1. Introduction
Plant roots can efficiently improve the soil shear strength of the slope and forms the root–soil composition of the nearby surrounding soil. The roots of vegetation are intermingled in the soil layers of the slope, making the soil of the slope a combined layer [
1,
2,
3,
4,
5]. The mechanism through which root-reinforced soil is related to concrete reinforcement mainly plays the role of shear stress and tensile strength effect. The effect of root reinforcement increases the cohesive strength of soil, decreases soil deformation, prevents the incidence of surface tension cracks, and can avoid slope failure initiated by triggering factors [
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16]. The shear stress is developed in the soil and transferred to the ground as tensile resistance in the roots, which ensures mechanical reinforcement by the roots [
17,
18,
19,
20,
21,
22,
23,
24].
The effect of plant roots on slope stability can be divided into hydrological and mechanical factors, which can be valuable to ensure slope stability [
1,
2,
5,
21,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37]. With regards to the hydrological effects, plants intercept rainfall allowing it to slowly infiltrate into the soil by reducing the runoff velocity and, thus, reduces soil erosion [
7,
8,
38]. Additionally, vegetation can reduce soil moisture by means of transpiration, which increase the matrix suction of the soil, resulting in an increase in the soil shear strength [
7,
14,
37]. In terms of mechanical aspects, vegetation increases the soil shear strength by transferring the shear stress developed in the soil to the roots fibers, through the tensile strength mobilized in the roots [
39,
40,
41,
42]. This study focused on the mechanical effect of plant roots on slope stabilization along mountainous transportation corridors by considering the effect of soil moisture variation.
In recent decades, a few powerful design tools have been created for slope stability analysis. The advanced technology has increased the use of finite element (FE) methods as it dominates a wide range of features. Geometry is discretized into finite elements. It also incorporates the effect of material stiffness in the analysis and can model non-linear stress–strain behaviour of materials and understand their stability on the basis of deformation characteristics. The finite element method can be used to assess the stability of slopes using a failure definition, such as the finite element strength reduction method. In the strength reduction method, soil strength parameters is reduced until the slope becomes unstable and, thus, the factor of safety is calculated as the ratio between the initial strength parameter and the critical strength parameter [
13,
41,
43,
44,
45,
46,
47,
48,
49,
50,
51].
Chock et al., [
33] conducted a slope stability analysis using the FE method. From the analysis, they reported that the computed factor of safety (FOS) was 1.05, without root reinforcement, which showed that the slope was marginally stable. When the plant was grown on the entire slope, the FOS increased from 1.05 to 1.25, indicating a 19% improvement in stability. The factor of safety (FOS) increased to 1.2 (14.3%) as the vegetation was grown on the surface and toe of the slope. However, when the plants grew only on the surface of the slope, the FOS was increased by only 3%. The FOS of the slope depended on the increment of the root cohesion (
Cr) value and the depth of root (
Hr). Similarly, Naghdi et al. [
2] found that when plants vegetated on the entire slope, the FOS increased significantly than when vegetation grew only on the slope toe, slope surface, on the top of slope. On the contrary, Habibah et al., [
10] stated that when a hillslope was vegetated with trees at the bottom of the slope, it became more effective in slope stabilization than at any other position on the slope. To investigate the contradictions between the previous studies, the spatial distribution of vegetation on the slope were conducted and compared among the selected plant species.
Slope failure disasters are an universal occurrence on a planet that, like Earth, is tectonically active [
7,
15,
16,
17]. Slope failure along road corridors is one of the critical problems in the mountainous terrain of Ethiopia. The losses by landslide in Ethiopia that occurred between 1993 and 1998 destroyed more than 200 houses, more than 500 km of roads, and caused the death of about 300 people. Slope failure problems in Ethiopia are mainly related to hilly and mountainous terrains of the highlands of Ethiopia, which is characterized by variable topographical, geological, and hydrological and land use conditions. Earthquake-triggered landslides are rarely reported in Ethiopia. Landslide related hazards are becoming thoughtful concerns to the public and to the planners and decision-makers at various levels of the government [
6].
In the case study area, the Jimma-Mizan asphalt concrete road, passes through the irregular topography of the south-western part of Ethiopia. The rugged geographical condition that is accompanied by the erratic rainfall leads to repeated slope failure along road corridors. This causes loss of human life, hindering of traffic movement, high maintenance cost, and destroys infrastructures. This led to the necessity of an investigation of slope safety along road corridors. Mechanical slope retaining structures are being implemented to stabilize slopes prone to failure. However, these structures are not environmentally friendly and it is not possible to address all of the road problems, due to financial limitations [
52]. Thus, the use of plant roots is considered a sustainable alternative to improve slope stability. However, there are no studies that have been conducted on the mechanical characteristics of vegetation to stabilize cut slopes along mountainous road corridors in Southwest Ethiopia.
Therefore, the objective of this study is to conduct slope stability analysis by considering spatial distribution of vegetation along the road cut slopes using numerical modelling, evaluate and select suitable plant species for slope stabilization. In addition, the parametric study is conducted to assess the sensitivity of a factor of safety of the slope to the different species of plants, variation in soil moisture content, vegetation spacing and geometry of the slope.
2. Materials and Methods
2.1. Topography of the Study Area
The study site is characterized by highly variable topographic features. In which, the steep hill slope and deep cut valley are dominant in the area. Since the hill slope are steep enough, external factors such as rainfall and road cut could trigger the slope to failure. As well, the land use land cover also potential for instability of slope in the study area. Less vegetated hill slope of the area also aggravates to slope instability than vegetated slope, which ensures less mass wasting process [
27]. The depth of failure plane of the slope is variable, which ranges from 0.5 m to 2 m. As shown from
Figure 1, the mode of failure is earthen slide and mudflow with shallow soil cover.
2.2. Study Area
This study is conducted in the sub-humid tropical area along Jimma-Mizan asphalt concrete road, South-western Ethiopia. It is located 430 km to the South-western of Addis Abeba, wich is capital city of Ethiopia. The study area is located between 7°27′25″ to 7°30′00″ latitudes and 36°24′55″ to 36°27′25″ longitudes (
Figure 2).
The area characterized by a significant rainfall lasting from May to end of September. The rainy phase peaks from July until the beginning of September and the average annual rainfall of study area is 1650 mm/year.
2.3. Sampling and Root Excavation Techniques
Five plant species were selected for root characterization and tensile strength test. The root excavation of each plant species were carried out manually within the area delineated by the vertical projection of the above ground biomass [
52] and to various root depth (
Figure 3). The size of excavated area is a function of the width of the above ground biomass in each plant species. During excavation, care was taken to avoid any damage to roots. After excavation, the roots were packed immediately in plastic bags to preserve their moisture content and then transported to the Jimma University Institute of Technology for tensile strength tests.
2.4. Selection of Plant Species
Selection of appropriate plant species for rehabilitation of degraded land and slope failure is based on their promising mechanical root characteristics [
53]. Five plant species, namely,
Eucalyptus globules (tree),
Psidium guajava (shrub),
Salix subserrata (shrub),
Chrysopogon zizanioides and
Pennisetum macrourum (grasses), which are the most dominant and native to the study area were selected.
Salix subserrata is the species growing fast, regenerating itself, and characterized by deeply penetrating tap root system with lateral thin roots.
Chrysopogon zizanioides and
Pennisetum macrourum are widespread grass species, which are characterized by a shallow root system with relatively short tap roots.
Eucalyptus globules is a tree species which develops deep taproots with long lateral roots, and which generates a large root system with relatively short taproot and long lateral roots [
53,
54]. As for
Psidium guajava, the roots of this species possess very thin and numerous roots with horse tail-like thin roots, where each of secondary and tertiary roots develops several smaller roots to anchor the alluvial soils near rivers [
54]. Therefore, the unique characteristics of roots makes the species a very promising candidate for slope stabilization along transportation corridors in sub-humid tropics like south-west Ethiopia.
2.5. Determination of Root Tensile Strength
Five plant species are selected to test the tensile strength of roots for the proposed slope stability analysis model. The test was conducted for different root diameter ranges between 0.25–6.5 mm. To ensure an accurate reflection of the mechanical roots property, all plant root specimen, which were collected from the field, were placed in sealed bags. The tensile test was done by using a Testometrics material-testing machine (serial no. 500–517, Testometric Co.Ltd, London, UK) with the test force ranges between 40–100 KN with testing speed of 20 mm/min. The root diameter was measured using digital calliper in three different points, and the mean diameter was calculated to assign the representative value corresponding to the breaking point of each sample. The tensile strength value of each root was determined by the machine load cell and recorded with the data logger. The influence of roots on the reinforced strength of soil can be expressed as a cohesion term [
5] in the Mohr-Columb failure criteria determined by Equation (1):
where
C′ is the effective cohesion of the soil, σ is the normal stress due to the weight of water and soil of sliding mass, μ is pore–water pressure developed in the soil, φ’ is the effective friction angle of the soil and Δ
S is the apparent cohesion provided due to the presence of roots. According to the study conducted by Genet et al., [
5] the additional soil cohesion provided by plants root can be calculated by Equation (2):
where
Tr is the average tensile strength of roots per unit area of the soil,
Ar|
A is root area ratio (%) and β is the angle of root distortion in the shear zone. Sensitivity analyses shows that the values of (
sinβ +
cosβ·
tanφ′) can be approximated as 1.2 for 30° < φ′ < 40° and 48° < β < 72° [
5]. The following formula was used to calculate the tensile strength as stated in Equation (3):
where,
Fmax is the maximum force (N) needed to break the root and
D is the mean root diameter (mm) before the break.
The model developed by De Baets et al. [
35] is used to estimate the increase in soil shear strength due to presence of roots. Their model assumes that vegetation roots grow vertically, so tension is exerted to them as soil is sheared. This model is also used by De Baets et al., [
55,
56] where they tested root tensile strength and root distribution for selected plant species.
2.6. Mechanism of Soil-Root Reinforcement
Figure 4 shows that vegetation enhances the soil shear strength by transferring the shear stress developed in the soil to roots fibers through the tensile strength mobilized in the roots. When a tree roots extends across a shear surface, or upwards beyond the potential failure mass, making a small angle with the downslope direction of the shear zone and the roots within the shear zone develop tension. In other words, shear stresses in the soil mobilize the tensile resistance in the root fiber, which in turn provides greater strength to the soil.
If the soil is rooted, the increased soil shear strength can be expressed as additional cohesion:
where
Tri is the tensile strength of an individual root (
i) and (
Ari|
A) is the root area ratio (RAR) or proportion of root cross-sectional area to soil cross-sectional area A. Soil cohesion due to roots (
Cr, kPa) was calculated from average T
r of each species and RAR [
31,
35].
The influence of vegetation roots on soil shear strength can be taken as part of the cohesive strength component of the soil-root system [
28,
37,
39,
44,
55,
56]. For the case when the phreatic surface is at the soil surface and location of the potential shear plane for infinite slope is at a depth
z below the soil surface [
11], the factor of safety is the ratio of activating force to driving force is determined by Equation (5):
where,
C′ and
ϕ′ are the effective soil strength parameters, ∆
C is the increased cohesion due to tree roots, α is slope angle, w
t vegetation surcharge
γsat is saturated unit weight,
γw unit weight of water,
z, effective root zone. To predict the slope failure threshold conditions, the soil strength parameters are estimated from the Mohr-Columb failure envelope derived from the peak values of a series of shear stress-displacement curves.
2.7. Determination of Unit Weight of Soil
Lab tests were performed to determine the in-situ density of undisturbed soil obtained by pushing or drilling a thin-walled cylinder. The test is conducted using the ASTM D 7263 standard method. The bulk density is the ratio of mass of moist soil to the volume of the soil sample. Whereas, the dry density is the ratio of the mass of dry soil to the volume of soil sample. The unit weight of soil is determined by the following procedures (1) the soil sample extruded from the cylinder using the extruder (2) representative soil specimen is cut from the extruded sample (3) the length (L), diameter (D), and mass (Mt) of the soil specimen determined and recorded (4) then the moisture content of the soil is determined (w).
2.8. Triaxial Compression Test for the Determination of Soil Parameter
This test is performed to determine the unconsolidated-undrained shear strength of unsaturated clay soil. The triaxial compresion test is conducted according to ASTM D2850 testing procedures. The unconsolidated-undrained (UU) triaxial compression test was conducted at different soil-water contents. Different confining pressures were applied to each specimen. The shear velocity was controlled at 1.27 mm/min in the test. The stress was recorded at intervals of 0.4 percent axial increment. When the peak value of deviator stress reaches the maximum, the test continued for an additional 5% axial strain. If no peak value is recorded, the test is stopped when total axial strain reached 20 percent.
From the triaxial compression tests, the fundamental slope stability analysis parameters were extracted. These are the friction angle, soil cohesion, elastic modulus (E) and Poisson’s ratio (υ). These parameters are then used in computer model to predict how the material behaves in the slope stability analysis. Soil and plant root parameters for numerical modelling are summarized in
Table 1 below.
2.9. Finite Element Slope Stability Analysis Method
The factor of safety was computed using the PLAXIS 8.2 Geotechnical software. The effect of plant root reinforcement on the slope material is carried out by using soil stiffness and shear strength parameters, soil moisture variation, and root parameters. The plain-strain Mohr-Columb model with 128 elements was used to mesh the soil material. All the slope boundary faces are open except for bottom of the slope which is closed (motionless). The generation of mesh is based on the triangulation procedure. Medium mesh discretization are defined for the model. Simple sketch of the slope with the geometry, dimensions, discretization and boundary conditions are shown in
Figure 5. To assess the impact of plant roots to the slope stability, the factor of safety (FOS) of the slope is calculated using finite element method (PLAXIS2D) [
31,
57]. The calculation of the FOS for the slope in the PLAXIS 2D package is based on the strength reduction (phi-c) procedures [
58,
59].
Two different soil moisture content are taken at different season. Namely, soil moisture content at 16% and 23%. The study intended to investigate weather systematic change in soil moisture content along hill slope affects soil-root reinforcement. The input parameters used for modelling are Young Modulus of elasticity (E), Poisson’s ratio (υ), cohesion (C’), friction angle of soil (φ’). The principal vegetation-related input parameters used in the PLAXIS model are, apparent root cohesion (Cr), effective depth of root zone (Hz) and root tensile strength (Tr). The boundary faces of 2D displacement imposed freely to move except for the bottom boundary of the slope face, which is assumed to be non-movable. In this study, the effect of the spatial distribution of vegetation on slope stability was evaluated. The FOS of the slope determined with homogenous slope (β = 45°) with a height, H, of 14 m was considered. The root cohesion, Cr, depth of root zone, Hr, and the tensile strength of the root, Tr, were variable for each plant species.
The soil type in this research is cohesive clay soil, of which some soil properties, such as unit weight, cohesion, friction angle, poisons ratio, modules of elasticity and moisture content were determined from laboratory tests. The parameters for soil moisture content (ω) =16%; are: unit weight (γ) = 16 kN/m
3; Young’s elastic modulus (E) = 3.125 MPa; Poison’s ratio (
υ) = 0.2; friction angle (ϕ) = 4°; cohesion (c) = 47 kPa; dilatancy angle = 0°; the parameters for soil moisture content (ω) = 23%; are: unit weight (γ) = 18 kN/m3; Young’s elastic modulus (E) = 3.10 MPa; Poison’s ratio (υ) = 0.2; friction angle (ϕ) = 3°; cohesion (c) = 50 kPa; diltancy angle = 0°. Other input parameters for the soil and plant species are given in
Table 1. The cohesion of clay soil increases with increase of water contents at certain limits above which they started to decrease. In other word, cementation (cohesion) force increase with increasing water contents up to certain limit. Above which this force decreases because of excessive water content. Therefore, 23% water content is not excessive to decrease cohesion of soil.
All selected plant species were considered in the simulation at the four locations of the slope. Namely, on the entire slope, on slope surface, only at the top, and only at the toe. The results from the finite element slope stability analysis were presented as “stable slope” when the factor of safety (FOS) is greater than 1.0, and “unstable slope” when the FOS is less than 1.0. The geometry of the slope was modelled using the PLAXIS 2D interface. The slope has uniform cross-section, the corresponding stress state and loading scheme over a certain length are perpendicular to the cross-section (z-direction). For in-plane strain, it is assumed that the strain and displacement in the z-direction are zero but the normal stresses are different from zero.
In PLAXIS-based finite element analysis, strength reduction technique is utilized to conduct slope stability analysis by incorporating the effect of plant roots as root-soil reinforcement. The strength reduction techniques for finite element slope stability analysis have been successfully adopted by many authors [
23,
38,
57]. This analysis method allows finding the FOS of slope by initiating a systematic reduction of shear strength parameters,
Cf and
φf, wich are defined in Equations (6) and (7):
where, SRF is the strength reduction factor. The factor of safety (FOS) for slope stability is the value of SRF to bring the slope to failure.
4. Conclusions
The findings of the study can be summarized as follows: for the same slope geometric configurations, the slope that was initially unstable without plant roots reinforcement became safe when reinforced by plant roots. Plant roots have a significant role in stabilizing shallow failure of the slope along road cut slopes. Generally, the stability of the slope has increased as the value of root cohesion and effective depth of root zone increased. In addition, the result showed that better FOS was obtained for slope with vegetation covered on entire slope surface than with plant-covered on the top, on the surface, and toe of the slope. The failure mechanism of the study area was initiated at a maximum depth of 2 m. As the depth of root penetration increased on the entire ground surface, the safety factor increased. Among studied plant species, the root of Salix subserrata can penetrate beyond the failure zone and produced a higher factor of safety and can reinforce soil up to the depth of 2.2 m.
The slope of the study area was more susceptible to failure for increased soil moisture content and this leads to a decrease in the factor of safety. On the contrary, as the soil moisture content decreases the factor of safety increases. This is because, as the water contacts with soil, the shear strength of soil declines. In general, the wet condition of the slope combined with steeper slope is the most critical situation for slope failure along road corridors. The analysis shows that roots distributed with smaller vegetation spacing, throughout the slope surface have a positive effect on slope stability, with a significant increment of the FOS. From PLAXIS 2D modelling the actual slope of the study area is unstable. Generally, decreasing in slope cut inclination along slope of mountainous area in combination with plant vegetation and providing gabion at the toe of the slope enhances slope stabilization along transportation corridors. Because, the stability is achieved by self-weight of gabion. In addition, gabion is advantage in filtering excess pore water developed in the slope. In conclusion, among the five-studied plant species, Salix subserrata is the most promising in slope stabilization due to its better root density and mechanical characteristics.