Print ISSN 2777-0168| Online ISSN 2777-0141| DOI prefix: 10.53893 https://journal.gpp.or.id/index.php/ijrvocas/index
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The Performance of Skewers Foundation Combined with Revetment under Rapid Drawdown Conditions on Balikpapan Coastal Area in Soft Soil using Numerical Analysis Undrained B
Sulardi
1, Tatag Yufitra Rus
2, *1Sekolah Tinggi Teknologi Migas, Balikpapan, Indonesia
2Politeknik Negeri Balikpapan, Balikpapan, Indonesia
Email address:
tatag.yufitra@poltekba.ac.id
*Corresponding author
To cite this article:
Sulardi, & Yufitra Rus, T. (2022). The Perfomance of Skewers Foundation Combined with Revetment under Rapid Drawdown Conditions on Balikpapan Coastal Area in Soft Soil using Numerical Analysis Undrained B. International Journal of Research in Vocational Studies (IJRVOCAS), 2(1), 21–25. https://doi.org/10.53893/ijrvocas.v2i1.100
Received: January 22, 2022; Accepted: April 04, 2022; Published: April 18, 2022
Abstract:
This research analyses the skewer foundation with Plaxis 2D considering the stability before and after drawdown conditions of the sea level with variations in the point load on the foundation. The study is carried out using the back-analysis method with the assumption that the protective building foundation is in a balanced state with the safety factor (SF) = 1.5. As a result, the stability of the rapid drawdown gave a different value which is the skewer foundation before rapid drawdown showed a better result of the performance of foundation during point load than after drawdown. Before drawdown conditions, the value of the point load (P) is 12 Tons. Meanwhile, after drawdown conditions, the skewer foundation is only able to accept a load of 5 tons. The value of the movement of the skewer foundation before and after the drawdown of sea-level conditions occurred at the same settlement value of about 0.0179 m = 1.18 cm. In this condition, the skewer foundation is considered capable of resisting the settlement issue where the maximum settlement value for shallow foundations for clay soils is not greater than 65 mm.Keywords:
Skewer Foundation, Balikpapan Coastal Area, Revetment, Undrained B1. Introduction
Balikpapan Bay is located in west of the Makassar Strait, or southwest of the Pacific Ocean. The Balikpapan Bay area is widely used as a residential area and as an industrial area.
The Balikpapan Bay area is dominated by very soft and soft soil types.
It is needed to study the proper design of the foundation to support buildings in the coastal area of Balikpapan Bay.
Innovations in planning the design of supporting buildings for the Balikpapan coastal area continued to be carried out.
The construction of shallow foundations is combined with revetment construction as wave protection which is located on + 3.5 meters above sea level (SWL) with uneven ground surface contours and visually sloping towards the coast.
The skewer foundation is a type of shallow foundation
with a square shape, consisting of layers of concrete slabs of a certain size with a hole in the center and assembled as the shallow foundation with timber pile. The timber pile with a certain depth locked by a beam and the construction of the concrete slab will perform together to reduce the load on the building above it so that the stress transmitted to the subgrade will be relatively small and safe. The foundation of the concrete slab is arranged in a triangular shape like the concept of the load distribution from the surface to the soil layer.
This study is an analysis of the skewer foundation performance in retaining the revetment construction with Plaxis 2D considering the stability before and after drawdown of sea level with variations in the point load on
22 the foundation using Undrained B. The skewer foundation is the newest invention of the shallow foundation so that it is a copyright research with originality or there is no similar study.
Figure 1. Industrial area in Balikpapan Bay coastal area
2. Description of Foundation Design and Soil Exploration
2.1. Foundation Design
The design of the skewer foundation combined with the revetment can be seen in Figure 2. The Skewers foundation is designed with 7 levels of the concrete slab. In the bottom of the concrete slabs is installed the single beam by timber with a length around 4 m which is useful as a foundation restrainer and provides additional strength to foundation stability. The foundation is given a hole in the middle to insert the timber pile to lock the arranged plates through the pile with a depth of 4 m to strengthen the foundation from sliding and overturning.
Figure 2. The design of the skewer foundation with revetment
According to USACE (1991), the construction of the revetment building is designed using 200 mm stone with a slope of 2.5 H: 1V and a construction thickness of 350 mm as suggested by Brown and Clyde (1989). This suggestion is also following the thickness of the revetment construction recommended by MOTH (1999).
2.2. Soil Exploration Data
The coastal area is identical to soft soil (alluvial) originating from downstream river sedimentation and erosion of the surrounding coastal area. Alluvial soil has a low bearing capacity and high compressibility so buildings on it will tend to occur settlement and the foundation will be damaged due to erosion.
From observations through the SPT-N test as shown in Figure 3, it is known that the soil consistency in the coastal area of the Balikpapan Bay coastal area is very soft to soft soil which is estimated as the subgrade soil at a depth of +0.00 to -28.00 m a type of sedimentary soil (alluvial) with clay soil which is very soft, and medium-high plasticity, and there is a coal bed which typically has the low bearing capacity, easily compressed, easily deformed, unstable and not suitable for residential area.
Figure 3. Information on Soil Investigation by SPT-N
The results of the SPT-N test in Figure 3, show that the soil condition in Balikpapan Bay coastal area has the consistency of very soft marine clay with hard soil depths at a depth of 25 m from the soil surface. At a depth of 0-17 m, the soil only has the N value of 2 so the consistency of the soil is classified as very soft soil. Furthermore, at a depth of 17-25 m, the soil is still relatively soft with the N value of no more than 6.
3. Numerical Analysis Modelling
The modeling of the skewer foundation analysis using Plaxis 2D can be seen in the results of the generated mesh model in Figure 5. The soil is divided into 3 layers with each distribution of consistency which can be seen in Table 2. In the numerical analysis of the skewer foundation, variations in the
Very Soft Clay
Hard Soil Soft Clay
0
5
10
15
20
25
30
0 20 40 60
Depth (m)
SPT-N
Pengujian SPT
addition of P load on the foundation are carried out. in a state of the before and after drawdown of sea level to determine the Pmax value achieved.
Figure 4. Generated Mesh Model in Numerical Analysis
3.1. Soil Parameter
Analysis of the Plaxis modeling was carried out using Mohr-Coulomb with drainage type parameters using Undrained B through effective soil modulus (E') and undrained soil shear strength (Su). Determination of the effective soil modulus value (E') is based on the value of the empirical approach, where the Architectural Institute of Japan (2001) suggests that for clay soil the value of E' = 2800 N.
Furthermore, the determination of the Su value uses an empirical approach based on the suggestion of Terzaghi &
Peck (1967), where the value of Su = 6.25 N.
Table 1. Soil Parameter Input with Mohr-Coulomb using Undrained B Depth
(m)
Soil Types
N Values
𝛾sat
(kN/m3) 𝛾unsat
(kN/m3) E’
(kN/m2) Su
0-17 Very soft
clay 2 17.1 16.2 8000 12,5
17-25 Soft clay 5 17.55 16.5 20000 31,25 25-30 Hard Soil 44 23.4 20.4 176000 275 Note: (v’ = 0,3)
3.2. Structure Parameter
The input parameters of the structure are divided into the concrete slab, revetment, and timber pile. The concrete slab and timber pile material use a Linear Plastic with Non-Porous drainage type modeling at Plaxis input. The unit weight of concrete is 24 kN/m3. The concrete used in the modeling is
with a concrete quality of K225 or Fc = 19 MPa. So the formula to get the modulus of elasticity in concrete is E = 4700 (Fc).
Furthermore, the determination of the pile parameter input is based on PPKI (1967), where the pile is a very high-quality timber (level 1), so the volume weight value is 10.4 kN/m3 with a modulus of elasticity of 12250000 kN/m2. Lastly, the revetment material is used for sea wave retaining structure with a stone unit weight 20 kN/m3 using the MC model with Non-Porous drainage type material.
Table 2. Input Parameter of Revetment Structure Material
Name
Material Model
𝛾 (kN/m3)
E
(kN/m2) v ∅ C kN/m2
Revetment MC 20 150000 0,3 38 0
Table 3. Input Parameter of concrete slab and pile Material
Name
Material Model
𝛾 (kN/m3)
E
(kN/m2) v
Concrete Non-Porous 24 20487000 0.15
Timber Pile Non-Porous 10.4 12250000 0.25
4. Results and Discussion
Figure 5. The Comparison of safety factor reached in before and after drawdown sea-level conditions
The results of the analysis show that the value of the safety factor before drawdown sea-level conditions has a greater value than after drawdown sea-level conditions. This is due to after drawdown sea level condition, the water on the face is lost and reducing the passive pressure stability of the skewer
1 2 3 4 5
0 5 10 15 20 25 30
MSf (Safety Factor)
Load P (Ton)
Pasang Surut
24 sea level condition, water loss of sea level affects decreasing the vertical load into potentially overturning and shearing failure of skewer foundation.
In the before drawdown conditions where the safety factor value is in a critical condition (SF, 1.5), the value of the point load (P) that can be carried by the foundation is 12 tons.
Meanwhile, in after drawdown sea-level conditions, the foundation is only able to accept a load of 5 tons. This proves that the foundation can be used for the coastal area in Balikpapan Bay.
In both before and after the drawdown of the sea-level conditions, there was no difference in the failure surface that occurred on the skewer foundation combined with the revetment. However, in Figure 6, it can be seen that the contours of total displacement before the drawdown sea-level condition hase a magnitude greater than the flow sea conditions. Where conditions before drawdown sea have a better safety factor value than after drawdown sea conditions with a given point load variation.
For the value of the settlement predicted in the analysis of before and after drawdown sea-level conditions with the value of SF = 1.5, the settlement value is about 0.0185 m = 1.18 cm.
In this condition, the skewer foundation is considered to be able to withstand the movement of the foundation where the maximum value for the shallow foundation allowed according to Skempton and MacDonald (1955), for clay soil not greater than 65 mm.
Table 4. Maximum Settlement of Shallow Foundation (Skempton &
MacDonald, 1965)
Foundation Types Maximum Settelemen Allowed (mm)
Isolated foundation in clay 65
Isolated foundation in sand 40
Mat/raft foundation in clay 65-100
Mat/Raft foundation in sand 40-65
5. Conclusions
This study uses the foundation analysis method through modeling the skewer foundation with Plaxis 2D considering the stability before and after drawdown conditions of the sea level with variations in the point load on the foundation.
The analysis method is carried out using the back-analysis method with the assumption that the protective building foundation is in a balanced state with the safety factor (SF) = 1.5. Based on the analysis results obtained in this study, it can be concluded as follows:
1. In both before and after drawdown sea-level conditions, there is no difference in the shading or landslide line of displacement contours that occurs on the skewer foundation. However, the value of displacement after drawdown conditions has a greater magnitude value than in before drawdown conditions.
2. The results of the analysis show that the safety factor value after drawdown has a greater value than before drawdown because there is water loss which affects the passive pressure that supports the skewer foundation.
3. In before drawdown conditions where the safety factor value is in critical condition (SF, 1.5) the value of the point load (P) is 12 Tons. Meanwhile, after drawdown conditions, the skewer foundation is only able to accept a load of 5 tons.
4. The value of the movement of the skewer foundation in
Figure 6. The contour of displacement: (a) before; (b) after rapid drawdown
`
(a)
(b)
before and after drawdown sea-level conditions with the value of SF = 1.5, occurred the same settlement value of about 0.0179 m = 1.18 cm. In this condition, the skewer foundation is considered capable of resisting the settlement issue where the maximum settlement value for shallow foundations according to Skempton and MacDonald (1965) for clay soils is not greater than 65 mm.
Acknowledgements
We would like to thank PT Refinery Pertamina Balikpapan (PT Kilang Pertamina Balikpapan) for providing soil exploration data. We also thanks for the support and permission to visit the project area in this research.
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