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

e impact of soil parameters and slope angle on the deformation and stability of cut slopes is critical for dening road project safety measurement. is study investigates the eect of soil properties and slope angle on the deformation and stability of cut slopes in general and the specic 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 dierent slope angles. Twenty-four (24) dataset samples were collected from six dierent cut-slope sites. ese dataset samples were categorized in two situations, i.e., before and after water saturation for each cut slope.e limit equilibriummethod (LEM) comparison clearly showed that the Spencer, Bishop, and Morgenstern-Price methods produced similar FOS. e Ordinary and Janbu approaches, on the other hand, underestimate the FOS. Most LEMs except Ordinary and Janbu methods that estimated higher FOS than nite 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 ne 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 ne-grained fractions. Moreover, attening the slope stabilizes the cut slopes based on the results obtained from both Plaxis 2D and Slope/W.


Introduction
e 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. ese frequently occur as a result of a lack of forethought while dealing with sloping terrain. e majority of the constructed roads pass through valleys, hilly, and mountainous terrain constructed from cutting and lling of the terrain [1][2][3][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][6][7].
Because the "limit equilibrium technique" and the " nite 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 veri cation of one method with the other. e 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. e 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. is research could help assess the performance and validity of slope stability analysis.

Materials and Methods
is 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][36][37][38].
e 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.
e 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.

Sampling, Sample Collection, and Laboratory Tests.
e 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].
e specific gravity of the study area ranges from 2.67 to 2.73. e 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%. e 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. e soil slope-2 is classified as low-plastic silt and high-plastic silt, with lowplastic 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 midslope and toe slope. e 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. e input data for the analysis of LEM and FEM were determined from in situ and laboratory tests. e 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.

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.  Advances in Civil Engineering FOS � resisting force(moment) driving force(moment) . (1) e Slope/W program was used to build the geometry. Morgenstern-Price, Ordinary, Spencer, Bishop, and Janbu methods were used to solve FOS. e 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.

Strength Parameters (C, ϕ).
e shear strength parameters were determined from representative testing of the soil in the laboratory. e samples were prepared by remolding the soil samples using their natural moisture content and in situ unit weight. ree 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. e 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)

Unit Weight (c).
e 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. e 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).

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. e numerical simulation approaches of the finite element method (FEM) and limit equilibrium method (LEM) were employed to investigate slopes with specified geometry [41].

Numerical Results.
e 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. e comparison was made using modeling data from Plaxis 2D (FEM-based) and Geo-Slope 2012 (LEMbased) 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. e input parameters for both LEM and FEM analysis were carefully determined from both laboratory and field tests. e 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.

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.

Characteristic Slope-1.
e 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.

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. 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.

Characteristic Slope-4.
e 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. e 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. 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. e 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. 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. e 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.

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].     Tables 8-13, respectively, using the different LEM methods. e 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. is 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.

Conclusions.
From the modeling results, the causes of cut-slope instability, the cut-slope geometry, water content, and soil properties were investigated. e 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. e 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%. e LEM estimates higher FOS than FEM analysis, except for the Janbu and Ordinary methods. e Janbu and Ordinary methods, when compared to the FEM, underestimate FOS by up to 7% on average.

Recommendations
e 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][44][45].

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

Conflicts of Interest
e authors declare that they have no conflicts of interest.