ \hline \multirow{3}{*}{Before air boosting} & \(-\)3\(\sim\)\(-\)2 h & 1.7 \\  & \(-\)2\(\sim\)1 h & 1.8 \\  & \(-\)1\(\sim\)0 h & 1.7 \\  & 0.5–1.5 h & 4.2 \\ After air boosting & 1.5–2.5 h & 2.8 \\  & 2.5–3.5 h & 1.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the water drainage before and after air boosting. A displacement field and image of the slurry are presented in Figure 9 for the time period 7-10 s of air boosting. There is an increase in the amplitude of the horizontal displacements: 3.5 mm in Figure 9a compared to 1.5 mm in Figure 8a. More importantly, the slurry above the booster plate has been mobilised by the pressurised air, contrasting with the observation from Figure 8a that observable displacements are concentrated only in zones fronting the booster plate. From the slurry image taken at \(t^{\star}\) = 10 s, small vertical cracks (extending along the axis of the booster plate) and oblique cracks (originating near the top of the booster plate) can be observed. Figure 8: PIV results for a time period of 0–7 s of air boosting: **(a)** coloured contour on horizontal displacement; **(b)** slurry picture taken on time instant \(t^{\star}\) = 7 s.  
From the horizontal displacement contour constructed for the time period 10-40 s (Figure 10a), it can be seen that the slurry was compressed with an incremental displacement amplitude of 1.5 mm. The displacement field has a more even distribution; that is, the slurry at the boundary of the observation window (\(x\) = 12 mm and \(x\) = 188 mm) was also effectively compressed compared to that at time period 7-10 s (Figure 9a). A closer observation of Figure 10a reveals that there were localised blue/red coloured zones within the red/blue coloured zones, which indicates opposite displacements occurring along the zone boundaries. This can be confirmed from the slurry picture taken at \(t^{*}\) = 40 s (Figure 10b) in which oblique cracks to both sides of the booster plate can be clearly observed. It is understood that the slurries on the two sides of the oblique crack should move in opposite directions. By comparing Figures 9b and 10b, it is observed that the vertical crack extends upward to connect with the oblique cracks. Moreover, there was an obvious increase in the opening widths of both the vertical and oblique cracks. Figure 9: PIV results for a time period of 7-10 s of air boosting: (**a**) coloured contour on horizontal displacement; (**b**) slurry picture taken on time instant \(t^{*}\) = 10 s.  
Within the following 30 s of air pressurisation (i.e., time period 40-70 s), the dominant slurry displacement field shrinks to be close to the booster plate (Figure 11a), with the largest displacement increments existing along lines oblique to the axis of the booster plate.