The air-booster vacuum preloading method has been applied to slurry ground improvement. It is based on the conventional vacuum preloading method but with an additional injection of pressurised air into the soil via pre-installed conductors. The drainage effect of air-booster vacuum preloading has been demonstrated by past studies; however, direct observations of the real-time behaviour of slurries subjected to boosted air remain lacking. This study used a combined monitoring technique that included particle image velocimetry, pore water/air pressure gauges, a vortex flowmeter and an electronic balance to conduct a laboratory test of air-booster vacuum consolidation of dredged slurry. The tests allowed analyses of (1) the real-time displacement field of the slurry, (2) the pressure–flux relationship of the pressurised air, and (3) the pore water pressure responses during air boosting. The first aspect allowed direct observation of small-crack initialisation and propagation during pressurisation; while the latter two confirmed the crack initiation based on drops in air and pore water pressures. The measured crack initiation pressure was verified by comparison with theoretical predictions. The results demonstrate that pressurised air induces cracks in soil, which promote the drainage consolidation of dredged slurry.

The vacuum preloading method is commonly adopted for improving the soft ground that the embankment of the railway line is laid on. The PIV (Particle Image Velocimetry) technique is a powerful tool in observing the formation of the soil column, a phenomenon that is unique to the dredged slurry when treated by vacuum preloading. However, it is not clear to what extent the motions of the slurry particles can be represented by the PIV tracers. In this paper, a mesoscopic model has been established by using the CFD-DEM method to reproduce the vacuum consolidation process of the slurry, in which the PVD (Prefabricated Vertical Drain) membrane, the slurry particles, and the tracers are described by the DEM, and the pore water is governed by the CFD method. Eight computational cases that can cover a broad range of material parameters governing the PIV model tests on the dredged slurry have been designed and studied by the established model. The representativeness of the PIV tracer is evaluated by comparing the statistic displacement of the tracer to that of the slurry particles. It is found that for the commonly used tracer, the carbon powder, can reliably represent the particle motions of the slurry since the difference in displacements of the tracer and the slurry particles is smaller than 6.5% if the diameter ratio between the tracer and the slurry particle is within 1.8.

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