## 1 Introduction  
With the continuous development of the economy, land resources are becoming increasingly scarce. As a result, coastal countries and regions commonly resort to land reclamation to expand usable land areas [1, 2]. The slurry used for land reclamation is primarily sourced from the nearby seabed and is characterized by high initial water content and extremely low strength [3, 4, 5, 6]. Therefore, the development of a rapid and effective dewatering method for dredged slurry has become an urgent challenge in geotechnical engineering [7]. Currently, common slurry dewatering techniques in engineering practice include vacuum preloading, surcharge preloading, and the geotextile tube method [8, 9, 10, 11]. Among these, the vacuum preloading method has been widely adopted in slurry treatment projects [12, 13, 14] due to its cost-effectiveness and relatively simple construction process [15, 16]. However, when applied to slurry with a high clay particle content, the dewatering efficiency of the vacuum preloading method is often significantly reduced [17]. This decline in efficiency is primarily attributed to the formation of clogging zones, where soil particles accumulate near the prefabricated vertical drain (PVD) under vacuum pressure, resulting in an extremely low-permeability region [18; 19]. The vacuum pressure experiences substantial losses as it propagates through the clogging zone, ultimately preventing the vacuum preloading method from achieving the desired soil reinforcement effect [20]. Consequently, mitigating clogging remains the most critical challenge in the application of vacuum preloading for slurry treatment. Flocculants are chemical additives that enhance slurry dewatering performance through mechanisms like electrostatic neutralization, adsorption bridging, and sweep flocculation. These processes reduce the interparticle distance within the slurry and promote the aggregation of particles into floc structures, thereby improving its dewatering efficiency [21; 22; 23; 24]. The integration of flocculants with vacuum preloading, known as the flocculation combined with vacuum preloading method, has been shown to significantly enhance slurry dewatering performance [12; 25; 26]. Numerous studies have investigated the treatment effects and underlying mechanisms of this method. [PERSON] et al. [27] conducted laboratory tests and found that APAM effectively mitigated the clogging of slurry while improving both the consolidation rate and the final consolidation effect. [PERSON] et al. [28] applied APAM and FeCl\({}_{3}\) in flocculation combined with vacuum preloading treatment of dredged slurry, reporting increases in water discharge volume by 56.8% and 46.5%, respectively, compared to traditional vacuum preloading. [PERSON] et al. [29] and [PERSON] et al. [30] investigated the vacuum preloading treatment of lime-flocculated slurry and observed that lime treatment transformed fine micrometer-scale particles into millimeter-scale aggregates. This modification significantly enhanced slurry permeability, reduced the clogging of slurry around prefabricated vertical drains (PVDs), and improved consolidation efficiency. [PERSON] et al. [31] further explored the dual use of lime and APAM in treating high-water-content slurry via vacuum preloading, demonstrating that flocculation treatment improved consolidation efficiency, increased slurry strength, and reduced clogging. In summary, existing research indicates that flocculation treatment effectively alleviates the clogging of slurry and enhances the efficiency of vacuum preloading. However, its influence on the development of the clogging zone during vacuum preloading tests has primarily been inferred through indirect indicators, such as the water discharge rate and consolidation efficiency. A direct observation method for assessing clogging zone evolution remains lacking. In recent years, the PIV technique has played a significant role in geotechnical engineering [32; 33; 34].