# Improved Mandel System for Prefabricated Vertical Drain Installation: A Macro to Micro Analysis
## 4 Results and Discussion
### Microstructure Analysis  
#### 4.2.1 PVD Structure  
1. PVD filter and pore interaction before vacuum preloading  
A PVD is typically composed of a geotextile filter-wrapped plastic band with molded channels designed to aid in the consolidation of soils. These serve as drainage pathways for pore water in soft compressible soils that consolidate more quickly when subjected to continuous vacuum preloading [33]. The PVD filter should meet the filter design criteria as they are important for enhancing the PVD performance. A commonly used criterion was given in previous studies [34; 35; 36; 37]. The SEM pictures from the three types of PVD before vacuum preloading showed the arrangement of the straight filter and pore area. At 500\(\times\) magnification, microscopic images in representative sections of the three types of PVD were captured, as shown in Figure 12. The pore size or the apparent opening size (AOS) must be small enough to prevent the fine particles of the soil from entering the filter and the drain. The standards [34; 35] commonly used can be expressed as  
\[O_{95}\leq(2-3)D_{85} \tag{13}\]  
\[O_{95}\leq(4-7.5)D_{85} \tag{14}\]  
\[O_{50}\leq(10-12)D_{50} \tag{15}\]  
where \(O_{95}\) is the AOS of the filter, \(D_{85}\) is the size larger than 50% of the fabric pores, and \(D_{85}\) and \(D_{50}\) refer to the sizes for 85% and 50% of particles in the soil by weight. \(O_{95}\leq 0.075\) m (or 75 mm) is often specified for a prefabricated vertical drain. As a result, to achieve the goals of the vacuum preloading effect, it is critical to select a suitable pore size of the filter. In addition, to interpret the SEM images, the pores and PVD filter size in the area were analyzed as shown in Figure 13.  
The results show that the pore size area of the filter for PVD types A, B, and C varied in the range of 0.1-12.136 \(\upmu\)m. The average \(O_{95}\) values for PVD filters types A, B, and C of 83, 116, and 265 \(\upmu\)m were employed in this study according to the criterion proposed in [35]. Figure 12: SEM images of PVD structure: (**a**) type A; (**b**) type B; (**c**) type C.  
In addition, quantitative analysis of the soil microstructure area showed that PVD type B had a large filter area of 55% and a pore area of 45%, whereas types A and C had a filter area of 43% and a higher pore area of 57%. Therefore, the SEM images could characterize PVD filters with different pore sizes. It is essential to observe the PVD type, and a large PVD filter ensures a higher drainage rate. A large PVD filter area in percentage means that more filter material is present per unit area of soil, thus increasing the filtering capacity of the system and helping prevent soil particles from clogging the PVD [38]. However, if the pore area is smaller in percentage, there is less space available for water to flow through the soil and into the PVDs; this can reduce the overall drainage capacity of the system and slow down the consolidation process. The balance between the PVD filter and pore areas is important in designing an effective PVD system. The filter material must be sufficient to prevent clogging and maintain long-term performance. At the same time, the available pore space must be adequate to allow for efficient drainage and consolidation. Therefore, selecting the PVD type is essential to ensure its successful implementation in the field. According to this study, PVD type B over types A and C according to the PVD filter structure. 2. The PVD filter and pore interaction after vacuum preloading  
The interaction between the PVD filter and the pores, also known as clogging, which is caused by vacuum preloading, depends on several factors, including the properties of the soil, the filter material, and the drainage capacity of the system [38; 39; 40]. On the other hand, if the filter material has a low permeability or the PVD system is not designed correctly, the interaction between the PVD filter and pores may not be optimal [41]. This can lead to clogging of the filter and a reduction in the drainage capacity of the system.