Slope Stability Analysis Using Plaxis & Slope/W (Case Study: Bagbagan-Jampangkulon Road Section STA 8+400, STA 8+200, and STA 13+000)

The Bagbagan - Jampang Kulon Road Section in Sukabumi Regency faces soil stability challenges primarily driven by natural factors. A safety factor exceeding 1.5 is considered secure. Analyses were performed using PLAXIS (Finite Element Method) and SLOPE/W (Limit Equilibrium Method). For the road section at various points (STA 8+200, STA 8+400, and STA 13+000) under Class I pavement loads (12.5 kN/m), PLAXIS yielded safety factors of SF=1.387, SF=1.315, and SF=1.265, while Class II loads (10 kN/m) produced SF=1.388, SF=1.318, and SF=1.267. SLOPE/W analysis under Class I loads yielded SF=1.295, SF=1.226, and SF=1.015, with Class II loads producing SF=1.297, SF=1.229, and SF=1.017. Minor discrepancies of approximately 8-9% at STA 8+200 and STA 8+400 and 1.2% at STA 13+000 were observed between the two programs. Despite these variances, PLAXIS and SLOPE/W provide reasonably close safety factor results. These differences may be attributed to each program’s distinct assumptions and calculation methods. Overall, both methods underscore the need for reinforcement to enhance slope stability in this critical road section.


Introduction
The development of road infrastructure development as a means of land transportation has had a significant impact on various aspects of people's lives.Infrastructure development can affect the economic growth of a region, helping to increase development and progress.As a country with many mountains and hills, Indonesia faces challenges in building roads that cross hilly areas.One problem that often arises is the existence of land with slopes and cliffs.
The land surface does not always form a flat plane or has elevation differences from one place to another, so it forms a slope.Slope is a topographic condition that is often found in various civil construction works.Slopes can occur naturally or deliberately created by humans with a specific purpose.
A landslide is one of the natural disasters that often occurs on natural and artificial slopes.Most of the slope failures occur during the rainy season.It occurs due to increased pore water pressure on the slope.This results in a decrease in the soil shear strength (c) and the internal shear angle ( υ ), which in turn causes a slide.
Slope stability analysis has an essential role in the planning of civil constructions.Unstable slopes are hazardous to the surrounding environment.Therefore, slope stability analysis is needed.Slope stability is known by calculating the safety factor.
The Bagbagan, Jampangkulon, Sukabumi Road section has various topographical conditions ranging from highlands, lowlands, hills, and mountains.Areas with hilly and mountainous topography are prone to landslides.Because the topography of the Bagbagan, Jampangkulon, and Sukabumi roads is hilly and mountainous, indicating that the area is prone to landslides.
Due to the landslide disaster on that road, the authors conducted a slope analysis at STA 8+200, STA 8+400, and STA 13+000 Jalan Bagbagan, Jampangkulon, Sukabumi Regency using the limit equilibrium method and the finite element method.Slope stability analysis with this method requires precision and perseverance to get accurate results, so the analysis can be done using a computer program.
Currently, many types of programs for soil mechanics calculations make it easier for users to analyze various soil conditions quickly.The programs used are SLOPE/W and Plaxis v8.

2.
Literature Review Landslides occur when slope-forming material, such as rock, soil, or other materials, moves down or out of the slope.Landslides begin when water seeps into the ground and adds to the weight of the soil.If the water penetrates the impermeable layer of soil, which acts as a shifting plane, the soil becomes slippery, so the weathered soil above it will move along the slope and out of the slope.The occurrence of landslides is influenced by factors such as the thrust force on the slope, which is greater than the resistance force.
The slope's inclination angle, water, loads, soil or rock-specific gravity, and earthquake activity influence the thrust.Meanwhile, the resistance force is affected by rock strength and soil density.The slope movement is also related to the rock and soil conditions that form the slopes, geological structure, rainfall patterns, vegetation cover, and land use.

Slope
A slope is an inclined land surface that forms a certain angle to a horizontal plane.In places with two ground surfaces of different heights, forces will act to push so that the higher ground tends to move downwards, called the gravitational potential force, which causes landslides [1].In every type of slope, the possibility of landslides is always there.Landslides occur when the driving force exceeds the opposing force originating from the shear strength of the soil along the plane of the failure [2].Technically, landslides occur when the safety factor does not meet (SF <1.5).

Slope Stability
Slope stability is one of the problems that often occurs in mining construction work, road construction, and others.Disturbed slope stability can cause environmental damage, damage to construction equipment, threaten worker safety, reduce production intensity, and disrupt the smooth running of mining [3].
To prevent slope collapse, it is necessary to perform slope stability analysis.One way to assess slope stability is to find the value of the safety factor.The safety factor is the ratio between the resisting and driving forces on the slope.
Retaining force is the strength of the material that inhibits the occurrence of landslides or slope failure.This resisting force is related to the physical characteristics of the rock or soil, such as cohesion (the bond strength between the particles) and the coefficient of friction (resistance to relative movement between the particles).The higher the value of cohesion and coefficient of friction, the higher the holding force possessed by the material.
On the other hand, the most significant driving force is the force of gravity, which is a force that acts in the direction of the slope and is directed outwards from the slope.This gravitational force is one of the driving factors that has the potential to cause slope failure if it exceeds the existing resisting force.The main objective of the analysis of slope stability is to ensure that the safety factor meets the requirements.Generally, the desired safety factor is greater than 1.If the safety factor is less than 1, it indicates that the slope is unstable and has the potential to fail.Where : The safety figure for slope stability: • F < 1.5, the slope is unstable • F = 1.5, the slope is critical.This means that with a little additional driving moment, the slope becomes unstable.• F > 1.5, the slope is stable.

Slope Stability Concept
This movement can involve soil masses, rock masses, or both.If the soil mass dominates the ground motion and occurs through a plane on the slope, be it an inclined plane or a curved plane, it is called a landslide.
Slope stability analysis involves the concept of slope stability, which is the application of knowledge about the shear strength of the soil.Shear failure in soil occurs due to relative motion between soil grains.Therefore, the shear strength of the soil depends on the forces acting between the soil grains.
In the analysis of slope stability, the principles of soil mechanics are used to study the forces acting on the slope and compare them with the shear strength of the soil.This is done to determine the safety factor of the slope, namely the ratio between the resisting force and the driving force.The slope is considered stable if the safety factor is greater than 1.However, if the safety factor is less than 1, the slope can fail.So it can be concluded that the shear strength consists of: -Cohesive Part, depending on the type of soil and grain bonds.-The frictional part, which is proportional to the effective stress acting on the shear plane.[2] Factor affecting slope stability -Topography: Significant topographical changes, such as cutting or loosening of soil on a slope, can change the distribution of loads and reduce slope stability.-Seismic activity: Earthquakes or other seismic activity can cause slope vibrations, which can reduce shear strength and cause failure.-Groundwater: Changes in groundwater flow, such as water seepage or excessive rain infiltration, can cause an increase in pore water pressure in the soil.High pore water pressure can reduce friction between soil particles and reduce shear strength, which in turn can cause failure.-Loss of strength: Factors such as changes in soil composition, drying out, decomposition, or specific chemical reactions can cause a loss of strength and stability of a slope.-Stress change: A change in the stress acting on a slope, for example, due to a change in load applied to the slope or the displacement of soil around the slope, can cause a stress imbalance and lead to failure.-Season/Climate/Weather: Variations in climate and weather, such as heavy rainfall or soil freezing and thawing, can affect soil moisture and cause slope strength and stability changes. (1)

Data Technical
This data is obtained from factual reports of KMP -PT.Karya Utama Citra Mandiri -DKI Jakarta -West Java National Road Implementation Center with point STA.13+000 (Road Bagbagan Section, Jampangkulon, Sukabumi Regency, West Java).

Finite Equilibrum Method (FEM) Finite Element Method
Plaxis is a series of finite element programs developed to analyze deformation and geotechnical stabilization in civil planning.The program allows users to create complex finite element models and provides detailed calculation results.
In Plaxis, a simple data input procedure allows the user to enter the required soil shear strength parameters.These parameters are then automatically reduced until a slide occurs.Thus, Plaxis can be used to analyze slope stability and obtain results regarding the safety factor and potential movement on the slope being analyzed.
Plaxis' strength lies in its ability to model and analyze complex geotechnical conditions and situations, including deformation changes, soil displacements, and slope stability.The program also provides visualization tools that help understand and analyze calculation results.A factor of safety (SF) for slope stability is: SLOPE/W, as part of Geostudio, SLOPE/W gives the user the flexibility to choose the equilibrium method according to the slope conditions to be analyzed.Thus, geotechnical engineers can use this program to evaluate slope stability more efficiently and accurately.

4.
Modeling and Analysis Results Slope stability modeling requires initial data from the drill point test results to obtain soil parameters.Figure 2 and Table 1 are the soil stratification at STA 8+200.Figure 3 and Table 2 are the soil stratification at STA 8+400.Figure 4 and Table 3 are the soil stratification at STA 13+000.

Analysis Slope Stability with Plaxis Program
Modeling for slope stability analysis with the Plaxis program can be seen in Figure 5, Figure 6, and  It consists of several stages, including the Initial Phase.At this stage, the slope conditions are in the early stages.Using the Finite Element Method, the calculation uses gravity loading to calculate the slope.Make Phase 1 (Safety Factor) in the case to get the results of the slope safety factor in terms of the initial phase, namely the initial phase, and select the type of calculation to be safe.The process starts by calculating each phase in plaxis to get the results.
The analysis results can be displayed in the form of curves, images, or tables-calculation info to get the value of the slope factor of safety.After correcting the calculation phases, the safety factor value will be obtained in the final phase.The safety factors for each model's total stress and adequate stress conditions are analyzed.

Stability Analysis with Slope/W Program
Selection of the analytical method used to calculate the safety factor for the Bagbagan -Jampang Kulon road slopes STA 8+200, 8+400, and 13+000.The method used is Limit Equilibrium.In selecting the type of analysis to be used, the writer chose the Morgenstern-Price analysis because, in this type, the resistance (moment equilibrium) and thrust (force equilibrium) are considered.Selection of the type of avalanche (slip & surface) The author chose the Entry and exit type because it is more practical and is defined from right to left.Modeling for stability analysis with the Slope/W program can be seen in Figure 13, Figure 14

Comparison Analysis Results
The analysis of slope stability using the Plaxis and SLOPE/W programs produces different safety factor values.The results of the analysis slope for STA 8+200 are in Table 4, for STA 8+400 in Table 5 and for STA 13+000 in Table 6.The difference in the value of the safety factor between the two programs can be caused by several factors, such as differences in the analytical methods used, the shear strength model implemented, or differences in the assumptions and input parameters used in the analysis.
It is important to note that the two safety factor values are still above the minimum value required to maintain slope stability.However, these differences indicate that there are variations in the analysis results between the two programs.In this case, further checking and evaluation of the assumptions and parameters used in the analysis can be carried out to ensure the reliability of the results of the slope stability analysis.

Conclusion
The author has compared the results of research on the factor of safety using the FEM and LEM methods using the Plaxis and Slope/W programs.Based on the results of the analysis obtained, the authors reach the following conclusions: 1. Safety Factor using the PLAXIS Program: The author obtained a safety factor with a Class I pavement load of 12.5 kN/m, resulting in a safety factor value of STA 8+200 (SF=1,387), STA 8+400 (SF=1,315), STA 13+000 (SF=1,265).While the Class II pavement load of 10 kN/m produces a safety factor value of STA 8+200 (SF=1.388),STA 8+400 (SF=1.318),STA 13+000 (SF=1.267).From the analysis, it is carried out with the help of the Plaxis program.This indicates that the slope stability does not meet the safety requirements because the safety factor obtained is less than 1.5.2. Safety Factor using the SLOPE/W Program: The author obtained a safety factor with a Class I pavement load of 12.5 kN/m with a safety factor value of STA 8+200 (SF=1,295), STA 8+400 (SF=1,226), STA 13+000 (SF=1,015).While the Class II pavement load of 10 kN/m produces a safety factor value of STA 8+200 (SF=1.297),STA 8+400 (SF=1.229),STA 13+000 (SF=1.017).
From the analysis it is carried out with the help of the Slope/W.Although the value of the factor of safety obtained is slightly lower than the results of the analysis with Plaxis, it is still below 1.5, which indicates that the slope stability still does not meet the safety requirements.3. Comparison of Difference in Safety Factor: From a comparison between the safety factors obtained from the Plaxis and Slope/W programs, the authors found that there was a difference in comparison at STA 8+400, which was 9%, at STA 8+200 was 8%, at STA 13+000 was 1.2%.This difference shows that the safety factor results between the Plaxis and Slope/W programs are not too far away.
The conclusion shows that the two programs, namely Plaxis and Slope/W, provide relatively close safety factor results to one another.Although there is a slight discrepancy, this could be due to differences in assumptions and calculation methods between the two programs.

Suggestion
The suggestions that the writer can give based on the analysis that has been carried out with the help of the Plaxis and Slope/W programs are: Slope Strengthening Research: The research that has been conducted focuses on the analysis of slope stability for the Bagbagan -Jampang Kulon -Sukabumi road section.However, it is important to consider the need for further research on slope reinforcement.Slope reinforcement can improve safety for road users and reduce the risk of slope failure.Slope reinforcement studies may include selection of appropriate reinforcement methods, design of reinforcement structures, and analysis of stability after reinforcement is applied.

3 F
= (Retaining Moment/Pushing Moment) = (RC.LAC)/(W.y)F = Safety Factor W = Weight of soil that will slide (kN) LAC = Arch Length (m) c = Cohesion (kN/m2 ) R = Radius of the observed landslide area (m) y = Distance of center of gravity W to O (m) φ inputs / tan φ reduction = c inputs / c reduction SF = (Available Shear Strength)/(Shear Strength during Landslides) = Mark ΣMsf at the time of the landslide (kN/m 2 ) φ inputs = angle of shear in the soil ( ° ) c reduction = reduced soil cohesion (kN/m 2 ) φ reduction = reduced internal shear angle ( ° ) 3.4 Finite Element Method Limit Equilibrum Method (LEM) SLOPE/W is a module of the Geostudio software developed by a private company in Canada.The primary function of SLOPE/W is to analyze the safety factor on slope stability.SLOPE/W can analyze slope stability problems in both simple and complex scales.This program uses the Limit Equilibrium method, which has eight methods to analyze various sloping slope surfaces.Using SLOPE/W, users can model and analyze slope stability by inputting relevant geotechnical data, such as soil shear strength parameters.This program then calculates the slope safety factor by considering the resisting and driving forces acting on the slope.

Figure 3 Table 2 1 No
Figure 3 Stratification of STA 8+400 Drill Test Points

Figure 4 Table 3
Figure 4 Stratification of STA 13+000 Drill Test Points

Figure 7 .
The pavement load Jalan Bagbagan, Jampangkulon, Sukabumi uses class I, Arterial, Collector, which is 100 kN/m, then the road width is divided by 8 meters into 12.5 kN/m and uses class II, Arterial, Collector, Local, Environment, which is 80 kN/m, then divided by 8-meter road width to 10 kN/m.

Figure 9 Figure 10 Figure 11
Deformation in the Initial Phase (a) STA 8+200, (b) STA 8+400, (c) STA 13+000 Safety Factor and Form of Sliding Bow: By using the phi-c reduction type of calculation, we can find out the safety factor of the slope by selecting the total incremental and looking at the Msf value (value of the factor of safety) and with the shading display to show the slope arc and the value of the safety factor for the sliding that occurs on the slopes of STA 8+200, 8+400 and 13+000 under load conditions.Form of STA 8+200 (a) Class I Load Sliding Bow (SF=1.387)(b) Load Class II Sliding Arc (SF=1.388)Form of STA 8+400 (a) Class I Load Sliding Bow (SF=1.315)(b) Load Class II Sliding Arc (SF=1.318)(a) (b) Figure 12 Form of STA 13+000 (a) Class I Load Sliding Bow (SF=1.265)(b) Form of STA 13+000 Load Class II Sliding Arc (SF=1.267) , and Figure 15.To determine the soil property, the Mohr-Coulomb strength model was selected.The strength of the Bedrock model was selected to analyze total stress and effective stress and hard soil layers.Determine the working load.The case in this Final Project uses class II, Arterial, Collector, which is equal to 100 kN/m, and then divides the road width of 8 meters into 12.5 kN/m and uses class II, Arterial, Collector, Local, Environment, which is equal to 80 kN/m then divides the road width of 8 meters into 10 kN/m.