Heave in the Soil due to Driving Pile Foundations

GIS 150 KV Tambak Lorok III Semarang is a coastal area, stabilization is carried out with piles to reduce land subsidence in the area. In the piling process the soil grains are forced vertically and laterally when the pile foundation is driven into the ground, thus increasing the water pressure in the soil pore system. Nearby buildings will be affected and may suffer damage. The pile driving location for the GIS 150 KV Tambak Lorok III Semarang project is adjacent to other structures. So, the amount of land that needs to be cleared around the project must be calculated. This study uses methodology that makes theoretical predictions about the magnitude of heave caused by piles. Subsequently, vertical ground movement measurements made using the Finite Element Method (Plaxis Program) were compared with the theoretical predictions. Pile depth variations of 45 and 50 meters were performed in the plaxis analysis. In conclusion, the results of the two approaches will be contrasted. In the GIS 150 KV Tambak Lorok III Semarang project, soldier piles with a diameter of about 600 mm were driven 45 meters deep into the ground with hydraulic jacks. The theory led to the conclusion that the heave values for the effect of piling at -45 m depth and at -50 m depth were 0.1285 m and 0.1284 m, respectively. Based on Plaxis pile measurements at a depth of 45 meters, the vertical soil movement value was 5.80 cm, while 4.90 cm was recorded for a depth of 50 meters.


Introduction
The foundation is a part of the structure that functions to distribute the structural load to the ground below [1].The implementation of piling foundation piles in the field is by hitting, drilling or jacking them into the ground and connecting them with a pile cap (poer).It also depends on the type of soil, material and characteristics of the pile load distribution [2].
Construction of the GIS 150 KV Tambak Lorok III Semarang project is a coastal area (Figure 1.), which is a PLN building covering an area of 86 m x 25 m with a total of ± 500 piles (Figure 2.).The problem that often occurs due to piling is ground movement which occurs due to the pole being inserted into the ground.The ground movement is vertical (upward) and lateral (sideways).Vertical ground movement (upward/protrusion of the ground) is called heave [3].
The large volume of piles inserted into the ground is directly proportional to the large ground movement that occurs [4], the area of the surrounding environment that is affected, and the large increase

Ground Movement
According to Olsson and Holm [6], the Hellman/Rehnman method uses a particular modeling and formulation.The area that is impacted by the driving of a group of piles with an area boundary in the shape of the outermost pile is said to be limited to a distance equal to the depth of one model pile in Figure 3 at the ground surface.Factor heave, η, describes the compressibility of clay soil which has a value between 0.5 and 1. Factor value heave inversely proportional to the level of compressibility of clay soil.Factor heave, α -δ, is a factor to consider the load of surrounding buildings.The α factor value represents the weight of the building in plane A Figure 3.The β factor value represents the weight of the building in plane B Figure 3.The γ factor value represents the weight of the building in plane C Figure 3.The δ factor value represents the weight of the building in plane D Figure 3.According to Olsson and Holm [6], factor value heave ranges from 0 to 1, with 1 denoting a light building and 0 denoting a heavy building.
The lateral ground movements can also be calculated using this calculation model.Vertical ground movement (heave) and lateral soil movement are seen as having similar importance at the ground surface [7].

Finite Element Method (Plaxis)
A computer programme called PLAXIS, which uses the two-dimensional finite element method, is used specifically to conduct deformation and stability analysis for a variety of geotechnical applications [8].Users can quickly generate geometric models and element meshes using this program's simple graphical interface technique and cross-sections of the circumstances they want to study.
Axisymmetric and Plane Strain Axisymmetric and plane strain are PLAXIS global geometry models selected based on which best represents conditions in the field.The vertical drain planting pattern is installed in local fields at certain distances, while in the PLAXIS program the vertical drain implementation facilities are continuous (plane strain) and radial (axisymmetric) as shown in Figure 4.

Research Methods
This research uses 2 methods, namely theoretically using equation ( 1), namely the Ollson & Holm [6] method and numerical analysis with the Plaxis program.The data required in theoretical calculations are pile dimensions and pile cap.Meanwhile, the soil properties data needed in the Numerim analysis is the Plaxis program.In the Plaxis analysis, variations in pile depth are used, namely 45 m and 50 m.After the theoretical and manual analysis results were obtained, the results of the deformation analysis and total settlement were compared.

Theoretical Calculations
Based on data at the project location, it was obtained : The value is obtained heave the x direction at a pile driving depth of -45 m is 0.1285 m and at a depth of -50 m is 0.1284 m based on the calculations in equation ( 1) above.From the results above it can be seen that the value heave at a depth of 45 m and 50 are almost the same

2D Plaxis Analysis at a Depth of 45 m
In the 2D plaxis analysis, the geometry is made first as follows: • After running, the results of vertical ground movement in Figure 6.are: In Figure 6, it can see the direction of the arrow in the moving image coming from the pole and then the direction of movement is in the vertical direction around the pole.This indicates a change in soil volume due to piling.The vertical displacement value is 5.8 cm. Figure 7, shows the valuesSafety Factor (SF) from pile driving is 2.41 (greater than 1.5).This means that the Displacement is still safe.

2D Plaxis Analysis at a Depth of 50 m
The geometric depiction at a depth of 50 m is the same as at 45 m, only the depth of pile driving is different.Following are the results of the analysis: The same as the pile at a depth of 45 m, the results of the plaxis analysis with a pile depth of 50 m show that vertical ground movement moves from the pile to the direction around the pile.The deformation value in the vertical direction is 4.9 cm.From the plaxis results Figure 9, it can be seen that the direction of soil movement is the same, namely from the pole to the vertical direction around the pole.Piles driven at a depth of 45 m have greater deformation and a smaller Safety Factor (SF).

Conclusion
From the results of calculations and analysis of plaxis the following conclusions can be made: a.According to a theoretical method, the heave value (vertical ground movement) at a depth of -45 m and -50 m is 0.1285 m and 0.1284 m, respectively.b.The deformation value in the vertical direction of the pile at a depth of 45 m based on Plaxis 2D results is 5.8 cm with Safety Factor 2.41.c.According to Plaxis 2D data, the pile has a vertical deformation of 4.9 cm with a Safety Factor of 2.47 at a depth of 50 m.d.From the plaxis results, it can be seen that the direction of soil movement is the same, namely from the pole to the vertical direction around the pole.e.The difference between the results from the theoretical approach and the plaxis results is quite significant, this is because the theoretical calculation area is smaller (1 pile cap), while the plaxis analysis describes a wider area coverage.

Table 1 .
Based on the theoretical approach in equation (1), the value is obtained heave in Table1: Calculation results heave.