Effects of Soil Properties and Slope Angle on Deformation and Stability of Cut Slopes

Habtemariam, Behailu G.; Shirago, Kelifa B.; Dirate, Democracy D.

doi:https://doi.org/10.1155/2022/4882095

Advances in Civil Engineering

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 4882095 | https://doi.org/10.1155/2022/4882095

Effects of Soil Properties and Slope Angle on Deformation and Stability of Cut Slopes

Behailu G. Habtemariam,¹Kelifa B. Shirago,¹and Democracy D. Dirate¹

Academic Editor: Paolo S. Valvo

Received22 Feb 2022

Revised28 Apr 2022

Accepted09 May 2022

Published07 Jun 2022

Abstract

The impact of soil parameters and slope angle on the deformation and stability of cut slopes is critical for defining road project safety measurement. This study investigates the effect of soil properties and slope angle on the deformation and stability of cut slopes in general and the specific Arba Minch-Chencha upgrading road project. Forty-eight (48) analyses were carried out both in Slope/W and Plaxis 2D software for six cut slopes and analyzed for four different slope angles. Twenty-four (24) dataset samples were collected from six different cut-slope sites. These dataset samples were categorized in two situations, i.e., before and after water saturation for each cut slope. The limit equilibrium method (LEM) comparison clearly showed that the Spencer, Bishop, and Morgenstern-Price methods produced similar FOS. The Ordinary and Janbu approaches, on the other hand, underestimate the FOS. Most LEMs except Ordinary and Janbu methods that estimated higher FOS than finite element method (FEM) analysis. It is observed that the main reasons for the cut-slope instability were the provision of steep cut-slope angles, the existence of a high proportion of fine soil, and moisture content, which was observed in both Plaxis 2D (FEM) and Slope/W (LEM). It was concluded that the slope is more stable for the soil having few fine-grained fractions. Moreover, flattening the slope stabilizes the cut slopes based on the results obtained from both Plaxis 2D and Slope/W.

1. Introduction

The Arba Minch-Chencha road is a national roadway that was built by cutting through the natural terrain of the area and creating a steady slope. Natural disasters such as landslides, soil collapse, and slope failures regularly occur throughout the world. These frequently occur as a result of a lack of forethought while dealing with sloping terrain. The majority of the constructed roads pass through valleys, hilly, and mountainous terrain constructed from cutting and filling of the terrain [1–4].

Evidence indicated that the demand for engineered cut slopes for construction activities grows, and the need for analytical approaches, investigative tools, and stabilization procedures to tackle slope instability requires attention. Man-made slopes usually disrupt the delicate balance of natural soil slopes. Slides can happen in practically every way imaginable, slowly or quickly, and with or without apparent provocation. References [1, 5–7].

Many studies in recent years have been performed to check the stability of the slope using the limit equilibrium method (LEM) or strength reduction methods by finite element method (FEM) [8–13]. An alternative approach was developed by [13–22] to evaluate the slope performance by probabilistic approach. Other researchers like [15, 20, 23–25] evaluated the slope for unsaturated soil.

However, few researchers have addressed the problem of slopes using the strength reduction method by using the finite element method (FEM) to evaluate the slope performance [17, 26–31].

Because the “limit equilibrium technique” and the “finite element method” are still in their early phases of application in many road and infrastructure projects, it is vital to employ these methods to prevent harm after and during construction. However, most of the research deals with either LEM or FEM, to check the stability of the slope, which lacks verification of one method with the other.

The purpose of this study is to investigate and assess the effects of soil properties, water, and slope angles on the stability of cut slopes using the Slope/W (LEM) and Plaxis 2D (FEM) models on the Arba Minch-Chencha route. For each slope, a large number of soil samples were evaluated against input values derived both in the laboratory and in the field. Slope/W and Plaxis 2D software were used to examine the slope geometry with the input parameters. The results can be used to investigate slope performance for various soil types, slope angles, and the impact of water on the cut slopes’ slope stability. This research could help assess the performance and validity of slope stability analysis.

2. Materials and Methods

This study was focused on a case-based investigation and analysis of road projects at the Arba Minch-Chencha site. Representative soil samples from six selected sites were collected for laboratory testing to experiment index and engineering properties of the representative soil samples. Limit equilibrium was used to analyze slope stability. [5, 32, 33].

Moreover, Liu et al. [34] analyzed the stability of the slope using limit equilibrium and numerical methods to analyze the slope stability. Many researchers used the strength reduction methods to analyze slopes. [35–38].

2.1. Description of the Study Area

Arba Minch (6° 02′ 24″ to 6° 03′ 57″ N and 37° 33′ 36″ to 37° 35′ 30″ E) and Chencha (6° 14′ 54″ to 6° 15′ 31″ N and 37° 22′ 33″ to 37° 34′ 2″ E) are two towns located in Ethiopia’s Southern Nations, Nationalities, and People’s (SNNP) regional state (Figure 1). Arba Minch town is located 480 km southwest of Addis Ababa, with Lake Abaya and Lake Chamo to the east. The bottom section of the region is located in the rift valley’s plane, while the middle and top portions are located in Ethiopia’s southwestern highlands. The Arba Minch-Chencha road section is an extension of the Morka-Gircha-Chencha Road Project, 21 km from Shara Kebele, and the Sodo-Arba Minch main road 15 km from Arba Minch town. The Arba Minch-Chencha upgrading road project begins at a junction on the Arba Minch-Sodo road, which is 470 km from the capital, and connects to the Morka-Gircha-Chencha Road Project, which runs in a westerly direction to the project’s end, Wacha Town (Gamo Zone), passing through many rural villages and covering a total length of 108.6 km.

2.2. Sampling, Sample Collection, and Laboratory Tests

The soil samples for this investigation were obtained from the Chano-Chencha section of the existing gravel road from Arba Minch to Chencha. Six sampling areas were selected from various points along the road to reflect all soils found in the surrounding based on a preliminary site study. A total of twenty-four pits were excavated to a maximum depth of three meters to visually observe the subsurface condition and collect representative samples (Figure 2). For this study, good-quality disturbed soil samples were easily collected, transported carefully to a laboratory for testing, and then tested to determine engineering properties, moisture content, specific gravity, Atterberg limit, grain size analysis, direct shear, and one-dimensional consolidation [39].

The specific gravity of the study area ranges from 2.67 to 2.73. The soils for almost all samples were fine-grained. From the sieve size analysis results, clay content ranges from 1.52% to 12.14%, silt content ranges from 30.64% to 59.86%, sand fractions range from 28.80% to 58.45%, and gravel content ranges from 1.5% to 9.2%. The soil in slope-1 was classified as silty or clayey gravel and sand in the crust slope using the American Association of State Highway and Transportation Officials (AASHTO) classification system. The soil slope-2 is classified as low-plastic silt and high-plastic silt, with low-plastic silt at the mid-slope and high-plastic silt at the toe slope. Slope-3 and slope-6 were grouped under silty or clayey gravel and sand at the crust slope and low-plastic silt at mid-slope and toe slope. The soil profile of slope-4 was also classified as high-plastic silt, and slope-5 was classified as having low-plastic silt and high-plastic silt. The input data for the analysis of LEM and FEM were determined from in situ and laboratory tests. The shear strength parameters were determined from a remolded sample. Young’s modulus and Poisson’s ratio were determined from an unconsolidated undrained test of the triaxial test. Young’s modulus is varied with water content, so to be conservative, Young’s modulus for the saturated condition is taken for both the analysis of before and after saturation condition. Table 1 shows laboratory and field test results for the input parameters for analysis of the model.

2.3. Analysis Methods

Methods of limit equilibrium using Slope/W are as follows: the factor of safety is calculated from a two-dimensional equilibrium condition such that the factor of safety is the ratio of resistance force or moment to driving force or moment.

The Slope/W program was used to build the geometry. Morgenstern-Price, Ordinary, Spencer, Bishop, and Janbu methods were used to solve FOS. The slope stability was further assessed using Plaxis 2D, which is a finite element method, and the factor of safety was determined by the strength reduction technique.

For both LEM and FEM, the input parameters were carefully determined from laboratory testing.

2.4. Strength Parameters (C, ϕ)

The shear strength parameters were determined from representative testing of the soil in the laboratory. The samples were prepared by remolding the soil samples using their natural moisture content and in situ unit weight. Three tests were conducted with parrying vertical stress such that the second sample is equivalent to the in situ vertical stress of the soil sample and applying shear stress until the sample failed.

The shear strength was determined for natural moisture content and saturated condition, and for the saturated condition, the soil sample was soaked till the sample is saturated. For a saturated soil sample, an additional increase in the stress on the sample is proportional to an increase in pore water pressure, and in other words, B = 1 [40].

2.5. Unit Weight (γ)

The unit weight of the soil was determined using a core cutter sampler, by dividing the weight of a known volume of the sample by the volume of the soil.

Young’s modulus and Poisson’s ratio (E, ʋ).

Young’s modulus and Poisson’s ratio were determined by conducting unconsolidated undrained triaxial tests on undisturbed soil samples using a Shelby sampler. The vertical load was applied until the samples failed, so the modulus of elasticity and Poisson’s ratio of the sample at a different location are determined using equations (3) and (4).

3. Results and Discussion

In this study, the critical slope and stability of the slope were analyzed for different soils, which were achieved through the accurate determination of input parameters. Before the laboratory test of soil samples, a field assessment was performed to recognize prone soil slopes. Subsequently, there will be in situ tests to determine the density of the field and collect representative samples for laboratory testing. We determine the input parameters such as cohesion, friction angle, Atterberg limit, and Young’s modulus for analysis of the model slope. The numerical simulation approaches of the finite element method (FEM) and limit equilibrium method (LEM) were employed to investigate slopes with specified geometry [41].

3.1. Numerical Results

The stability of the slopes was analyzed using the FEM and LEM for comparison and verification of the slope condition. Using the LEM and FEM software, factor of safety (FOS) and slope deformation were determined. The comparison was made using modeling data from Plaxis 2D (FEM-based) and Geo-Slope 2012 (LEM-based) software. Furthermore, the condition of the slope before saturation and after saturation was analyzed so that the stability of the slope and the deformation of the corresponding state of the soil were determined. The input parameters for both LEM and FEM analysis were carefully determined from both laboratory and field tests. The whole set of the input parameters for both Plaxis 2D and Slope/W software for specified slopes was used to model the geometry of the slope.

3.2. Plaxis 2D Analysis

Plaxis 2D is a very useful finite element software to solve the FOS of the slope using the strength reduction method at the finite element level, whereas LEM analyzes the stability of the slope from equilibrium condition for the resisting force or moment to force or moment causing instability. From the analysis of the Plaxis 2D, it is indicated that as the water level rises, the slope deforms more; this indicated that the slope becomes more unstable with high water content. Many researchers like [42] indicated that water hurts the long-term stability of the embankment due to the development of pore water pressure. As depicted in Figure 3, the most extreme total displacement for the slope angle (1H: 1V) is 905 × 10⁻³ m as determined by the shear strength parameters after saturation.

3.2.1. Characteristic Slope-1

The FOS and deformation of CS1 for different slope angles before saturation and after saturation are shown in Table 2. After modification of the slope, it showed much deformation, which indicated the soil is more deformable both before and after saturation conditions.

3.2.2. Characteristic Slope-2

For slope angles of (1H: 1V), (1.5H: 1V), (2H: 1V), and (2.5H: 1V), Table 3 shows FOS and slope deformation values for slope-2 using shear parameters by Plaxis 2D for the case of before saturation and after saturation.

3.2.3. Characteristic Slope-3

Table 4 shows the FOS and slope deformation values for slope-3 using shear parameters by Plaxis 2D for the case of before saturation and after saturation.

3.2.4. Characteristic Slope-4

The FOS before saturation for slope (1H: 1V), (1.5H: 1V), (2H: 1V), and (2.5H: 1V) was 1.205, 1.345, 1.490, and 1.674, respectively, while the FOS of the fully saturated condition was 0.977, 1.154, 1.385, and 1.480, respectively. The highest extreme total displacement determined using shear parameters after saturation for a slope angle of (1H: 1V) is 943 × × 10⁻³ m. Table 5 shows FOS and slope deformation values for slope-4 using shear parameters by Plaxis 2D for the case of before saturation and after saturation.

3.2.5. Characteristic Slope-5

Table 6 shows FOS and slope deformation values for slope-5 using shear parameters by Plaxis 2D for the case of before saturation and after saturation.

The maximum total deformation attained using shear parameters (1H: 1V), (1.5H: 1V), (2H: 1V), and (2.5H: 1V) is larger after saturation. In contrast, the maximum total deformation attained using shear parameters before saturation is more negligible, and the factor of safety value is the opposite. Using the shear characteristics after saturation, the maximum extreme total displacement for the slope angle (1H: 1V) is 996 × 10⁻³ m.

3.2.6. Characteristic Slope-6

Table 7 shows FOS and slope deformation values for slope-6 using shear parameters by Plaxis 2D for the case of before saturation and after saturation.

The maximum total deformation attained using shear parameters for slope is reduced when the slope angle flattens, which is analogous to many researchers. Moreover, both horizontal deformation and vertical deformation increased after saturation. In contrast, the maximum total deformation attained using shear parameters before saturation is more negligible, the factor of safety value is the opposite, and this suggested that deformation and factor of safety are inversely proportional for the slope under dry conditions and saturated conditions (Table 7). Using the shear characteristics after saturation, the maximum total displacement for the slope angle (1H: 1V) is 958 × 10⁻³ m.

3.3. Slope/W Analysis

Slope/W analyzes the stability of the slope using the LEM. Five different methods (Sarma, Bishop, Ordinary, Janbu, and Morgenstern-Price) were taken to check the stability of the slopes. Each method analyzes the slopes based on equilibrium conditions, some methods considered equilibrium for the moment and force in the horizontal and vertical direction (Sarma and Morgenstern-Price), others considered only moment equilibrium (Ordinary and Bishop), and Janbu considered force equilibrium in the horizontal and vertical directions [2].

From the results of the Slope/W, slip surfaces for slopes CS1(a), CS2(b), CS3(c), CS4(d), CS5(e), and CS6(f) for the case of slope angle (1H: 1V) after saturation of the soil are shown in Figure 4.

(a)

(b)

(c)

(d)

(e)

(f)

The FOS for slopes CS1(a), CS2(b), CS3(c), CS4(d), CS5(e), and CS6(f) for different slope angles before and after saturation is indicated in Tables 8–13, respectively, using the different LEM methods.

The results presented in Tables 8–13 clearly show that the FOS is higher than 1 for the case of before saturation for the given different slope angles. This indicates that all slopes are considered stable under dry conditions. However, FOS values for all six slopes for the geometric slope angle of (1H: 1V) with water saturation are less than 1, and this depicts that the slopes are not stable under saturated conditions.

4. Conclusions and Recommendations

4.1. Conclusions

From the modeling results, the causes of cut-slope instability, the cut-slope geometry, water content, and soil properties were investigated.

The minimal factor of safety value in the result table clearly illustrates that the factor of safety reduces as the amount of water in the slope increases, which indicated that the slope becomes more unstable as the number of water increases. As the slope angle decreases, the amount of factor of safety increases.

The steepness was the primary cause of instability and then are a lower factor of safety and a higher maximum predicted displacement of a cut slope. Moreover, flattening the slope from (1H: 1V) to (1.5H: 1V), (2H: 1V), and (2.5H: 1V) stabilizes the slopes in both Plaxis 2D and Slope/W.

When the modeling results were compared, it was demonstrated that the Spencer, Bishop, and Morgenstern-Price approaches produced nearly the same FOS. However, Janbu and Ordinary methods may underestimate the FOS ranging from 5% to 6%. The LEM estimates higher FOS than FEM analysis, except for the Janbu and Ordinary methods. The Janbu and Ordinary methods, when compared to the FEM, underestimate FOS by up to 7% on average.

5. Recommendations

The factors of safety and cut-slope displacements were calculated using finite element analysis methods and the two-dimensional limit equilibrium method. To provide a more realistic simulation of the problem, the slope side effect should be incorporated into three-dimensional finite element analysis methods.

In this study, two conditions of the soil state condition are considered: a dry condition and a saturated condition, which are extreme conditions, so it is recommended to check the unsaturated stability condition considering the suction stress in unsaturated soil, the soil-water characteristic curve (SWCC), and the suction stress characteristic curve (SSCC) of the unsaturated soil obtained from the study area. [24, 43–45].

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors would like to extend their heartfelt gratitude to the Arba Minch University staff for their support at our disposal for laboratory testing and manuscript preparation for the accomplishment of this paper. This work was a duty as an academic job in our institute, Arba Minch University.

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Copyright © 2022 Behailu G. Habtemariam et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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