Volume 10, Number 4 (July 2023):4779-4790, doi:10.15243/jdmlm.2023.104.4779 ISSN: 2339-076X (p); 2502-2458 (e), www.jdmlm.ub.ac.id
Open Access 4779 Research Article
Modelling of mechanical roots on slope stability
Yulia Amirul Fata1,6, Hendrayanto2*, Erizal3, Suria Darma Tarigan4, Takeshi Katsumi5
1 Forest Management Science, Graduate Study Program, IPB University, Bogor 16680, Indonesia
2 Forest Management Department, IPB University, Bogor, 16680, Indonesia
3 Civil and Environmental Engineering Department, IPB University, Bogor 16680, Indonesia
4 Soil and Land Resources Science Department, IPB University, Bogor 16680, Indonesia
5 Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
6 Soil Department, Faculty of Agriculture, Brawijaya University, Jl. Veteran, Malang 65145, Indonesia
*corresponding author: [email protected]
Abstract Article history:
Received 11 January 2023 Accepted 1 April 2023 Published 1 July 2023
Root system mechanical reinforcement through root-soil cohesion on slope stability is important. However, the root cohesion of Tectona grandis, Maesopsis eminii, and shrubs (Chromolaena odorata) on slope stability is rarely studied and modelled. This study aimed to model the mechanical effect of vegetation through root cohesion, namely teak (Tectona grandis), Maesopsis eminii, and shrubs (Chromolaena odorata). The study was conducted in a simultaneous landslide on January 1, 2020, that dominantly occurred on vegetated slopes of Sukajaya District, Bogor Regency, West Java. The Wu model's root cohesion (CR) was modelled on slope stability using a modified Bishop model. The modelling used the data from field and laboratory-measured. The study found that the presence of a root system increases slope stability's factor of safety (FOS). The root system of young Maesopsiss eminii produces the largest effect of FOS compared to the root system of shrubs, teak, and old Maesopsis eminii. The slope stability of vegetated slopes is a function of the CR and the effective root zone depth. The highest total CR of vegetation was teak with 0.398 kPa, followed by shrubs, young Maesopsis eminii, and old Maesopsis eminii with 0.202 kPa, 0.191 kPa, and 0.087 kPa, respectively. The effective root zone of teak, young Maesopsis eminii, and shrub were 500, 230, 140, and 66 cm, respectively.
Keywords:
Chromolaena odorata Maesopsis eminii mechanical effect root cohesion slope stability Tectona grandis
To cite this article: Fata, Y.A., Hendrayanto, Erizal, Tarigan, S.D. and Katsumi, T. 2023. Modelling of mechanical roots on slope stability. Journal of Degraded and Mining Lands Management 10(4):4779-4790, doi:10.15243/jdmlm.2023.104.4779.
Introduction
Vegetation affects slope stability through complex mechanical and hydrological mechanisms such as the canopy cover that reduces the amount of rainfall and kinetic energy of rainfall reaching the soil surface, the association of organic matter with soil that changes soil properties in retaining and flowing water, also through mechanical mechanisms on the above ground and underground. The above-ground biomass brings pressure on the soil and slopes through the weight of vegetation surcharge and the wind transferred as moments and forces through above-ground biomass
towards the soil by the root system (Kim et al., 2020).
Meanwhile, the underground biomass increases soil aggregates through additional root cohesion (Cazzuffi and Crippa, 2005; Emadi-Tafti et al., 2021). Root cohesion (CR) is vegetation's dominant mechanical effect in increasing slope stability compared to other parameters (Frei, 2009; Kokutse et al., 2016; Nguyen et al., 2018; Fata et al., 2021a).
The vegetation type, age, spacing, and characteristics of the growth sites (climate, soil type, and land management) can influence CR. Vegetation type affects the characteristics of the root, such as morphology, tensile strength (TR), length, diameter,
Open Access 4780 water content, depth, and stem diameter (Cazzuffi and
Crippa 2005; Adhikari et al., 2013; Mehtab et al., 2021). The vegetation age affects the TR and root length. The TR rises with vegetation age (Voottipruex et al., 2008; Rajesh et al., 2017), yet it can also decrease the root area ratio (RAR) due to root decay which can decrease CR (Meng et al., 2014; Tadsuwan et al., 2017). Vegetation spacing affects root biomass and growth (Ni et al., 2019). Meanwhile, external conditions such as land management and growth sites affect plant growth and root production (Fata et al., 2022).
The root cohesion of shrubs is generally greater than trees. Cazzuffi and Crippa (2005) showed that the CR of shrubs (Elygrass, Eragrass, Pangrass, and Vetiver grass) in Italy was greater than that of trees (Green alder and Willow tree) in Switzerland.
Voottipruex et al. (2008) showed that the CR of vetiver grass was greater than the CR of Acacia mangium Willd trees in Thailand, as well as shrubs (Rhodomyrtus tomentosa and Melastoma sanguineum) had greater CR than trees (Reevesia thyrsoidea) in Hong Kong (Leung et al., 2015). However, the root cohesion of the shrub has more effect on shallow slope stability than trees.
In the past few decades, studies on vegetation's role in slope stability have focussed on the importance of the mechanical root in the shallow landslide process, and many researchers have modelled the effect of strengthening roots from the slope scale to the regional scale (Masi et al., 2021). Slope stability is defined as a factor of safety (FOS), the soil's shear strength ratio to shear stress. FOS on vegetated slopes is more significant than on slopes without vegetation (Cazzuffi and Crippa, 2005; Leung et al., 2015; Wang et al., 2019; Islam et al., 2020). FOS is influenced by the presence of additional cohesion by the roots in the soil (CR), which further increases the soil shear strength (Gray and Sotir, 1996). The mechanical vegetation impact can be implemented into standard slope design by including the CR parameter in the Mohr–Coulomb shear strength model. When the fibres (roots) penetrate the soil, the soil displacement generated by shear stress creates friction between the soil particles and the root surface, allowing the roots to mobilize their TR. Furthermore, shear stress is distributed from the soil towards the roots, strengthening the soil matrix (Khalilnejad et al., 2012).
Studies related to root cohesion on slope stability, especially the root effects of teak (Tectona grandis), Maesopsis eminii, and shrubs (Chromolaena odorata), have yet to be widely developed in Indonesia and abroad. Temgoua et al. (2017) and Hairiah et al. (2020) investigated the TR of teak roots. The TR of teak in the Togo area, West Africa, at the age of 8-54 months’
ranges from 21.56 ± 11.21 MPa (Temgoua et al., 2017), while the TR of teak roots in the East Java region, Indonesia, ranges from 2.4-4 MPa (Hairiah et al., 2020). According to the previous study, it is necessary to research the CR of teak, Maesopsis eminii,
and shrubs because these plants are commonly found in Indonesia. There are few studies on these three plant species in slope stability modelling.
The mechanical vegetation influence is necessary to determine root cohesion in slope stability, especially in landslide prevention. The objective of this study was to model the mechanical effects of teak (Tectona grandis) roots, Maesopsis eminii, and shrubs (Chromolaena odorata) roots which majority grow in landslide locations in the mountainous area of Mount Halimun Salak, West Java, Indonesia.
Materials and Methods Study site
The study was conducted in the mountainous area of Mount Halimun Salak, in Pasir Madang Village, Sukajaya District, Bogor Regency, West Java Province, Indonesia (Figure 1). Landslides often occur at this location, and in January 2020, simultaneous landslides dominantly occurred on vegetated slopes.
Landslides that occurred in teak (Tectona grandis) and Maesopsis eminii forest plantations are presented in Figure 2.
Field investigation
Field studies were conducted to reconstruct the landslide profile, which included measuring the landslide geometry, slope steepness, and coordinates with a roll meter and a Real-Time Kinematic-Global Positioning System (RTK-GPS), and measuring slope steepness with a clinometer. Moreover, there is the measurement of the Electrical Resistivity Tomography (ERT) and root and soil sampling activity (Fata et al., 2021b). Geophysical investigations through the ERT method are based on analyzing the physical rock properties. The ERT method estimates groundwater depth (seepage water table as phreatic line occurrence) by measuring the resistivity of soil and rock. A multi- channel ARES resistivity meter was used to measure the resistivity in a Dipole-Dipole configuration. The electrode distance is 3 meters, the resistivity depth of approximately 20-50 meters and an ERT line length of 141 meters. The RES2DINV program is applied to generate resistivity in a 2D ground model. The ERT investigation was carried out in the preliminary study by Fata et al. (2021b) and then used to reconstruct the slope/geometry profile in the slope stability modelling.
Soil and root sampling
The roots of teak (Tectona grandis), Maesopsis eminii, and shrubs (Chromolaena odorata) were sampled in January 2021. Soil samples were collected at landslide and non-landslide sites on the slope's crest, middle, and toe. Six sample locations on non-landslide slopes and three on landslide slopes were taken at 0-50 cm and 51-100 cm. Two soil sample types were taken, namely, disturbed and undisturbed. Undisturbed soil samples were collected with a sample ring of 8 cm in diameter
Open Access 4781 and 40 cm in height (Fata et al., 2021b) to measure soil
shear strength to determine the soil's mechanical properties. Meanwhile, the disturbed soil sample was taken at about 3 kg that used to measure the specific gravity of soil particles and the Atterberg limit for soil classification. The root samples were selected to represent three root diameter classes, including diameter class I (>5-≤10 mm), diameter class II (≥2.5-
≤5 mm), and diameter class III (<2.5 mm). Root
cohesion was estimated using vegetation root diameters ranging from 1 to 10 mm.
The area and depth of the sample plots of vegetation roots were measured as same as the size and depth of each vegetation root. Furthermore, purposive root samples were taken in each plot representing root diameter classes I, II, and III. Before the root tensile strength laboratory test, the roots were soaked in water for 14 days before being air-dried (Fata et al., 2022).
Figure 1. Study site in Sukajaya District, Bogor Regency, West Java Province, Indonesia.
Figure 2. Research location of (A) young Maesopsi eminii, old Maesopsis eminii, and shrubs (Chromolaena odorata), and (B) Tectona grandis.
Open Access 4782 Root cohesion estimation based on root tensile
strength
A Universal Testing Machine (UTM) was used to determine the root tensile strength with a capacity of 3 tons at room temperature 25 oC. The ASTM D638-14 was used to set the test speed of the UTM machine based on the root diameter class. The test speed for the root diameter class of 3.18 mm is 1 mm/minute, whereas the test speed for the root diameter class of
>3.18 mm is 5 mm/minute. Before taking measurements, the UTM device was calibrated, and the precision of the root tensile strength is 1/10000 (Fata et al., 2022).
Fixing clamps made by steel grooved beams were used to reduce root slippage during the test. The black rubber is attached between the clamps to cover a gap between the steel groove and the root, protecting the root from slipping. When the roots were damaged near or at a clamping position during testing because of slipping, the test was replicated with new roots with similar diameter class and length.
Roots about 15 cm in length were tested with two repetitions in 3 different diameter classes, namely diameter classes >5-≤10 mm (I), ≥2.5-≤5 mm (II), and
<2.5 mm (III). The root diameter I and II were tested with a test speed of 5 mm/s, while the root diameter class III was tested with a test speed of 1 mm/s.
The root tensile strength (TR) was determined using formula (1) in kgf/cm2. Root cohesion (CR) was determined using formula (2) in kPa (Wu et al., 1979).
4
2 max
D TR F
1
) cos (sin tg t
CR R 2
A T A tR R R
3
) cos (sin tg A
T A
CR R R 4
R
R t
C 1,2 5
n
i i i
R i
h a RAR d A A
1 2 4
6
where Fmax is the maximum force required to break roots (kgf), D is root diameter (cm2), tR is the TR
increase, AR/A is the root area ratio (RAR), AR is the root fibre area (mm2), and A is the soils’ effective cross-sectional area (mm2). (sincostg)has values varied from 1 to 1.3; Wu et al. (1979) chose the value 1.2, which was also adopted in the analysis. d is the root diameter (mm), a is the cross-sectional root width, h is the root depth (mm), and i is the number of roots.
Soil laboratory analysis
The undisturbed soil samples were used in the laboratory for a soil shear strength test using the direct shear method. The direct shear test refers to SNI 3420:2016 (method of direct shear strength test of soil in unconsolidated and undrained conditions) and ASTM D3080 (standard test method for the direct shear test of soils under consolidated drained conditions). The test object was three soil samples taken from each soil sample ring. The normal loads used were 0.5, 1, and 1.5 kgf. If the shear strength test results using normal loads 1 and 1.5 kgf are insignificant, the normal load increases to 2 kgf.
The direct shear test provides a graph representing the relationship between normal stress ( ) and maximum shear stress () obtained from equation (7) and equation (8):
A
N
(kg/cm2) 7
A
P
(kg/cm2) 8 where N is the normal force (kN), A is the specimens’
cross-sectional area (cm2), and P is the maximum shear force (kN).
Site profile
The slope geometry in slope stability modelling results from a geophysical investigation using the ERT method on landslides and non-landslide slopes.
Preliminary studies have been carried out previously to build 2D and 3D ground models by Fata et al.
(2021b). Because landslides are complex 3D phenomena, analysis of underground structures by 3D models can more precisely reconstruct landslide processes. The 3D model adds more complexity to the 2D ground model by combining surface geomorphology data. The 3D model's limitation is that the outermost 3D outputs result from interpolation of the outermost ERT lines; therefore, they cannot represent the ERT lines' resistivity within the area (Chambers et al., 2011). Dark blue and dark purple represent the lowest resistivity (1 ohm.m) and maximum resistivity (1050 ohm.m), respectively.
Based on their resistivity values, lithological rocks are classified into three resistivity types; low for tuff lithology (0-350 ohm.m), moderate for breccias lithology (350-772 ohm.m), and high for rocks (>722 ohm.m). The resistivity value indicates the lithology variation for each soil type, moisture content, and materials sensitivities.
The slope geometry used in the modelling results from the 3D ground model located on the A4 Line (Figure 3a) and the 2D ground model (Figure 3b).
Based on the 2D and 3D ground models, a sloping geometry with a depth of 10 m from the surface was selected, which can represent two layers of soil depth (0-50 cm) and (51-100 cm) and has a low resistivity
Open Access 4783 layer which indicates a seepage water table as a water-
saturated layer. The seepage water table was used as a
phreatic line in the slope stability modelling (Figure 4), representing the groundwater table's presence.
Figure 3. (a) 3D ground model and (b) selected 2D ground model (A4 line), (c) slope geometry used (Fata et al., 2021b).
Slope stability modelling
The limit equilibrium method (LEM) is used in slope stability modelling to determine the landslide area's factor of safety (FOS) (Geo-Slope International Ltd, 2021). One of the LEM methods is the slice method used in inhomogeneous soils, and seepage flows occur in uncertain soils. The FOS is the ratio between the resisting moment and the failure-causing moment. The FOS of the Bishop method is used to calculate the force around the slice soil plane called the Simplified Bishop method with a FOS approximately field
conditions (Liong and Herman, 2012) and a FOS approximately model based on the balance of moments and forces (Geo-Slope International Ltd, 2002). The FOS was calculated through slope stability modelling using the GeoStudio-Slope/W program. GeoStudio- Slope/W can model slope stability for various soil types, complex stratigraphy, surface slip planes, and pore water pressure conditions (Geoslope Office, 2021). The slope geometry based on the 2D ground model (Figure 3c) for slope stability modelling is presented in Figure 4.
Figure 4. (a) the slope geometry of the bare land slopes illustrated using GeoStudio-Slope/W software, (b) the simulation of slope stability analysis that shows a shear plane area and FOS value.
Bishop's safety factor is presented in equation (9).
Substituting the vegetation effect on slope stability as root cohesion (CR) in the Bishop method of the FOS equation forms a new FOS equation (11).
n
i i i
n i
i i wi i i
W ma
tg b u W b c F
1 1
sin
' '
9
Open Access 4784 mai i sin iFtan 'i
cos )
(
10
n
i i i
n i
i i wi i i R
W ma
tg b u W b c c F
1 1
sin
' '
'
11
where F is the factor of safety (FOS), Wi is the i-th sectional weight (kN), c' is the effective soil cohesion (kN/m2), φ′ is the effective soil friction angle (o), θi is the soil section angle (o), uw is the pore water pressure (kN/m2), and b is the soil slice width (m).
The root cohesion effect on the FOS was modelled using slope stability analysis. Five scenarios were modelled, namely scenario (1) The slope stability modelling of bare land slopes or without the vegetation influence (Figure 4a), scenario (2) The slope stability modelling of Teak (Tectona grandis) influences (maximum root length = 230 cm), scenario (3) The slope stability modelling of old Maesopsis eminii (maximum root length = 500 cm), scenario (4) The slope stability modelling of young Maesopsis eminii (maximum root length = 140 cm), and scenario (5) The slope stability modelling of shrubs (Chromolaena odorata) (maximum root length = 66 cm) as presented in Figure 5.
Figure 5. The geometry slope profiles illustrated using GeoStudio-Slope/W software that affected by vegetation roots of trees (young Maesopsis eminii, old Maesopsis eminii, and Tectona grandis), and shrubs
(Chromolaena odorata).
Results and Discussion Mechanical soil properties
The direct shear test produces maximum shear stress and normal stress parameters. The Mohr stress diagram defines the shear strength function as the failure circle under different stress-strain conditions
(Khalilnejad et al., 2012). Based on the graph, the correlation between the maximum shear stress and the normal stress, the regression equation was obtained to determine the effective cohesion (c') and the effective soil friction angle ( '). The soil mechanics parameters from the laboratory test are presented in Table 1.
Table 1 The recapitulation of mechanical soil parameters for slope stability modelling.
Soil mechanic parameter Layer I Layer II
Minimum Average Maximum Minimum Average Maximum '
c (kPa) 1.68 3.06 4.87 1.08 2.53 4.87
' (◦) 24.42 51.39 61.08 24.42 53.35 61.08
sat (kN/m3) 14.01 14.84 15.63 14.41 15.35 15.93
Table 1 shows the soil mechanical parameters at 0-50 cm (layer I) and a depth of 51-100 cm (layer II). Soil
mechanics parameters consist of minimum, average, and maximum values obtained from laboratory tests.
Open Access 4785 The effective cohesion value in layer II is relatively
smaller than that of layer I, which indicates that the shear strength of layer I is greater than that of layer II.
Meanwhile, saturated soil layer II has a larger effective internal friction angle and saturated unit weight than layer I. The greater the effective soil friction angle and unit weight, the denser the soil. Steep slopes with colluvial soils (susceptible to shallow landslides,
specifically in the middle latitudes) have relatively low soil friction angles (a few degrees) (Masi et al., 2021).
Root characteristics and root cohesion
Vegetation characteristics such as root architecture of trees (young Maesopsis eminii (A-1), old Maesopsis eminii (A-2), Tectona grandis (C)) and shrubs (Chromolaena odorata) (B) are presented in Figure 6.
Figure 6. The root architecture of trees (A-1) young Maesopsis eminii, (A-2) old Maesopsis eminii, and (C) Tectona grandis; and shrub (B) Chromolaena odorata.
Table 2. The root characteristics.
Vegetation
Diameter Class
l d Test
Speed
Ѳ Fmax TR i a h RAR tR CR Total CR
(mm) (cm) (mm) (mm/s) (%) (kgf) (Mpa) (N- roots)
(cm) (cm) (%) (Mpa) (kPa) (kPa)
Tectona grandis
>5 - ≤10 15 7.00 5 38.71 172.15 48.15 23 120 150 0.049 2.37 0.284
0.398
≥2.5 - ≤5 15 4.10 1 37.41 37.90 52.58 21 200 180 0.008 0.40 0.049
<2.5 15 1.20 1 26.24 3.20 93.60 296 250 230 0.006 0.54 0.065 Old
Maesopsi eminii
>5 - ≤10 15 8.25 5 43.35 21.90 3.88 15 50 175 0.092 0.36 0.043
0.087
≥2.5 - ≤5 15 4.00 1 58.25 25.85 19.02 37 100 250 0.019 0.35 0.042
<2.5 15 1.70 1 37.19 1.15 5.29 116 160 500 0.003 0.02 0.002 Young
Maesopsi eminii
>5 - ≤10 15 7.55 5 43.76 75.60 13.17 3 30 40 0.112 1.47 0.177
0.191
≥2.5 - ≤5 15 3.75 1 35.36 14.50 9.00 5 50 120 0.009 0.08 0.010
<2.5 15 0.90 1 19.03 2.10 32.67 22 85 140 0.001 0.04 0.005 Shrubs
>5 - ≤10 15 9.00 5 62.31 3970 6.07 7 53 40 0.212 1.29 0.154
0.202
≥2.5 - ≤5 15 4.00 1 67.59 8.70 6.18 10 90 65 0.021 0.13 0.016
<2.5 15 0.75 1 33.19 1.05 55.43 104 145 66 0.005 0.27 0.032
Meanwhile, root length (l), root diameter (d), TR
testing speed, water content (Ѳ), the maximum force required to break roots (Fmax), root tensile strength
(TR), number of roots (i), cross-sectional root width (a), and root depth (h), root area ratio (RAR), the TR
increase (tR), and the root cohesion (CR) for each root
Open Access 4786 diameter class are presented in Table 2. The root
architecture of vegetation, namely young Maesopsis eminii, old Maesopsis eminii, teak (Tectona grandis), and Chromolaena odorata, have root diameter characteristics of ≤5 mm to >50 mm (Figure 6).
Based on the diversity of root diameters, only root diameters ≤10 mm was tested for TR to calculate root cohesion. Based on Table 2, the maximum root length is old Maesopsis eminii reaching 500 cm, followed by teak, young Maesopsis eminii, and shrubs with root lengths of 230 cm, 140 cm, and 66 cm, respectively. Meanwhile, the greatest lateral root distribution is teak, with a length of 250 cm, followed by old Maesopsis eminii, shrubs, and young Maesopsis eminii, with lateral root distributions of 160 cm, 145 cm, and 85 cm, respectively. The maximum root depth depends on the interaction between soil structure, soil saturation zone, rainfall, tree competition, and bedrock depth which can also affect root architectural characteristics (Mehtab et al., 2021).
Based on Table 2, the root water content ranged from 19.03-67.59%, where the highest root water content was shrubs, followed by old Maesopsis eminii, teak, and young Maesopsis eminii. The smallest root water content was in root diameter class III (<2.5 mm), while in root diameter class I and II, the water content was relatively getting bigger. The root water content (Ѳ) and root length (l) have no relationship.
The smaller the root diameter, the higher the number of roots. The maximum number of roots was in the root diameter class <2.5 mm, wherein each teak, old Maesopsis eminii, young Maesopsis eminii, and shrub were found with 296, 116, 22, and 104 roots. The number of roots, root width, and root depth are a function of the root area ratio (RAR). RAR is greatest in diameter class I and decreases in classes II and III.
The maximum RAR for teak, old Maesopsis eminii, young Maesopsis eminii, and shrub were 0.049, 0.092, 0.112, and 0.212%, respectively.
Root tensile strength (TR) is affected by root diameter (d), root length (l), root water content (Ѳ), and the maximum force required to break the root (Fmax). TR and d generally have a negative exponential relationship, where the increasing root diameter can decrease TR (Fata et al., 2022). Based on Table 2, old Maesopsis eminii and young Maesopsis eminii have varying TR that the chemical and biological compounds can influence in the roots from lignin and cellulose compounds (Zhang et al., 2014).
The diversity of TR values is also influenced by the conditions of root preparation before testing and the method of TR testing performed (Mehtab et al., 2021, Fata et al., 2022).
Wu's (1979) model of root cohesion is influenced by the TR and the root area ratio (RAR). The RAR is a function of the number of roots (i), root diameter (d), cross-sectional width (a), and root depth (h) (equation 6). Table 2 shows that the largest RAR is in diameter class I (>5 - ≤10 mm) in all vegetation. The RAR value significantly affects CR compared to TR. The greatest TR is in root diameter class III, while the highest CR is the same as the highest RAR value in root diameter class I. The root diameter is a factor that plays an important role in calculating RAR (Mehtab et al.
2021).
Maximum root cohesion was in diameter class I, which in teak, old Maesopsis eminii, young Maesopsis eminii, and shrub was 0.284, 0.043, 0.177, and 0.154 kPa, respectively. CR in each root diameter class or CR
in each vegetation is added to obtain total CR. The most significant total CR was in teak at 0.398 kPa, followed by shrubs, young Maesopsis eminii, and old Maesopsis eminii with total CR of 0.202, 0.191, and 0.087 kPa, respectively (Table 2). The higher the total CR, the greater the roots system contribution to enhancing soil shear strength. Shear strength increases mainly in soils with root biomass or depending on the root depth of each vegetation. Total CR and root depth are considered in the slope stability modelling.
The factor of safety (FOS)
Based on Limit Equilibrium Method (LEM) analysis, the Bishop method was used to model the mechanical vegetation effect on slope stability by root cohesion.
Modelling of the mechanical vegetation effect was carried out in 5 scenarios, namely scenario (1) The slope stability modelling of bare land slopes, or without the vegetation influence (Figure 4a), scenario (2) The slope stability modelling of teak (Tectona grandis) influences, scenario (3) The slope stability modelling of old Maesopsis eminii, scenario (4) The slope stability modelling of young Maesopsis eminii, and scenario (5) The slope stability modelling of shrubs (Chromolaena odorata), each of which has a different root depth (Figure 5). Slope stability modelling produces FOS numbers presented in Table 3, and the relationship between FOS and root depth is presented in Figure 7.
Table 3. The recapitulation of the factor of safety (FOS).
Types FOS
Min % Average % Max %
Without vegetation 1.010 - 2.581 - 3.754 -
With vegetation
Tectona grandis 1.043 3.3 2.621 1.5 3.791 1.0
Old Maesopsi eminii 1.019 0.9 2.591 0.4 3.763 0.2
Young Maesopsis eminii 1.069 5.8 2.732 5.9 3.966 5.6
Shrubs 1.041 3.1 2.658 3.0 3.864 2.9
Open Access 4787 Figure 7. The relationship between FOS and root depth.
Based on Table 3, slope stability has increased with the presence of vegetation of 0.2-5.9%. The roots of the young Maesopsis eminii have the highest increase of FOS than other plants by 5.6-5.8%, followed by shrubs, teak, and old Maesopsis eminii. The increase in slope stability is affected by root depth and root cohesion. The largest FOS in young Maesopsis eminii has a maximum root depth of 140 cm with a total CR
of 0.191 kPa (Figure 7).
Root cohesion (CR) at a depth of <100 cm with a root diameter of ≤5 mm was smaller than in a root diameter class >5-≤10 mm. CR at 100-200 cm depth in young Maesopsis eminii with root diameters ≤5 mm is very low compared to CR in other vegetation at the same depth. The highest CR value at a depth of 100- 200 cm is for teak in the >5-≤10 mm diameter class, which is 0.284 kPa. The CR value at a 200-300 cm depth in old Maesopsis eminii with a root diameter class of ≥2.5-≤5 mm was smaller than the CR in teak with a root diameter class of <2.5 mm. While at a root depth of up to 500 cm, CR has the lowest value of 0.002 kPa in old Maesopsis eminii with a root diameter of
<2.5 mm. In this case, root depth is the availability of root biomass and root cohesion that influences the increase in FOS in slope stability modelling. The largest increase in FOS is not in the largest total CR
value or vegetation with the deepest root depth.
The slope stability raising of young trees is larger than that of shrubs. This research is in line with the results of the study by Leung et al. (2015), where young vegetation can bring unsafe slopes to marginal safety (a slightly greater FOS). However, slope stability is affected by biomass distribution and root cohesion at each depth.
Young Maesopsis eminii has the greatest FOS with a maximum effective root depth of 140 cm.
Shrubs with the highest FOS than teak and old
Maesopsis eminii have the shallowest effective root depth of 66 cm. Meanwhile, old Maesopsis eminii has the lowest FOS and the deepest effective root depth of 500 cm. The root biomass depth to soil depth ratio is the most important factor in the role of vegetation in enhancing slope stability. To verify the statement, the authors modelled with a soil depth of 60 m according to the slope profile of Figure 3b. The analysis results show that the FOS number is very low and causes the slope to become unsafe (FOS <1) at minimum soil mechanics parameter conditions and has an increasing FOS of 0-0.8%. This shows that the ratio of root zone depth to slope geometry profile affects the effectiveness of vegetation's mechanical effect through root cohesion in increasing slope stability. This also confirms that vegetation can be considered in preventing shallow landslide events and has a limited impact on deep landslides (Emadi-Tafti et al., 2021).
The mechanical vegetation effect can only increase the slope stability within the root zone, while the hydrological vegetation effect can increase the slope stability in deeper zones (Feng et al., 2020).
Several related studies in modelling mechanical vegetation effect on slope stability between tree and shrub species are presented in Table 4. Cazzuffi and Crippa (2005) compared the slope stability of bare land slope and slope with the vegetation effect of tree and shrub species which showed a FOS increase of up to two times at 0.5 m depth, and the FOS increase decreases with increasing depth. Islam et al. (2020) showed that the FOS increase by vegetation is affected by the soil type, whereas in clay soil, the FOS increase is ignored because it is insignificant. Kokutse et al.
(2016) stated that the mechanical effect of vegetation through root cohesion is the second most important parameter after the slope angle as a geometric parameter.
0 1 2 3 4 5
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
FOS
Root Depth (cm)
Without vegetation Shrubs
Young Maesopsis eminii Tectona grandis Old Maesopsi eminii maximum
average
minimum
Open Access 4788 Table 4. Recapitulation of trees and shrubs root cohesion effect on slope stability.
Vegetation Location Age (years) CR (kPa) FOS without vegetation FOS with Vegetation Source Trees
Green alder
Switzerland 10 10b,1 a a
Cazzuffi and Crippa (2005)
Willow Tree 10 10b,1 a a
Cordata alder
Florence 10 21b,1 >1 >1,2c,1
Turkey oak 15 50b,1 >1 >1,2c,1
Schefflera heptaphylla
Hongkong
a <1.1b,3 c,2 c,2
Leung et al. (2015)
Reevesia thyrsoidea a <0.8b,3 c,2 c,2
Populus tomentosa
China
10 3.13-19.8b,3 c,3 c,3
Wang et al. (2019)
Robinia pseudoacacia 10 4.81-29.97b,3 c,3 c,3
Olea europaea 10 1.63-8.3b,3 c,3 c,3
Casuarina glauca
Australia
a 2.16-34.03b,3 a a
Docker (2003)
Eucalyptus amplifolia a 9.43-47.84b,3 a a
Eucalyptus elata a 3.64-83.25b,3 a a
Acacia floribunda a 1.17-54.67b,3 a a
Shrubs Elygrass
Italy
2 15b,1 >4 >4c,1
Cazzuffi and Crippa (2005)
Eragrass 2 15b,1 a a
Pangrass 2 15b,1 >4 >4c,1
Vetiver 2 15b,1 >4 >4c,1
Vetiver Bangladesh 0.1-0.5 1.92-12.6 c,4 c,4 Islam et al. (2020)
Rhodomyrtus tomentosa
Hongkong
a <1b,3 c,2 c,2
Leung et al. (2015)
Melastoma sanguineum a <1.15b,3 c,2 c,2
Pennisetum pedicellatum
Malaysia 0.5 6.1 a a
Ettbeb et al. (2020)
Pennisetum polystachion 0.5 7.5 a a
Note:
a: unknown/not specified.
b: method; b,1: laboratory test. b,2: in situ shear test. b,3: Wu Waldron Model (WWM).
c,1: The relationship between depth and FOS showed that the vegetated slope increased almost two times more than the non-vegetated slope.
c,2: Young vegetation can bring an unsafe slope to marginal safety (FOS slightly larger than unity). Furthermore, the tree species studied did not exceed the shrubs.
c,3: Because of increased root cohesion, the slope planted with R. pseudoacacia is considerably more stable than those planted with P. tomentosa or O. europaea.
c,4: Vetiver grass on hill slopes enhances FS by up to 2-15% for sandy soils, but FS is negligible for clayey soils.
Open Access 4789 Slope stabilization that considers the potential slip
surface depth influences the roots' mechanical properties, the number of roots, root diameter, TR, and root bending strength crossing the soil slip plane. TR
mainly increases with decreasing root diameter; a big number of fine roots improves soil stability more efficiently than a small number of coarse roots.
(Reubens et al., 2007). Field studies on vegetated slopes show that a root diameter of 1-20 mm most contributes to slope strengthening (Styczen and Morgan, 1995). This study modelled the root cohesion effect on root diameters of 1-10 mm because a root diameter of <10 mm has a role as tensile fibre during slope failure, contributing to slope stability. However, root diameters of >10 mm have a role as anchors rather than tension fibres because of their stiffness (Tosi, 2007).
The question of the effectiveness of fine roots against coarse roots as optimal slope reinforcement has the same argument in the choice of trees against shrubs. Shrubs generally have shallow and dense fine roots, so they have greater root cohesion. Recent case study research will be important for verifying and validating the method, analysis results, and slope stability modelling. This last point is very important because modelling the mechanical effect of root cohesion in slope stability depends on the lithology characteristics, climatological, and vegetation characteristics, thus making the modelling and analysis results very site-specific.
Conclusion
Root-soil cohesion of the root system of teak (Tectona grandis), old and young Maesopsis eminii, and shrubs (Chromolaena odorata) varies. The highest total root cohesion (CR) of vegetation was teak with 0.398 kPa (TRmax = 93.6 MPa, RARmax = 0.049%), followed by shrubs with 0.202 kPa (TRmax = 55.43 MPa, RARmax = 0.212 %), young Maesopsis eminii 0.191 kPa (TRmax = 32.7 MPa, RARmax = 0.112%), and old Maesopsis eminii 0.087 kPa (TRmax = 19.2 MPa, RARmax = 0.092%). The largest RAR was for shrubs of 0.212%, and the largest TR was for teak of 0.284 kPa in root diameter class I (>5 - ≤10 mm).
The results of slope stability modelling with 5 scenarios, namely bare land, teak, old and young Maesopsis eminii, and shrubs, show a FOS increase in the range of 0.2-5.9% through mechanical vegetation effect. The largest average FOS increase occurred on the slopes with young Maesopsis eminii (5.9%), shrubs (3%), teak (2%), and old Maesopsis eminii (1%). This study shows that vegetation can affect unsafe slope stability to marginal safety (not significant).
The increase in FOS by vegetation roots is affected by the ratio of the root zone depth to the slope geometry. The greater the root versus slope ratio, the lower the increase in FOS. The stability of vegetated slopes is a function of the effective depth of the root zone as indicated by the FOS value. In this case, old Maesopsis eminii has the largest effective root zone
depth of 500 cm, followed by teak, young Maesopsis eminii, and shrub, with an effective root zone depth of each, is 230 cm, 140 cm and 66 cm, respectively.
Acknowledgements
This research was funded by the Program of Master leading to PhD (PMDSU) of The Ministry of Education, Culture, Research and Technology, Republic of Indonesia. The authors thank Herman, Peby Apriliano Mahaputra, Yoga Adhi Pratama, Safira Nazila Rahmah, and Ni Made Ayu Rianti, who helped to collect data and fieldwork.
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