###### Abstract  
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. CFD-DEM; dredged slurry; tracer; particle image velocimetry; particle movement 26 August from the natural soil that is of stable soil skeleton, the dredged slurry is of extremely high-water content, and thus the fine particles in the slurry are generally in the suspended or loose-contacting status. Under the vacuum gradient, the slurry particles would migrate towards and then accumulate onto the PVD membrane, which forms the soil column with the PVD along its axis [7; 8]. The soil column that is densely packed by the vacuum gradient is of extremely low permeability and thus impedes the drainage of the surrounding slurry [9]. With the realization of the physical importance of particle movements in explaining the inefficiency problem, both numerical and model-test studies have been carried out in the literature to characterize the particle motion and packing driven by the vacuum gradient in the slurry. Since the slurry contains both the solid and liquid phases, the current numerical studies generally adopt the two-phase approaches, e.g., the PFC software and the CFD-DEM method, in which the slurry particle is described by the DEM (Discrete Element Method) and the pore water is governed by the CFD (Computational Fluid Dynamics). The dynamic interactions between the particle and the fluid phases can be readily taken into account through the coupled CFD-DEM solver [10]. Based on the method, the problems of upward seepage [11], the soil consolidation [12], the underwater sedimentation [13], and the pipe erosion [14] can be modeled. Recently, a CFD-DEM-based model was established by [PERSON] et al. [8] for reproducing the soil-column forming process under different vacuum pressures. Moreover, more refined models were built by [PERSON] et al. [15] and [PERSON] et al. [16] for investigating the jamming problem of the orifice of PVD membrane by the slurry particles. The CFD-DEM method is superior in explicitly resolving the motions of each slurry particle, which can help to understand the soil column phenomenon from a microscopic-scale view. However, the method may be subjected to severe limitations when it is used for modeling a real engineering-scale problem since enormous particles would be involved, and then the computational cost would be too high. Alternatively, the model test is a generally accepted method for investigating problems of practical-engineering scale.