# Improved Mandel System for Prefabricated Vertical Drain Installation: A Macro to Micro Analysis
###### Abstract  
Increasing development of infrastructure in Indonesia has driven the need for effective ground improvement methods to accelerate the consolidation of soft soil, which is estimated to occupy around 10% of the country's land area. A prefabricated vertical drain combined with vacuum preloading is among the most effective methods for this purpose. However, the prefabricated vertical drain creates a smear zone in the surrounding soil area during installation. This study examines the effectiveness of a newly developed mandrel system in reducing the smear zone during prefabricated vertical drain installation. Large-scale consolidation tests at a macro level and microstructure analysis using scanning electron microscopy at a micro level were employed to investigate the effect of soil water content and shear strength. The results show that the water content and shear strength of the soft soil gradually increased in the inner smear zone and transition zone, while both decreased in the radial distance. Furthermore, the soil structure underwent a transformation in which the particle area and pore area became a closed flake structure, and apparent agglomeration occurred. The test results indicate that the newly developed mandrel system can effectively reduce the smear zone. The macro to micro test results demonstrated that the mandrel system is successful in reducing the smear zone effect.  
ground improvement; vacuum preloading; PVD; microstructure analysis; smear zone Article Improved Mandel System for Prefabricated Vertical Drain Installation: A Macro to Micro Analysis  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], A. Improved Mandel System for Prefabricated Vertical Drain Installation: A Macro to Micro Analysis  
Analysis. April. 2020213673  
the smear zone surrounding the PVD, while also remarkably affecting the consolidation rate [10]. Lastly, the smear zone acts as a barrier to the flow toward the PVD by reducing the permeability, and affecting the consolidation requires a far longer time [11].  
The size and shape of the mandrel are among the factors influencing the smear zone effect in the field [12]. Various studies have shown that choosing a mandrel with optimal size and shape can reduce soil disturbance during installation and improve drainage performance [13; 14; 15]. The actual band shape of the PVD and hexagonal zone of influence cause less disturbance compared to other shapes, emphasizing the need for appropriate mandrel selection [16]. The radial extent of the smear zone is typically measured in terms of the equivalent mandrel diameter, which is calculated by equating the mandrel's actual cross-sectional area to an equivalent circular area [17; 18]. Developing mandrel systems based on predetermined shapes and sizes is one of the solutions to reduce the smear zone effect during PVD installation. Additionally, selecting an appropriate PVD core is critical, as it influences the consolidation rate [19; 20]. Although previous studies provided valuable insights, there is still a need for further research to improve mandrel systems that can effectively minimize the smear zone during PVD installation and improve drainage performance.  
In this paper, we used one designed rectangular mandrel system and three types of PVD cores by large-scale consolidation testing in the laboratory (macro level). Microstructure analysis using a scanning electron microscope (SEM) examined the effect of the mandrel system due to PVD combined with vacuum preloading performance (micro level). The effects of water content, shear strength, pore area, and particle area are also discussed in this study. For field application, the experimental results have guided relevance for developing an effective method to reduce the smear zone.