Comparative Analysis of underconsolidated soil improvement with Pile Raft and Pile Bent Structures on Toll Road Project in Tangerang

Soft soils undergo consolidation when subjected to additional loads, owing to their low shear strength and high compressibility. This research conducts a comparative analysis of soil improvement techniques, specifically examining the application of pile raft and pile bent structures, with a focus on safety factors and construction time. The study employs the finite element method, incorporating hardening soil parameters to simulate the soil behavior and embedded beam parameters to model the pile raft and pile bent structures. The analysis adheres to the SNI Geotechnical 8460:2017 standard for assessing geotechnical safety. The findings of this investigation reveal that the utilization of pile bent structures offers greater efficiency in terms of accelerated construction timelines and favorable safety factors when compared to meticulous soil improvement practices. The latter necessitates an extended duration due to the substantial embankment process, often reaching heights of up to 12 meters, while pile bent structures only require a 3-4 meter embankment as a platform, with the remainder of the structure designed to meet the required elevation. This research underscores the advantages of employing pile bent structures in projects involving underconsolidated soil, offering valuable insights for the construction industry and potentially influencing future engineering practices in similar contexts.


Introduction
The definition of soft soil according to Rachlan [1] and Bina Marga [2] is soil, which generally consists of clay, including very poor foundation material owing to its high water content, low permeability, and very compressible soil that can be visually penetrated with a minimum thumb.as deep as ± 25 mm, or has a shear strength of 40 kPa based on field propeller shear tests.According to (Seotjiono, 2008;Pasaribu, 2008), soft soil is weak and naturally formed from the deposition process as an alluvial layer, usually found on alluvial plains, which are very compressive and have low shear strength, which is the undrained shear strength of the field.less than 40 kPa and high compressibility [1].
IOP Publishing doi:10.1088/1755-1315/1324/1/012037 2 Pile Raft is a method of improving the soil carrying capacity by fixing mini piles at the planned points and configurations, and then attaching a concrete slab to the top of the mini pile.The consolidation process certainly occurs in soft soils, but the pile raft method slows down the consolidation rate in the long term with small settlements.This is due to the carrying capacity of the mini pile and concrete slab which seems to be floating above it [2].Pile Bent is a soil improvement solution with high compressibility.The pile bent system is almost the same as the slab pile system, which uses the same pile foundation for the fixed-length and free-length sections.The difference lies in the type of pole used.A pile bent uses drilled piles, whereas a pile slab uses prefabricated piles.As for the upper structure, they both use slabs made of PCI girder.A pile Slab or Pile bent structure consists of a slab, pile head and pile foundation.The standard loading on the structure refers to SNI T-02-2005 the Standard loading for Bridges.The pile foundation modelling uses an elastic spring support model, which can represent the power support of the pile foundation.

Soil
Soil is defined as a material consisting of aggregates (granules) of solid minerals that are not cemented (chemically bound) to one another and decayed organic matter (which has solid particles) accompanied by liquid and gas that fills the spaces.empty space between these solid particles [3].
According to the Geotechnical Guide 1, 2001 the use of the term "soft soil" relates to soils which, if not properly identified and investigated, may cause intolerable long-term problems of instability and settlement, such soils have low shear strength and high compressibility.As for one type of soil that is included in the type of soft soil is peat and soft clay [3].
The definition of soft soil according to Rachlan [1] and Bina Marga [2] is soil, which generally consists of clay, including very poor foundation material owing to its high water content, low permeability, and very compressible soil that can be visually penetrated with a minimum thumb.as deep as ± 25 mm, or has a shear strength of 40 kPa based on field propeller shear tests.According to Seotjiono (2008) and Pasaribu (2008), soft soil is weak and naturally formed from the deposition process as an alluvial layer, usually found on alluvial plains, which are very compressive and have low shear strength, which is the undrained shear strength of the field.less than 40 kPa and high compressibility [1].

Soil Problem
One of the main problems with soft soil in construction work is large land subsidence.The large decrease was caused by a decrease in consolidation in the subsoil.The ability of soft soil to support embankments without the failure of excessive shear or settlement is highly dependent on the shear strength.Land subsidence lasts for a very long time; therefore there will be a real differential settlement [3].
Another problem that arises in the construction of soft soils is shearing.The mechanism of balance loss can occur in soils with low carrying capacity, resulting from the weight of the soil itself.Other problems are usually in the form of uplift, which occurs in many layers of clay (clay) and silt (silt) owing to differences in water pressure frequent occurrence of surface settlement (settlement), and frequent problems.This is generally caused by the heavy load borne by soft soil [3].

Foundation
For many years, bamboo or gelam piles with raft have been used by Indonesians in the construction of embankment on soft soils.The main consideration is for stability reason and in certain cases to reduce the settlement [8].Recently Pile Plate supported embankment has been used in practice by Indonesian engineers since 1990s as repalcement of the conventional bamboo or gelam piles to stabilise fill on top of soft to very soft soils or organics.These mini concrete piles can be used with or without cap.The use of pile cap is more effective if upper soil layer is sufficiently stiff or a blanket of sand is used under the pile cap [8].
According to Katzenbach et al [9], CPRF (Combine Pile Raft Foundation) is a composite geotechnical construction whose support effect is between the raft foundation elements and the pile foundation by considering the interaction between the foundation elements and subgrade [9].
Cast-in-situ, or cast-in-pile are builds by making a hole in the ground and then filling it with concrete.Various types of cast-in-place concrete pile are currently used in construction, and most of them have been patented by their manufactures.These piles may be devided into two broad categories: case and uncased.Both types may have a pedestal at the bottom [10].

Safety Factor of Embankment
The presumed shape of the landslide plane in the form of a circle is intended to mathematically simplify the calculation of its stability analysis and is considered to be close to the actual shape of the landslide plane that often occurs in nature.According to Bowles (1984), Hardiyatmo (1994) errors in slope stability analysis are not mostly caused by the presumed shape of the landslide plane, but many errors are caused by determining soil properties and finding critical landslide locations.In determining the stability of a slope, the term safety factor is known, which is the ratio between the forces that resist movement against the forces that move the soil, which is considered stable, when formulated as follows: Safety Factor (SF) = Holding Force / Driving Force Where : SF > 1.0 : the slope is in good condition, no landslide occurred SF = 1.0 : the slope is in a balanced state, there is a possibility of landslides SF < 1.0:The slope is not strong, and landslides will occur.Therefore, analyzing the stability of the slope is always related to the calculation of the safety factor of the slope [3].

Safety factor for pile foundation
Pile foundations are used to support buildings when strong soil layers are deep.This type of foundation can be used to support buildings that resist upward lifting forces, especially high-rise buildings, which are affected by overturning forces due to wind loads (Hardiyatmo, 2010).
To obtain the allowable capacity of a drilled pile with diameter d < 2m, Tomlinson [5] divided the ultimate capacity of the pile by a safety factor of two without enlargement at the bottom of the pile tip; if the base of the pile was enlarged, the factor of safety used was 2.5.Therefore, the value of the safety factor was used for drilled pole 2 [5].

Finite element method
PLAXIS 2D is a special purpose two-dimensional finite element program used to perform deformation, stability and flow analysis for various types of geotechnical applications.Real situations may be modelled either by a plane strain or an axisymmetric model.The program uses a convenient graphical user interface that enables users to quickly generate a geometry model and finite element mesh based on a representative vertical cross section of the situation at hand.
The finite element method is a computer application program based on the 2D finite element method, which is specially designed to analyze the deformation and stability of various geotechnical applications, such as soil capacity.In the modelling, a staged construction analysis can be permormed.Staged construction is very useful for analyzing excavation, earth filling or anchoring during the construction of geotechnical structures.The main outputs in the finite element calculation were the displacement at the nodal point and stress at the stress point.In addition, the stresses and moments acting on the structural elements can be determined to determine the effective dimensions of these elements.The basic law of the finite element method is Hooke's law of linear elasticity; isotropic, for example, can be considered as the simplest stress-strain relationship.This is because it involves only two input parameters: Young's modulus, elastic modulus, and Poisson's ratio.
In modelling with the finite element method, the field implementation phase is intended to be implemented into the working phase of the modelling program, and it is hoped that the implementation in the field is as close as possible to the program.Therefore, the results provide by the program can be considered representative the actual situation that occurs in the program.Although many experiments and validations have been carried out using the program, it is possible that it will be error-free.The simulation of geotechnical problems using the finite element method implicitly results in unavoidable numerical modelling and errors.Therefore, it is necessary to make accurate estimates of real-world situations, depending on user experience in modelling problems, such as understanding soil models, limitations of soil models, determining model parameters, and the ability to realize computer results.

Research methodology
This study was conducted by identifying soil from the borlog results.From the results of the borlog, there is soft soil that is sufficiently deep with a high embankment plan.In the first stage, a soil improvement analysis was carried out using a concrete mat (pile raft) with a preloading height of 2 m.Then, the next analysis is performed by replacing the high pile with a structure that can be called a pile bent.The results of the two studies were then compared in terms of time speed, cost, and safety factor.

Pile Raft Analysis
At Drilling Log BH-07, there were four types of soil: Silty Clay, Sandy Silt, Clayey Silt and Sand.Each type of soil has different E' and N-SPT value.Therefore, in the calculations using the finite element method program, the N-SPT data on each layer, but with the same soil type, is averaged so that it only becomes an 11-layer and uses an undrained and drained modelling analysis.Groundwater level at an elevation of ± 1.20 m.
The design of soil parameters can be determined by correlating the N-SPT data.At point BH-07, the test results can be seen in the table below, and soil data were used to determine the soil parameters.The soil parameters needed to determine the deformation and tensile capacity are the undrained shear strength (Cu) for undrained and effective shear strength (cˈ) for drained, soil elasticity modulus (Eu) for drained and effective soil elasticity modulus (Eˈ) for drained, soil unit weight (γ), internal friction angle (ɸ) for undrained and effective internal friction angle (ɸˈ) for drained, and Poisson's number (ʋ).Table 1 list the results of the N-SPT test [7].

Elevation
Type of Soil N-SPT To determine the value of the deformation and tensile capacity of the foundation, finite element modelling was used, namely direct foundation modelling without going through the construction process.The pile parameters are presented in Table 2.After obtaining the types of soil, the data obtained in each layer were used as an average and were further classified into 11 layers.Soil data were entered into the finite element method program by creating a profile of each soil and the thickness of each layer, as shown in Table 4.

Deformation analysis (Pile Raft)
The pile raft model uses the embedded beam row parameter, which is combined with the concrete mattress parameter as a load transfer from the above.The length of the minipile is 12 m with the addition of a surcharge load on the design embankment as high as 2 m. for heaps that enter the river area using granular material.The soil was entered into the finite element method according to the correlation in Table 4 by creating a profile of each soil and the distance of the soil.This model is illustrated in Fig. 1.

Figure 1. Initial modelling of finite element method
After modelling the retaining wall and the load is applied, the mesh generation process is performed to divide the elements in so that the calculation can be carried out.This analysis is illustrated in Figure 2    The safety factor obtained using pile raft analysis was 1.94 for the short term and 1.91 for the long term.The largest deformation location was on the right side of the embankment.

Deformation analysis (Pile Bent)
The pile bent model uses the embedded beam row parameter, which is combined with the concrete mattress parameter as a load transfer from the above.The length of the Pile Bent was 30 m with transverse intervals of 3.2 m and longitudinal intervals of 16 m.The soil was entered into the finite element method according to the correlation in Table 4 by creating a profile of each soil and the distance of the soil.This model is illustrated in Figure 6.After modelling the pile bent and the load is applied, the mesh generation process is performed to divide the elements in so that the calculation can be carried out.This analysis is illustrated in Figure 7 and Figure 8, respectively.In the short term, the safety factor obtained using pile bent analysis is 1.32 for the short term and 1.37 for the long term.The largest deformation location was on the left side of the embankment platform area.

Matrix Comparation Pile Raft vs Pile Bent
After calculation using a finite element, a comparison matrix can be made between the handling of the pile raft and the pile bent.The matrix is shown in the Table 5. Requires a platform for piling tools with a high enough carrying capacity for the stability of the tool when carrying out pile driving

Conclusions
From the results of the above analysis using the finite element method accompanied by two types of soil improvement, it can be concluded that 1. Soil handling using pile rafts requires a granular platform for landfilling on the left side of the pile, because the area is a swamp area.
2. The safety factor of the pile raft is relatively large because it uses granular material (1.91) with a settlement of 1.84 m and the location of the deformation factor of safety is not on the river slope area.3. Pile raft implementation requires quite a long time because it requires 2 m of preloading and design piles > 10 m, which will certainly affect the stability of the construction phase.4. Replacing the heap with a pile bent structure requires a relatively short time because it only fills 4 m platform with a freestanding height of ± 6 m. 5.The pile bent structure uses a 1 m diameter of bor pile.There is a hard soil lens that cannot be penetrated if mini piles are used.6.The pile bent safety factor only occurred in the embankment platform, which was equal to 1.37 with settlement of 0.019 m, which did not affect the safety of the pile bent structure.7. From the comparison matrix for improvement with pile raft and pile bent, can be used pile bent, because it shortens implementation time and minimizes the risk of settlement.

Figure 2 . 3 .
Figure 2. Deformation result for short term Figure 3. Deformation result for long term

Figure 4 . 5 .
Figure 4. Safety factor for short term Figure 5. Safety factor for long term

Figure 6 .
Figure 6.Initial modelling of finite element method

Figure 7 . 8 .
Figure 7. Deformation result for short term Figure 8. Deformation result for long term

Figure 9 .
Figure 9. Safety factor for short term Figure 10.Safety factor for long term