Deformation and Stability Analysis of Clay‐Geomembrane Lined Landfill Slope: A Numerical Approach

Abstract

The study emphasizes landfill slope deformation and stability, pointing out that excessive instability can pose environmental and health risks. Existing knowledge lacks sufficient consideration for the complex time‐dependent behavioral and compositional changes while modeling numerical simulations. This paper aims to simulate a single clay‐geomembrane composite lined landfill slope deformation and stability using an advanced numerical approach. Slope angle, waste composition by integrating aging effects, configuration, and leachate level are considered as influencing factors. Thus, material properties are collected through a rigorous review process. A total of seven material models and eight submodels were developed using PLAXIS 2D by integrating Mohr–Coulomb, soft soil, and soft soil creep models to capture real waste composition and heterogeneity. Accordingly, results of vertical deformation rise from 0.44 to 1.41 m as the slope angle increases from 1V:4H to 3V:4H, but the factor of safety decreases from 1.82 to a critical 1.06, suggesting high instability at steeper gradients. Landfills with horizontal configurations offered better results than cellular filling. Because of age‐based heterogeneity and compaction inconsistency, cellular filling caused a 23% decrease in Fs and a 49% increase in vertical deformation. Aging showed that between fresh and 5‐year‐old waste, factors of safety decreased from 1.56 to 1.36, but after 5 years, waste properties maintained a smooth variation. PWP increased from 4.50 to 32 kPa as the leachate level rose from the drainage base to the ground surface, leading to a decrease in Fs from 1.60 to 1.26 and an increase in deformation from 0.75 to 1.12 m. In saturated conditions, long‐term creep deformation under sustained loading reached 1.55 m over 4.80 years, demonstrating the crucial role that time‐dependent analysis plays in landfill performance. The model is verified through backward analysis and comparative estimation with previously published research findings and ensures close agreement for its applicability for landfill slopes. In conclusion, this study highlights that landfill stability is controlled by changing material composition and behavior over time and elevated leachate conditions, in addition to slope geometry. By incorporating interface shear behavior, waste heterogeneity, and creeping into landfill design, PLAXIS 2D significantly enhances the performance prediction of landfill system analysis.

1. Introduction

1.1. Background

Landfill safety is essential in modern waste management systems, as failures can lead to structural and environmental risks, creating environmental contamination and health issues through gas migration and leachate leakage [1]. Numerous factors, including waste composition, moisture content, liner system characteristics, and external loads, affect the stability of landfill slopes [2]. By limiting seepage forces, preventing leachate infiltration, and boosting overall structural integrity, lining systems are essential for increasing slope stability [3]. The performance of landfills under both static and dynamic loading conditions is impacted by the mechanical and hydraulic responses of various liners [4, 5].

Large volumes of solid waste are produced by growing urbanization and industrialization, making effective waste management an increasingly pressing global issue. Because they offer a controlled environment for waste containment, decomposition, and eventual stabilization, landfill systems continue to be the most popular waste disposal technique globally. Contaminated fills, as opposed to open dumping, use sophisticated lining, leachate collection, and gas management systems to reduce environmental hazards, such as air pollution, groundwater contamination, and landfill deformation and instability [6]. Landfill systems are crucial to present waste management because they can minimize harmful environmental effects while offering a sustainable and safe method of disposing of waste. Additionally, by lowering greenhouse gas emissions, methane gas collection and utilization systems have converted landfills into possible renewable energy sources, aiding in the fight against climate change [7].

Landfill slope deformation and stability are influenced by various geotechnical and environmental factors, including waste composition, leachate accumulation, gas generation, and external loads. The interaction between landfill materials and foundation soils creates complex mechanical and hydraulic conditions that demand advanced analytical approaches. Studies have shown that waste degradation and gas generation induce settlement, which can alter stress distributions and compromise slope stability [8–10]. Additionally, the stability of solid waste landfill slopes is a crucial consideration in waste management, as slope failures can result in severe environmental contamination, economic loss, and risks to human health [11, 12]. With increasing waste generation worldwide, landfills are often pushed to their capacity, leading to steeper and higher slopes that demand advanced stability assessments [13, 14]. Traditional approaches to slope stability analysis, primarily limit equilibrium models (LEM), often fail to account for the complex interactions and heterogeneities in waste material, leachate, and reinforcement systems [15].

The design and construction of landfills play a crucial role in ensuring safe, efficient, and environmentally sustainable waste disposal. A well-engineered landfill consists of multiple layers, including bottom liners, leachate collection systems, waste fill, gas extraction systems, and final cover layers, all designed to prevent contamination and facilitate waste stabilization [16]. The construction process typically begins with site selection and preparation, followed by liner installation using geosynthetic materials and compacted clay to create an impermeable barrier. Proper waste and compaction in a limited layer optimize space and minimize excessive deformation, while leachate and gas management systems ensure efficient drainage and methane recovery [17]. However, landfill failure can result from slope instability and excessive deformation, leading to environmental contamination, structural instability, and safety hazards due to complex geotechnical and hydromechanical interactions, waste materials, and decomposition settlements. Shear failure and excessive settlement of waste masses because of slope angle, fill height, poor drainage, rise in leachate level, incorrect compaction, and compromised liner interfaces result in landfill disasters that can cause an abrupt mass movement of waste and potential groundwater and environmental contamination [9, 18].

As stated, potential studies were conducted on landfill failure mechanisms and their effect on environmental impact from various perspectives and provided significant contributions to the research and practical world to enhance design and construction strategies for the long-term stability and serviceability of the system. However, there are limitations on the consideration of the nature and behavior of waste materials and proper analysis approaches during landfill slope deformation and stability computations; hence, they remain challenging issues for the long-term stability and serviceability of the system. The existing knowledge commonly focuses on simple analytical approaches to analyze the slope and deformation of landfills rather than considering complex waste behavior that integrates accurate predictions using advanced numerical tools. Additionally, waste material is potentially heterogeneous, anisotropic, and nonlinear but is mostly modeled using the Mohr–Coulomb approach, which considers materials as exhibiting elastic behavior while assuming linear failure criteria. Studies also lack sufficient numerical estimation of landfill instability and deformation as it relates to the aging of waste materials. Furthermore, long-term creep deformation of landfills is also a potential area to be assessed.

Therefore, this numerical simulation aims to address some areas of the described limitations by employing a parametric numerical simulation using PLAXIS 2D software. The research will focus on analyzing the deformation and stability of landfill slopes under varying conditions and developing effective remediation recommendations to mitigate risks. Soft soil (SSM) and soft soil creep (SSC) models are employed to simulate nonhomogeneous landfill material system to examine the material interfaces, filling configurations, and levels of leachate by rainfall infiltration to evaluate their coupled influence on slope deformation and stability using a finite element methodology. Through these rigorous analysis steps, factors of safety, total vertical deformation, and creep deformation are estimated. Eventually, possible conclusions and recommendations for future trends are drawn.

2. Materials and Methodology

2.1. Analysis Design

The analysis methodology was planned to gather properties of landfill and foundation soil to model deformation and stability under various influencing variables, as shown in Figure 1. This simulation design focused on deformation and stability, emphasizing how soft landfills deform under influencing variables.

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Various models were developed by considering slope angles, filling configuration, integrating waste age, and leachate level rise as simulation variables. The evaluation of model outputs was conducted using the results of deformation and stability within various models, computing total and lateral deformation, and factors of safety were estimated. Eventually, based on evaluations and existing practical frameworks, model validation, conclusions, and future suggestions were drawn.

2.2. Input Parameters for Simulation

Input properties of slope geometry, landfill composition, liner and cover material, and foundation soil properties were collected from related articles through a rigorous literature review. Key parameters of these materials include unit weight, shear strength, permeability, and consolidation properties. These properties were investigated in the field and laboratory using direct shear, triaxial shear tests, large-scale consolidation, and compression tests [19]. Accordingly, software input properties are collected and summarized in Table 1.

TABLE 1 Input properties of foundation, clay (liner and cover), sand, and gravel [20–23].

Soil property	Foundation soil, ML	Bottom liner and final cover materials
Bottom clay liner/top clay barrier	Sand leachate collection layer	Bottom and top gravel protective layer	Vegetable layer (silty clay)
Model type	SSM	SSM	MC	MC	SSM
Thickness (m)	5.00	0.5∗/0.3∗∗	0.50	0.50	0.30
Unit weight (kN/m3)	16.50	16.00	18.00	18.50	14.50
Satu. unit weight (kN/m3)	18.00	17.50	19.00	19.50	16.00
Soil classification	ML	CH	SP	GP	CL
Cohesion (c′, kN/m2)	10	28	0	0	20
Friction angle (φ′)	26	16	32	34	12
Dilatancy angle (ψ)	0	0	2	4	0
Elastic modulus E (kPa)	—	—	45,000	50,000	—
Poisson’s ratio (υ)	0.22	0.30	0.32	0.34	0.25
Modified compression index (λ∗)	4.20 × 10−2	1.26 × 10−1	—	—	7.50 × 10−2
Modified swelling index (k∗)	1.71 × 10−2	2.40 × 10−2	—	—	1.50 × 10−2
Modified creep index (μ∗)	7.50 × 10−3	1.50 × 10−2	—	—	1.20 × 10−2
Hydraulic conductivity K (m/day)	7.50 × 10−3	5.50 × 10−5	6.50 × 101	7.40 × 101	8.5 × 10−3

Materials are modeled in different approaches based on their properties. Accordingly, the leachate collection sand layer and top and bottom protective gravel layers are modeled using the Mohr–Coulomb approach, whereas the foundation soil, clay liner, and vegetation cover are modeled using the SSM to capture the variability of the materials in the landfill system.

Similarly, waste material properties were also gathered based on their age to account for the composition inside the landfill system. Some of the values of material properties are adjusted through PLAXIS 2D backward simulation due to large variations in the results obtained from the review process. For instance, there is a large variation in the results of the unit weight of waste from 4.0 kN/m³ to 16.0 kN/m³ along the age ranges. Thus, simple normalization using average approximation is done after back analysis to minimize the bias of simulation outputs, as shown in Figures 2 and 3.

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SSM parameters for waste material with age-based are computed from consolidation test results. Accordingly, the modified compression index, λ^∗, is determined from C_c, while the modified swelling index, κ^∗, is determined from C_s, as shown in the natural logarithmic relation presented here. Similarly, the modified creep parameter, μ^∗, was calculated from the secondary compression index (C_a) by monitoring it over time. Accordingly, parameters are computed as λ^∗ = (C_c/2.3∗(1 + e)),  k^∗ = (2∗C_s)/(2.3∗(1 + e)) ≈ C_s/(1 + e) and μ^∗ = C_a/2.3∗(1 + e), and results are depicted in Figure 3(a). The void ratio considered in this analysis ranges from 0.30 to 4.0 of waste materials [30].

In fact, solid waste materials have a large value of hydraulic conductivity at fresh ages that are overviewed during data collection. However, due to insufficient integration with important input properties of waste materials, this paper considered hydraulic conductivity values from inclusively investigated papers and adjusted by back analysis. Accordingly, the hydraulic conductivity of waste used ranges from 9.20 × 10¹ m/day for fresh waste to 2.40 × 10⁻³ m/day for 15 years of age waste, as shown in Figure 3. As the results indicate, the hydraulic conductivity potentially decreases with the age of solid waste due to a sufficient decrease in the void ratio in the waste volume.

Waste material parameters, such as modified compression, λ^∗, swelling, κ^∗, creep, μ^∗, indices, unit weight ( Ƴ and Ƴ_s), and shear parameters (c′ and ф′), are collected by accounting for aging effects. Biological (biodegradation and gas generation), chemical (leachate compositional changes including pH and salinity), and mechanical (changes in density, compressibility, and shear strength) processes are incorporated under aging effects on the landfill system. These processes collectively vary the hydraulic and mechanical properties of waste over time, influencing the stability and settlement of the landfill. Hence, incorporating aging effects ensures that the model realistically represents long-term landfill behavior.

In this paper, HDPE geomembrane was used by adopting a single liner system. Geomembrane liner of 1.50 mm thickness, 0.94 g/cc density with yielding tensile strength of 22 kN/m, break tensile strength of 53 kN/m with hydraulic conductivity of 2.0 × 10⁻¹³ cm/s placed at the surface of the clay liner. This geosynthetic material is manufactured from polymers or hydrocarbon chains that are used for a wide range of engineering applications with high resistance to temperature and adverse climates for almost 50 years without damage. Geomembranes and geosynthetic clay liners are thin (≤ 10 mm), relatively impervious geosynthetics that are used as barriers in containment applications for waste disposal and in situ remediation as well.

2.3. Numerical Modeling Approach

In this analysis, waste materials with their components were modeled using the PLAXIS 2D tool, considering various approaches to capture the real-world behavior of the landfill system. PLAXIS is identified as a potential tool for effective and efficient modeling of geomaterials with a wide range, including rock to soft clay soil under various environmental and loading conditions. PLAXIS (SSM and SSC model) will be utilized to capture nonlinear, heterogeneous, anisotropic conditions of the landfill. It also incorporates realistic material properties, boundary conditions, and loading scenarios.

Hydraulic and mechanical boundary conditions are adopted for both the model and waste material inside the model. However, the landfill configuration selected for this analysis is above- and belowground fill configuration type. Such landfill configuration is applicable when the groundwater table lies at great depth below the base of landfill and the foundation is low in permeability. Thus, no flow is considered for the model from base through the liner system (geomembrane and clay), with an interface element to limit leakage. No flow along the sides of the model was assumed because of landfill configuration and plane symmetry, but surface ditch is assigned to accommodate surface water. However, mechanical boundary conditions are defined by fixing both the base and sides of the model for vertical and horizontal displacements. Similarly, both hydraulic and mechanical boundary conditions were applied for waste material inside the model at the base and sides. Hydraulic boundary is limited at base by the liner system, which was treated as no flow because of very small hydraulic conductivity interfaces, and drainage was indicated by a high-permeability layer connected to head-controlled line drains. Although at the sides, the leachate head is assigned as hydraulic boundaries. Mechanical boundaries comprised fixed displacements at the base and free conditions on the vertical sides, with interface elements along the liner and cover.

In this paper, this software is adopted for special cases to account for waste material modeled as SSM and SSC model, allowing fine mesh to have accurate analysis with detailed adjustment of stiffness parameters through modified compression and swelling indices. Accordingly, seven models were developed considering three landfill failures inducing variables. These variables are slope angles, waste composition based on age, filling configuration, and leachate level rise, as presented in Figure 4.

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The SSM will be employed to evaluate the deformation and stability of landfill slopes. Model inputs will include material properties, boundary conditions, and loading scenarios. The suitability of the SSM will be assessed based on its ability to replicate observed behaviors in landfill slopes.

Using models 1 to 5, the slope angle’s influence on landfill deformation and instability was assessed. Failure mechanism at the interface behavior between the geomembrane liner and adjacent material issues was also assessed in these first five models during slope stability simulation [6, 31]. Additionally, differential settlement due to filling configurations was estimated using Model 2 with horizontal filling and Model 5 with cellular filling approaches. Eventually, the leachate level’s influence was addressed using Models 2, 6, and 7 as described in Figure 4. These simulations integrate geotechnical principles with advanced computational tools to create a sustainable landfill system and possible enhancements for waste management practices [32].

2.4. Verification of the Model

Verification was conducted to ensure that the developed model aligned with accepted theories and previously published results. It was performed by focusing on the model’s performance in line with established knowledge and benchmarking relevant research findings rather than direct comparison with experimental or observational data. In fact, the relevant articles used for verification in this paper analyzed their problems using experimental and observational data to ensure average waste material properties in the current model. Accordingly, the results of the verification computations and outputs were presented, comparing the current model with the findings of relevant papers. This procedure strengthened the validity of the model’s underlying assumptions by confirming that the estimations of the model were consistent with the approach and outputs that were found.

Moreover, verification of the waste material properties of this model was conducted using back analysis to ensure the reality behind the actual scenario in the deformation and stability of the landfill system. Hence, model calibration was made based on creep deformation and the factor of safety (Fs) by adopting Model 2. Selection of Model 2 is based on the assumption that the defined slope angle, waste configuration, and leachate level are considered in the current model developed using average waste material properties. The material properties of the subgrade, clay-geomembrane liner, sand leachate collection layer, top and bottom gravel protection layers, and final silty clayey cover are considered the same as initial forward analysis in the verification section. The only calibration data are based on waste material properties because it is the basic focus of the investigation in this paper and the most affecting material influencing instability in the landfill system. PLAXIS 2D back analysis was conducted through rigorous iterations, and waste material properties were adjusted for input into the software accordingly. While analyzing the model, simulation was allowed for systematic iterations of waste material properties. Furthermore, a systematic normalization of the results from the used research papers has been made to align their findings with the current model while verifying the model. Accordingly, waste material behaviors, such as unit weight, shear parameters, modified compression index, creep index, and hydraulic conductivity values, are rigorously calibrated, and the model was verified.

3. Results and Discussion

As detailed in the methodology section, this section discusses the results of numerical analysis and evaluation of the landfill deformation and stability behavior considering various conditions under gravity loading. The results will present deformation patterns, factors of safety, and potential failure modes under various conditions. The discussion will compare the performance of the SSM against other approaches, highlighting its advantages and limitations in landfill applications.

3.1. Slope Angle Effect on Landfill Deformation and Stability

3.1.1. Deformation

Deformation and stability influenced by slope angles were evaluated using four different models with varying slope angles to capture a real-world landfilling approach. Accordingly, Model 1 for 1V:4H, Model 2 for 1V: 3H, Model 3 for 1V:2H, and Model 4 for 3V:4H slopes are adopted in this analysis, as shown in Figure 5. The height of the model is 10 m from the ground surface and 15 m from the base of the clay liner, with horizontal slope lengths varying from 13.50 to 40 m. Computations of slope angles are conducted using fill height from the ground surface only. An overall width of 120 m landfill system is modeled, including surface runoff drainage systems. The plan-stain analysis approach is adopted in this paper.

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Model 2 and 4 deformation analysis outputs are used for presentation, and the remaining models, 1 and 3, were analyzed in a similar manner as shown in Figure 6. As indicated, the deformation of the landfill potentially increased with an increase in slope angle. Accordingly, the maximum vertical deformation (U) results of 0.44, 0.75, 0.94, and 1.41 m are obtained from Models 1, 2, 3, and 4, respectively. This shows that there is a high potential for failure of the landfill system with such escalated deformations, particularly for the last three models having deformation greater than 0.75 m, which is 8% of fill height from the ground surface or 5% of the full height of the landfill from the base of the bottom liner system. Even though, in extreme conditions, the maximum vertical deformation of 50% of total landfill height is anticipated over time, this paper adopted 8% possible deformation obtained from Model 2 with a slope angle of 1:3. This limit is also essential to account for its effect on the safety factor.

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3.1.2. Slope Stability

As presented in the deformation analysis, the slope angle has a major influence on the stability of the landfill slope. Landfill is more vulnerable with an increase in slope angle. The analysis outputs revealed that the Fs is inversely associated with the slope angle, which was achieved by different models as presented in Figure 7.

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Various deformation migration patterns are also observed during stability analysis. As shown in Figure 8, the deformation at the slope face is localized due to the cover geomembrane interface, indicating that the initiation of failure is anticipated along the interface and then to the fill volume of the system. Therefore, it is better to consider such problems by providing textured geomembrane types to minimize interface failures earlier. Additionally, runoff drainage systems have their own effect on the deformation pattern and stability, which should be accounted for in the long-term serviceability of the system.

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Slope stability analysis of four models revealed different results for safety factors, as shown in Figure 8. Models 1 and 2 are sufficiently stable with Fs of 1.82 and 1.60, which are greater than 1.50 for landfill stability requirements.

However, Model 3 and 4 computations with larger slope angles provided Fs of 1.31 and 1.06, respectively, which do not satisfy the Fs for landfill slope stability. Therefore, it is possible to use Model 2, fixing the slope angle at 1V:3H to evaluate other models because this model satisfies stability requirements and addresses deformation issues as well.

Overall, in landfill analysis, the selection of slope angles is quite an essential parameter for the stability and service of the system. As shown in Figure 9, the simulation output revealed that deformation in the vertical as well as lateral increases potentially, whereas the factor of factor decreases with increased slope angles from 14 (1V:4H) to 37 (3V:4H). Accordingly, total vertical and lateral deformation increased from 0.44, 0.75, 0.94, 1.41, 0.17, 0.25, 0.33, and 0.50 m, whereas the Fs decreased from 1.80, 1.60, 1.31, and 1.06 for slope angles 1:4, 1:3, 1:2, and 3:4, respectively.

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3.2. Landfill Configuration and Aging Effect

An evaluation of landfill configuration and aging effect is analyzed to estimate differential settlement along a horizontal arbitrary section upon the landfill volume of the model created by horizontal and cellular landfilling mechanisms, which are common in the practical world. The analysis was conducted using Model 2 for horizontal configuration and Model 5 for cellular configuration by adopting a slope of 1V:3H for both models. As presented in Figures 10(a) and 10(b), the material domain is arranged in age order, and the mesh profile utilizes fine mesh for the global model and very fine mesh for the liner system to capture realistic material response. Model 2 has already been analyzed; results are presented in Section 3.1; and only results are needed in this computation.

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Accordingly, 0.75 and 1.12 m maximum total deformation (U) and 0.25 and 0.37 m lateral deformation (Ux) are achieved for horizontal and cellular filled models, respectively. This indicates insight for the practical world that the deformation and vulnerability of slope failure are higher for the cellular filling than the horizontal filling approach. The reason for this potentially arises from variation in waste age along a given horizontal surface and lack of uniform compaction during the construction stage.

Additionally, differential settlement is evaluated upon two models by taking arbitrary horizontal sections A-A^∗ and B-B^∗ at 3.0 and 9.0 m from ground level, respectively. Figure 11 is used for demonstration purposes from the deformation output of Model 2 with two horizontal sections that Model 5 is also simulated in a similar manner.

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Analysis results of deformations at each point along the proposed cross sections are sorted and presented in Figure 12. The output results indicate a considerable difference in differential settlement results observed between horizontal and cellular filling approaches. It was revealed that cellular fillings have maximum deformation at the center, indicated by a concave downward curve relative to horizontal filling, resulting in higher differential settlement. In fact, there is a slight differential settlement difference in horizontally filled landfills, which is very small relative to cellular filling.

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In conclusion, it was revealed that cellular filling has a higher risk for differential settlement than the horizontally filled landfill system and has a high possibility for slope failure leading to environmental contamination.

The Fs is also evaluated for the two configurations. Accordingly, the Fs of Model 2 is 1.60 from Section 3.1 and Model 5 is 1.23, which is 31% difference between the two models, as shown in Figure 13. This indicates that the practical world must account for the filling configuration effect.

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The aging of waste materials has its own effect on landfill deformation and slope stability. To evaluate waste aging, different waste age ranges are used in the analysis. Consequently, fresh waste ages of 3, 5, 8, 12, and 15 years are applied in the model. Waste ages below two or three years are considered fresh (young). The horizontal filling approach is considered with 1:3 slope.

Accordingly, six submodels considering each waste age under Model 2 were developed, and analyses for deformation and safety factors were estimated and are presented in Figure 13.

As presented in Figure 14, there were the drastic increase in total vertical deformation and a decrease in Fs from fresh to 5 years of waste age. Lesser deformation and higher factors of safety at initial stages are achieved by the apparent stability of waste material, which becomes looser with age due to decomposition. Accordingly, deformation rises from 0.61 to 0.80 m and the Fs falls from 1.56 to 1.36 for fresh to 5 years aged waste models. After this age, both deformation and the Fs maintain a slight increase. These trends indicate that organic matter is largely decomposed, and settlement slows down because of compression through a reducing void ratio. At a waste age of 15 years, total and lateral deformations of 0.87 m and 0.39 m are achieved, respectively. The average value of the Fs for six submodels is 1.52, which is slightly lower than 1.60 achieved from Model 2, where older waste is at the bottom and fresh at the top.

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Practically, filling uniform waste at once may be rare, however, under certain conditions, such as shifting an old landfill to another location due to an essential project. For this reason, knowing the possible age and risk of deformation while integrating instability issues can provide insight into both the practical and research worlds of environmental geotechnics.

3.3. Leachate Level Influence on Landfill Deformation and Stability

Three leachate level models were accounted for to simulate the impact of leachate level rise on the landfill system, inducing instability and excessive deformation. As a result, the leachate level at the leachate collection layer is estimated using Model 2, and the leachate levels at the bottom protective layer and at ground surface level are measured using Models 6 and 7, respectively. Age-wise horizontal landfill layout with a 1:3 slope is used for all models. Computation of Model 2 has already been conducted in Sections 3.1 and 3.2 with expected leachate levels; only results are needed in this section’s evaluation. Models 6 and 7 were analyzed by taking leachate levels, as discussed in the methodology section.

Due to the rise in leachate levels in the landfill system, pore water pressure (PWP) is potentially increased in the landfill system. As presented in Figure 15, PWP increased from 4.50 to 12 and 32 kPa when the leachate level was raised from the leachate collection level to the bottom protective layer and ground surface, respectively. The limitation here is that the flow boundary is considered at the base of the model, not at the HDPE membrane, to simplify the analysis.

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As presented in Figures 16, 17, and 18, the increase in leachate level increases in deformation from the localized interface to the landfill system. The deformation migrates more from Model 2 to Models 6 and 7 resulting in landfill instability and providing a reduced Fs. As a result, total vertical deformation is raised from 0.75 to 0.91 and 1.12 m, and Fs is reduced from 1.60 to 1.44 and 1.26 from Model 2 to Models 6 and 7, respectively. This indicates that the rise in leachate levels significantly escalates the failure of the landfill system.

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As indicated in Figure 19, long-term creep deformation for Models 2, 6, and 7 was also estimated for 4.80 years (1750 days) by adopting a soft creep model. Accordingly, the leachate level (L.L) at the ground surface (G.S) has creep deformation under sustained gravity load of 1.55 m for Model 7, whereas 1.24 m for Model 6 and 0.87 m for Model 2 are achieved. Long-term creep is due to the rearrangement of materials and microbial decomposition over time.

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The innovative aspect here is that creep deformation analysis under sustained gravity load of the landfill system were waste materials. No leachate effect is considered in the software simulation. However, the results here reveal that there is an influence of creep on consolidation and vice versa, which is an existing hypothesis in geotechnical engineering problems today.

3.4. Verification of the Model

As illustrated in the methodology section, creep deformation and factors of safety are considered to estimate the reality of the current model through back analysis for waste material properties. Model verification was conducted using a comparative approach by associating it with selected relevant research findings, which are analyzed from experimental and observational data.

Figures 20(a), 20(b), and 20(c) indicates the results of back analysis, showing both waste material properties and stability performance. Accordingly, the analysis model is defined by Model 2 as shown in Figure 20(a), while the waste material properties, such as bulk unit weight (Ƴ), effective cohesion (c^′), and effective friction angle (ф^′) in Figure 20(b) and, similarly, in Figure 20(c), the waste mechanical parameters λ^∗, k^∗, and μ^∗, along with the hydraulic conductivity (K), exhibit differences between the assumed and back-calculated values. It also indicates that the back-analyzed values are consistently higher than the initially assumed input parameters, suggesting that the model slightly underestimates waste behavior. These comparisons emphasize the calibration role of back analysis in adjusting material parameters to achieve realistic stability conditions, ensuring that the modeled response better reflects observed slope behavior.

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The performance of the current model was evaluated by comparing its calculations with previously published findings on creep deformation and Fs, as shown in Figures 21(a), 21(b), 21(c), and 21(d). The creep deformation response predicted by the proposed model is shown in Figure 21(a) and compared with the experimental and numerical findings of Simões and Catapreta [38], He et al. [19], and Bareither and Kwak [33]. The results demonstrate that the model effectively captures the overall time-dependent deformation trends, with the closest agreement obtained against previous findings. This verification is further supported by the RMSE values presented in Figure 21(c), where the model achieved an RMSE of 2% with Simões and Catapreta [38], while slightly higher deviations were observed when compared to He et al. [36] and Bareither and Kwak [33], yielding RMSE values of 8% and 13%, respectively. However, this paper considers a 20% RMSE difference between current and previously obtained results to account for potential behavioral variation in waste material properties.

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Similarly, the stability performance of the model, expressed in Fs, is presented in Figure 21(b) and benchmarked against the results of Ering and Sivakumar Babu [17], Feng et al. [35], Keskin and Kezer [30], and Ismail et al. [37]. The model response stabilizes around an Fs of 1.5, consistent with the reported literature values. The corresponding RMSE analysis in Figure 21(d) highlights the degree of agreement, with values of 6.7%, 4.6%, 5.3%, and 3.0% when compared to Feng et al. [35], Ering and Sivakumar Babu [34], Keskin and Kezer [30], and Ismail et al. [37], respectively. Remarkably, the lowest RMSE was observed with Ismail et al. [37], confirming the strong predictive capability of the proposed model in capturing stability behavior.

Accordingly, verification concludes that using back analysis and comparative valuation confirms that the current PLAXIS 2D model offers reliable estimates of both creep deformation and stability, ensuring strong agreement with previously published research papers. These outcomes demonstrate the robustness and applicability of the model for long-term performance evaluation of geotechnical systems in the field of landfill systems and management.

4. Conclusions

This analysis demonstrates the significance of using advanced numerical simulation to evaluate landfill slope stability and deformation under various design and operation scenarios. Using PLAXIS 2D with the SSM and SSC model, the analysis effectively captured the nonlinear, time-dependent, and heterogeneous behavior of landfill components and municipal solid waste (MSW). Accordingly, the following conclusions are made:

• •

  The stability of a landfill is greatly impacted by the slope angle. Vertical deformation rises from 0.44 to 1.41 m as the slope angle increases from 1V:4H to 3V:4H, but the Fs decreases from 1.82 to a critical 1.06, suggesting high instability at steeper gradients. To maintain equilibrium with an Fs of 1.60 and controllable deformation (0.75 m), a slope angle of 1V:3H is advised.

• •

  Landfills with a horizontal filling configuration overtook those with cellular filling. Because of age-based heterogeneity and compaction inconsistency, cellular filling caused a 23% decrease in Fs (1.23 vs. 1.60) and a 49% increase in vertical deformation (1.12 vs. 0.75 m).

• •

  Aging showed stage degradation and waste structural softening in the landfill models. Between fresh and 5-year-old waste, Fs decreased from 1.56 to 1.36, but after five years, waste properties level out because of the decreased void ratio, and at fifteen years, long-term deformation reaches a plateau of about 0.87 m.

• •

  Stability is greatly reduced by increasing leachate levels. PWP increased from 4.5 to 32 kPa as the leachate level rose from the drainage base to the ground surface. This led to a decrease in Fs (1.60 to 1.26) and an increase in deformation (0.75 to 1.12 m). This emphasizes the necessity of effective pore pressure control and drainage systems.

• •

  In saturated conditions, long-term creep deformation under sustained loading reached 1.55 m over 7.65 years, demonstrating the crucial role that time-dependent analysis plays in landfill performance.

• •

  Verification through back analysis and comparative valuation affirms the PLAXIS 2D model’s capability for predicting creep deformation and stability, ensuring a strong agreement with prior research, highlighting its suitability for assessing the long-term performance of landfill systems.

Acknowledgments

The authors would like to thank King Fahd University of Petroleum and Minerals and Arba Minch University for providing all logistical support for this research work.

Funding

No funding was received for this research.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

All the supporting data are included in this paper.

References

1 Honcharenko I., Anishchenko L., and Pisnia L., Expert-Analytical Estimation of Environmental Safety of Solid Household Waste Handling Processes, Eastern-European Journal of Enterprise Technologies. (2020) 1, no. 10, 63–76, https://doi.org/10.15587/1729-4061.2020.197007.

2 Stark T. D., Eid H. T., Evans W. D., and Sherry P. E., Municipal Solid Waste Slope Failure. II: Stability Analyses, Journal of Geotechnical and Geoenvironmental Engineering. (2000) 126, no. 5, 408–419, https://doi.org/10.1061/(asce)1090-0241(2000)126:5(408), 2-s2.0-0033827880.

3 McWatters R. S., Jones D. D., Rowe R. K., and Markle J. M., Investigation of a Decommissioned Landfill Barrier System Containing Polychlorinated Biphenyl Waste After 25 Years in Service, Canadian Geotechnical Journal. (2020) 57, no. 6, 882–902, https://doi.org/10.1139/cgj-2018-0784.

4 Celik B., Girgin S., Yazici A., and Unlu K., A Decision Support System for Assessing Landfill Performance, Waste Management. (2010) 30, no. 1, 72–81, https://doi.org/10.1016/j.wasman.2009.09.016, 2-s2.0-70649088852.

5 Tarhan B. and Unlu K., Performance-Based Landfill Design: Development of a Design Component Selection Matrix Using GIS and System Simulation Models, Environmental Geology. (2005) 49, no. 1, 133–147, https://doi.org/10.1007/s00254-005-0073-4, 2-s2.0-27944486152.

6 Gao W., Bian X.-C., Xu W.-J., and Chen Y.-M., Storage Capacity and Slope Stability Analysis of Municipal Solid Waste Landfills, Journal of Performance of Constructed Facilities. (2018) 32, no. 4, 1–20, https://doi.org/10.1061/(asce)cf.1943-5509.0001180, 2-s2.0-85046018440.

7 Blight G. E. and Fourie A. B., Catastrophe Revisited—Disastrous Flow Failures of Mine and Municipal Solid Waste, Geotechnical and Geological Engineering. (2005) 23, no. 3, 219–248, https://doi.org/10.1007/s10706-004-7067-y, 2-s2.0-17844383727.

8 Durmusoglu E., Corapcioglu M. Y., and Tuncay K., Landfill Settlement With Decomposition and Gas Generation, Journal of Environmental Engineering. (2005) 131, no. 9, 1311–1321, https://doi.org/10.1061/(asce)0733-9372(2005)131:9(1311), 2-s2.0-27544435591.

9 Edil T. B., Ranguette V. J., and Wuellner W. W., Settlement of municipal refuse, ASTM Special Technical Publication. (1990) 1070, 225–239.

10 Sharma H. D., Asce F., De A., and Asce M., Municipal Solid Waste Landfill Settlement: Postclosure Perspectives, Journal of Geotechnical and Geoenvironmental Engineering. (2007) 133, no. 6, 619–629, https://doi.org/10.1061/(asce)1090-0241(2007)133:6(619), 2-s2.0-34249084943.

11 Feng S. J., Wu S. J., Fu W. D., Zheng Q. T., and Zhang X. L., Slope Stability Analysis of a Landfill Subjected to Leachate Recirculation and Aeration Considering Bio-Hydro Coupled Processes, Geoenvironmental Disasters. (2021) 8, no. 1, 1–16, https://doi.org/10.1186/s40677-021-00201-2.

12 Goel A., Metya S., and Bhattacharya G., Probabilistic Slope Stability Analysis of Bioreactor Landfills, Lecture Notes in Civil Engineering. (2023) 303, 207–218.

13 Gurjar D. K. and Jha A. K., Influence of Slope Geometry on the Stability of Bioreactor Landfills: A Numerical Analysis, Lecture Notes in Civil Engineering. (2023) 298, 343–356.

14 Hossain M. S. and Haque M. A., Stability Analysis of Municipal Solid Waste Landfills With Decomposition, Geotechnical and Geological Engineering. (2009) 27, no. 6, 659–666, https://doi.org/10.1007/s10706-009-9265-0, 2-s2.0-70849134859.

15 Jun Z., Junsheng O., Wannian T., Huanlin Z., and Tongxi W. U., Numerical Analysis of Creep Effects on the Settlement of PHC Pipe Pile in Soft Clay Treated by Deep Mixing Method, Geotechnical and Geological Engineering. (2024) 42, no. 7, 5967–5977, https://doi.org/10.1007/s10706-024-02872-8.

16 Jessberger H. L., Geotechnical Aspects of Landfill Design and Construction. Part 2: Material Parameters and Test Methods, Proceedings of the Institution of Civil Engineers: Geotechnical Engineering. (1994) 107, no. 2, 105–113, https://doi.org/10.1680/igeng.1994.26378, 2-s2.0-0028167089.

17 Ering P. and Sivakumar Babu G. L., Slope Stability and Deformation Analysis of Bangalore MSW Landfills Using a Constitutive Model, International Journal of Geomechanics. (2016) 16, no. 4, 1–9.

18 Machado S. L., Vilar O. M., and Carvalho M. F., Constitutive Model for Long Term Municipal Solid Waste Mechanical Behavior, Computers and Geotechnics. (2008) 35, no. 5, 775–790, https://doi.org/10.1016/j.compgeo.2007.11.008, 2-s2.0-51449121698.

19 He H., Wu T., Wang X., Qiu Z., and Lan J., Study on Compressibility and Settlement of a Landfill With Aged Municipal Solid Waste: A Case Study in Taizhou, Sustainabilty. (2021) 13, no. 13, 1–13, https://doi.org/10.3390/su13094831.

20 Khoshand A. and Fall M., Geotechnical Characterization of Peat-Based Landfill Cover Materials, Journal of Rock Mechanics and Geotechnical Engineering. (2016) 8, no. 5, 596–604, https://doi.org/10.1016/j.jrmge.2016.05.007, 2-s2.0-84989928261.

21 Machado S. L., Karimpour-Fard M., Shariatmadari N., Carvalho M. F., and Nascimento J. C. F. D., Evaluation of the Geotechnical Properties of MSW in Two Brazilian Landfills, Waste Management. (2010) 30, no. 12, 2579–2591, https://doi.org/10.1016/j.wasman.2010.07.019, 2-s2.0-77957673650.

22 Pereira J. P., Geotechnical Studies of Waste Deposits and Landfills, 2000, Melbourne, Australia, 1–6.

23 Prathapan R. and Ki L. C., Proceedings 3rd International Symposium on Advances in Civil and Environmental Engineering Practices for Sustainable Development ACEPS 2015, Advances in Civil and Environmental Engineering Practices for Sustainable Development. (2015) 3, no. 4, 63–69.

24 Hyun et al., Geotechnical Considerations for End-Use of Old Municipal Solid Waste Landfills, International Journal of Environmental Research. (2011) 5, no. 3, 573–584.

25 Xie Y., Xue J., Gnanendran C. T., and Xie K., Geotechnical Properties of Fresh Municipal Solid Wastes With Different Compositions Under Leachate Exposure, Waste Management. (2022) 149, 207–217, https://doi.org/10.1016/j.wasman.2022.06.020.

26 Zekkos D., Bray J. D., Kavazanjian E. et al., Unit Weight of Municipal Solid Waste, Journal of Geotechnical and Geoenvironmental Engineering. (2006) 132, no. 10, 1250–1261, https://doi.org/10.1061/(asce)1090-0241(2006)132:10(1250), 2-s2.0-33748795570.

27 Hyun I. I., Geotechnical Considerations for End-Use of Old Municipal Solid Waste Landfills, International Journal of Environmental Research. (2011) 5, no. 3, 573–584.

28 Karimpour-Fard M., Machado S. L., Shariatmadari N., and Noorzad A., A Laboratory Study on the MSW Mechanical Behavior in Triaxial Apparatus, Waste Management. (2011) 31, no. 8, 1807–1819, https://doi.org/10.1016/j.wasman.2011.03.011, 2-s2.0-79958252728.

29 Mondal S. and Rafizul I. M., Analyzing the Impact of Age on Waste Properties at an Open Dumpsite in Khulna, Cleaner Engineering and Technology. (2024) 23, no. 1, 1–18, https://doi.org/10.1016/j.clet.2024.100833.

30 Keskin M. S. and Kezer S., Stability of MSW Landfill Slopes Reinforced With Geogrids, Applied Sciences. (2022) 12, no. 2, 1–13, https://doi.org/10.3390/app122211866.

31 Shi J., Shu S., Ai Y., Jiang Z., Li Y., and Xu G., Effect of Elevated Temperature on Solid Waste Shear Strength and Landfill Slope Stability, Waste Management and Research. (2021) 39, no. 2, 351–359, https://doi.org/10.1177/0734242x20958065.

32 El-Fadel M. and Khoury R., Modeling Settlement in MSW Landfills: A Critical Review, Critical Reviews in Environmental Science and Technology. (2000) 30, no. 3, 327–361, https://doi.org/10.1080/10643380091184200, 2-s2.0-0033869902.

33 Bareither C. A. and Kwak S., Assessment of Municipal Solid Waste Settlement Models Based on Field-Scale Data Analysis, Waste Management. (2015) 42, no. 5, 101–117, https://doi.org/10.1016/j.wasman.2015.04.011, 2-s2.0-84983119443.

34 Ering P. and Sivakumar Babu G. L., Slope Stability and Deformation Analysis of Bangalore MSW Landfills Using a Constitutive Model, International Journal of Geomechanics. (2016) 16, no. 4, 1–9.

35 Feng S.-J., Ai S.-G., and Huang R.-Q., Stability Analysis of Landfill Cover Systems Considering Reinforcement Geogrid Reinforcement Force Uh Resultant of the Pore Pressures Acting on the Interwedge Surfaces, Environmental Earth Sciences. (2016) 75, no. 10, 1–10.

36 He H., Wu T., Wang X., Qiu Z., and Lan J., Study on Compressibility and Settlement of a Landfill With Aged Municipal Solid Waste: A Case Study in Taizhou, Sustainability. (2021) 13, no. 9, 4831–4846, https://doi.org/10.3390/su13094831.

37 Ismail S., Taib A. M., Rahman N. A., Hasbollah D. Z. A., and Ramli A. B., Slope Stability of Landfill With Waste Degradation, International Journal of Innovative Technology and Exploring Engineering. (2019) 9, no. 1, 393–398, https://doi.org/10.35940/ijitee.a4148.119119.

38 Simões G. F. and Catapreta C. A. A., Monitoring and Modeling of Long-Term Settlements of an Experimental Landfill in Brazil, Waste Management. (2013) 33, no. 2, 420–430.