# Real-Time Behaviour of Dredged Slurry Treated by Air-Booster Vacuum Consolidation
## 1 Introduction  
Tideland reclamation and dredging operations in river channels produce large volumes of dredged slurry characterised by its high water content (up to 120%), large void ratio and high compressibility [1]. The dredged slurry from reclamation sites must be improved before it can be used at a construction site. Similarly, the slurry generated by river mud dredging should be reduced in volume (by drainage) before it is transported and handled. Vacuum preloading is a common engineering practice used to drain and improve soft soil [2]. The vacuum preloading method is generally considered superior to the surcharge preloading method in terms of ground stability. Specifically, vacuum pressure (~85 kPa) can be applied at a single stage for vacuum preloading, while for surcharge preloading, the load has to be applied in several stages to avoid possible stability failure of the soft ground [3,4].  
However, when the vacuum preloading method is used for dredged slurry, severe clogging often occurs, such that the rates of water drainage, pore pressure dissipation and settlement decrease substantially, even at the early or middle stage of the designed treatment period [5]. As a result, the improvement in the soil strength is severely limited and the requirements of bearing capacity and postconstruction settlement are difficult to meet. It has been noted that the clogging phenomenon does not exist in naturally consolidated soft soils (as opposed to the artificial ground made by dredging slurry) when treated by vacuum preloading. The mechanisms of clogging are still under investigation [6]and it is generally agreed that the migration of soil particles (driven by vacuum pressure) is the main cause of the clogging problem inherent to dredged slurry [7,8].  
Engineering solutions to the clogging phenomenon mentioned above include electro-osmosis [9], lime treatment [5], flocculant treatment [10,11] and air-booster vacuum preloading [1,12,13]. It has been shown in both laboratory and field tests that the air-booster system can promote the drainage consolidation of dredged slurry [13], resulting in the increased settlement, pore water pressure dissipation and shear strength of the soil. Specifically, the influences of pressurisation position [14] and duration [15] on slurry consolidation efficiency have been studied through laboratory tests, based on which optimal positions and duration times have been suggested. There are two explanations for the drainage mechanism of the air-booster system: (1) incremental stress induced by air pressure [1,12] and (2) soil cracks generated by airflow disturbance [13]. The first explanation was ascertained by in situ observations of the pore water pressure (PWP) increment slurry soil [13] during the air-boosting period, which generally lasts for 2-4 h per day for 5-7 consecutive days. However, the second explanation is only conceptual, and no direct observations of cracks have been made.  
In this study, a laboratory test of air-booster vacuum consolidation of dredged slurry was carried out. With the help of particle image velocimetry (PIV), the real-time displacement field of the slurry soil during air pressurisation was obtained, and cracks were clearly visible in the test slurry. The real-time air flux, air pressure, and PWP responses were also monitored and analysed. Theoretical predictions of crack-initiation pressure are provided and compared to those obtained from the tests. It is the first time that the cracks induced by the pressurised air are observed, which provides strong evidence for the exploration of drainage-promoting mechanism of the air-booster vacuum preloading method.