# Real-Time Behaviour of Dredged Slurry Treated by Air-Booster Vacuum Consolidation
## 8 Discussion  
### Stages of Soil Behaviour Subjected to Air Pressurisation  
According to the real-time observations of the boosting air flux and pressure, the displacement contours of the soil and excess PWP responses and the soil behaviours under air pressurisation can be qualitatively divided into four stages:  
1. Cavity expansion stage (0-7 s): The soil surrounding the booster plate expands radially in this stage, i.e., the pressurised air continuously compresses the surrounding soil along the direction perpendicular to the face of the booster plate. As a result, the soils to the left and right sides of the booster plate move to the left and right, respectively, as shown in Figure 8. The farther the soil is from the booster plate, the smaller its displacement is. No cracks are generated in this stage, since the air pressure rises steeply, while the air flux is negligible (Figure 7).
2. Crack initiation stage (7-10 s): The air pressure drops suddenly and the air flux increases substantially, which indicates that cracks have been generated in the surrounding soil, which provides airflow paths. Accompanying the increased air flow, the air pressure drops [19; 20; 21]. Figure 9b shows an image of the cracks. Combined with the displacement contours in Figure 9a, it is found that a vertical crack was generated in the soils right in front of the booster plate. An oblique crack appears in the soil on top of the plate, as schematically shown in Figure 15. The vertical and oblique cracks might be caused by tensile and shear failures of the soil, respectively. From Figure 7b, the crack initiation pressure of the slurry soil can be estimated as 8 kPa.
3. Crack propagation stage (10-70 s): With continuous air inflation, the vertical and oblique cracks become elongated and connected, as shown in Figure 10b. The air flux increases due to the greater number of connected crack channels, as indicated in Figure 16. In addition, the air pressure increases, since additional pressure is needed for continuous soil fracturing at the crack tips (Figure 7). In the first 30 s of this stage, crack propagation is mainly reflected as a widening of the cracks, as shown in Figure 11b. Correspondingly, steady increases in both the air flux and pressure can be observed. For the remaining 30 s of this stage (i.e., \(t^{\star}\) = 40-70 s), the vertical and oblique cracks propagate and become connected. Thus, the air flux increases slightly  
Figure 14: Time history of the excess pore water pressure during the air boosting.  
in a fluctuant manner, while the air pressure decreases gradually. Figure 7b shows that the pressure required for the formation of Y-shaped cracks is approximately 11 kPa.
4. Crack stabilisation stage (70 s and after): The air pressure and flux remain almost constant during 70-90 s of air boosting (Figure 7). Moreover, the cracks in the image in Figure 12b are almost the same as those in Figure 11b. These observations indicate that the soil cracks have entered into a steady state. The air pressure required to maintain the crack opening is approximately 10.5 kPa (Figure 7b), which is slightly less than that required for crack propagation.  
At the end of pressurisation, elastic rebound will occur in the soils on both sides of the cracks, since the supporting air pressure is lost. After elastic rebound, the Y-shaped crack remains; however, its opening width reduces significantly, as shown schematically in Figure 17. The unclosed cracks serve as a fast track for pore water transportation, which promotes seepage consolidation of the slurry during the vacuum preloading period following air boosting. This explains the variations in water yield described in Section 7.2.  
Figure 16: Schematic view of the connected cracks during the crack propagating stage.  
Figure 15: Schematic view of the crack distributions during the soil cracking stage.