Effect of Installation Pattern of Prefabricated Vertical Drain (PVD) on Degree of Consolidation in Soft Soils

ABSTRACT


INTRODUCTION
Prefabricated Vertical Drain (PVD) is a soil improvement method that functions to drain water from the soil pores, thus accelerating the process of land subsidence. This occurs in soft clay soils with high groundwater levels. With the presence of PVD, the degree of consolidation increases in line with changes in pore water pressure and also increases soil shear strength so that PVD can be used as a soil improvement method for very soft clay soils. [1]. PVD installation is usually combined with preloading to speed up the settlement process. Combining embankment and PVD on the weir can improve embankment performance compared to using only embankment alone [2] [10]. Preloading loads can be in the form of temporary and permanent loads and can be carried out in stages. According to Wang et al. [3], the secondary consolidation settlement continues in a gradual embankment after unloading, but the settlement amount is minimal. The main advantage of PVD on soft soils is that it can significantly accelerate soil settlement [4] [11] [12] [13] [14] [15]. The time needed to achieve a degree of consolidation of 90% is highly dependent on the PVD installation pattern and the distance between the PVD installations. Padang -Pekanbaru Toll Road Section I Section I Padang -Sicincin passes through several areas with subgrade conditions which are soft soil and are dominated by swampland with high groundwater levels. Based on the soil investigation results using the Standard Penetration Test (SPT) and drilling logs at STA 3+850, soft soil is located at 8.4 m. If the embankment is placed on soft ground, the soil will experience excessive consolidation settlement for a long time. This condition will damage the road pavement structure built on the piledoil [5]. This paper discusses the effect of the PVD installation pattern on changes in the degree of consolidation (U90). PVD installed with a triangular pattern and a rectangular pattern with variations in the distance between the PVD (1 m, 1.3 m, 1.6 m, 1.9, 2.2 m, 2.5 m, 2.8 m, and 3.1 m) with a preload of 5.8 m. The results of this study are expected to determine a more effective installation pattern and spacing to be applied in the field.
PVD is an artificial drainage system installed vertically in a soft soil layer. This vertical drainage system has the form of a belt with a rectangular cross-section, consisting of an outer part in the form of a filter made of synthetic materials such as geotextiles, paper, or jute and an inner part that functions as a water flow medium made of plastic or organic fibers. PVD serves to accelerate the process of settlement in soft soils. The rise/lower time using a PVD depends highly on the distance and installation pattern. The presence of PVD will cause radial/horizontal pore water flow in addition to vertical flow, which causes pore water to be removed more quickly [6]. Guo et al. (2018) examined comparing the consolidation degree with the proposed Asoka methods. The analysis showed that the proposed method's ultimate settlement value was smaller than the Asoka method. The proposed method could predict by considering horizontal and vertical flow directions [7].
According to Aspara et al., achieving a degree of consolidation of 90% on clay at the Riau PLTU Project site takes six months with an installation configuration with a Triangle pattern. [6]. Installation of PVD with preloading at Juanda Airport in Surabaya at the taxiway location. PVD is installed in a triangular pattern with a distance of 1 m and a depth of 20 m. The degree of consolidation of 90% is achieved within 11.5 weeks; without using PVD, 0% is completed within 87, 78 years [8].

Secondary Data
This study's secondary data comprised the boring log and N-SPT obtained from the Padang-Sicincin toll road construction project (PT. Hutama Karya).

Correlation of N-SPT Values to Soil Parameters.
The soil parameter values were obtained from the results of the correlation of N-SPT values consisting of cohesion values (c) and internal shear angles (), and consolidation coefficient (Cv).

Land subsidence (Settlement)
Land subsidence consists of immediate settlement, primary consolidation settlement, and secondary consolidation settlement [9]

Immediate drop(immediate settlement)
Settlement occurs immediately after workload, and soil grains are deformed without changing the water content.

Primary consolidation decline
Primary consolidation settlement occurs after the load is applied and there is a change in excess pore pressure, resulting in a reduction in volume which causes land subsidence.

Depreciation Secondary consolidation
Secondary consolidation settlement occurs after the pore water has been completely dissipated, and the settlement occurs only due to the adjustment of the soil grains.

Vertical Consolidation Coefficient (Cv)
To calculate the magnitude of the consolidation coefficient used, the Taylor method.

Planning of Prefabricated Vertical Drain (PVD)
PVD serves to shorten the path of the water to the drain (radial and vertical directions) so that the time to reach the 90% degree of consolidation can be shorter ( figure 2). The flow will be steep if you don't use PVD (figure 1).

Consolidation Time (t) with PVD
The determination of consolidation time (t) when using PVD has been carried out by combining the theories of Kjellman and Baron (1948), as has been done by Hansbo (1970). The equation expresses the result for consolidation time: Where: t: the time required to reach the degree of consolidation D: equivalent diameter of the circle of influence Ch: horizontal consolidation coefficient Uh: degree of consolidation in the horizontal direction

The average degree of consolidation (U average)
For drainage in the vertical and radial directions, to obtain the combined average degree of consolidation expressed by the equation (Carillo, 1942) )]

Basic Soil data analysis
Field test results include Standard Penetration Test (SPT) data, drill logs, and laboratory tests on soil physical and mechanical properties.Padang-Sicincin toll road.

Time required to reach 90% degree of consolidation (without PVD)
At the research location of the Padang -Sicincin toll road STA 3+850. The analysis results show that the average consolidation coefficient for each layer is 3.986 m2/year, with 2-way drainage (double drainage). To reach a degree of consolidation of 90%, it takes 3,153 years ( Figure 5).

Prefabricated Vertical Drain (PVD)
This paper's PVD dimensions are a = 100 mm and b = 5 mm. Triangular pattern and quadrilateral pattern with variations in the distance between the PVDs, namely 1 m, 1.3 m, 1.6 m, 1.9 m, and 2.2 m, with an embankment height of 5.8 m.

The average degree of consolidation
The results of the analysis show that the average degree of consolidation (Average) is more significant with increasing time (tables 3 and 4).  In the rectangular installation pattern (Figure 7) with an installation distance between PVDs of 1 m, a degree of consolidation of 90% was achieved in 1 month, and a PVD installation distance of 3.1 m was completed in 9.5 months  Tables 5 and 6 show that the triangular pairing pattern is faster to reach a degree of consolidation of 90% compared to the quadrilateral pattern.  Figure 8 shows that with increasing installation distance between PVDs (s) and significantly increasing time (t) to reach a 90% degree of consolidation, overall installation with a triangular pattern requires a shorter time when compared with a rectangular installation pattern.

CONCLUSION
If the soil receives preloading loads, consolidation increases with time. The average degree of consolidation is inversely proportional to the distance between the PVDs. The greater the distance between the PVDs, the smaller the moderate degree of consolidation and vice versa. To achieve a degree of consolidation of 90%, the PVD installation pattern with a triangular pattern is more effective when compared to a rectangular installation pattern. From the results of this study, several suggestions need to be considered. Further research on various soil types and reflections on the PVD material is required.