50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India EFFECT OF VEGETATION ON STABILITY OF SLOPES S. Anaswara 1, Graduate Student, NITK Surathkal, Mangalore - 575025, email: anaswaras6@gmail.com R. Shivashankar 2, Professor, NITK Surathkal, Mangalore - 575025, email: shivashankar.surathkal@gmail.com ABSTRACT: In this paper the role of plant roots in the mechanical stabilization of the vegetated slope is being studied numerically, including effect of wind loads. Vegetation, in the form of grass on surface of slope and trees located at different locations on slope are investigated to determine the stability of slopes. The turf on slope is simulated as a composite soil zone with an additional cohesion due to grass root system, and the roots of trees are considered as intrusions in the soil. Self weight of tree and wind loads on the tree canopy are also considered. Factor of safety values for different cases are evaluated. The results indicate that in general vegetation contributes significantly to the increase in the factor of safety values. Turfed slope with tree at toe is found to give the maximum benefit. INTRODUCTION Earth Slopes An earth slope is an inclined surface of a soil mass. Slopes either occur naturally or are engineered. Very often non-engineered slopes are cut or built by humans. Ensuring stability of slopes is a challenging task for civil engineers. Slope stability issues have been faced throughout history when humans or nature have disrupted the delicate balance of natural soil slopes, and in case of non-engineered slopes. In order to build stable/safe slopes and to ensure safety of natural slopes, there is a need to understand the analytical methods, investigation tools, and stabilization methods that involve specialty construction techniques. Vegetation on slopes and stability of vegetated slopes Vegetation is multifunctional, relatively inexpensive, visually attractive and does not require heavy or elaborate equipment for its installation. The stability of the slope is increased by vegetation in terms of mechanical and hydrological mechanisms. Vegetation helps to delay 'mass wasting' (or landslides) on hill side slopes. Slope engineering these days, in addition to geotechnical engineering and hydrology, is also encompassing Bioengineering, Biotechnical Engineering' and 'Geosynthetics Engineering'. Bio-engineering or biotechnical engineering involves the handling and manipulation of living components to produce useful products. Figure 1 shows engineering role of vegetation on ground surface [1]. Vegetation can affect the balance of stresses in a slope due to mechanical reinforcement from the root system of trees. It also affects the stability of Fig.1 Engineering role of vegetation on ground surface [1]
S.Anaswara, R.Shivashankar slopes by modifying the hydrologic regime of the soil. Since plants and grass absorb different amounts of water depending on the type of soil that they grow in, there are several different criteria for the selection of the most appropriate species. A general rule of thumb is to use local plants and grass that are adaptable to local climate. In the case of a tree on slope, its self weight (gravity load) and wind loads are the major destabilizing (and in some cases also stabilizing) forces with regard to the stability of the slopes. It involves a complex set of soil and structural interactions. Resistance to wind and gravity loading is distributed and shared throughout a tree and associated soil [2]. Fan and Lai (2013) [3] investigated vegetation located at the upper, middle and lower slopes (similar to Figs. 2, 3 and 4 respectively) to determine the influence of the spatial layout of planting on the stability of slopes. The soil arching, at proper tree spacing, was found to play an important role in the stability of slopes. The plant root system provides buttressing and reduces the soil displacement for the soil at the upslope of the tree. The rooted soil zone in the slope experiences greater shear stresses and shear strains with respect to the adjacent non-rooted soil zone. The factor of safety of vegetated slope decreases with increasing tree spacing. Vegetation at the upslope and mid slope provides better reinforcement to the slope with respect to that of downslope if the root system penetrates into the firm ground in the slope [3]. Stability of vegetated slopes may be affected by the distribution pattern of the root system architecture. Fig.2 Vegetation at top of slope Fig.3 Vegetation at middle of slope Fig.4 Vegetation at toe of slope A typical vegetation cover on slope should include trees and shrubs, in addition to grass. The selection of indigenous tree species on the basis of their root properties is an essential part of biotechnical slope protection [4]. In this study vetiver grass (Figs.5, 6) and neem tree (Fig.7) are considered for the analyses. Vetiver grass (Vetiveria zizanioides) has been used very successfully in erosion control in these coastal areas of Karnataka and Kerala in peninsular India. It is an excellent plant for slope works. Vegetation induced ground improvement technique is very often actually found to be more effective technique than most other ground improvement methods. Roots of vetiver grass' (Vetiveria zizanioides) which can grow up to 3 m long into the soil, behave like live soil nails' in top layers of soil, similar to the trees and some shrubs (Fig.6). Due to its dense massive root system, vetiver offers better shear strength increase on slopes and is widely used for soil slope stabilization. Tension tests conducted on vetiver grass roots (0.7-0.8mm diameter) showed a mean tensile strength of 75MPa (i.e. about one-sixth the strength of mild steel) [8]. Since the end of eighth decade of the last century, practice proved that dense green hedgerows of vetiver system can effectively slow down surface runoff, filter and to
50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India trap eroded sediments; Its deep and extensive root system can fix soil, anti-slide and anti-scour, protect slope, foreland, dike or river bank. Trees increase the matric suction of the soil via root water uptake in conjunction with the evapotranspiration of their canopy [9]. From ground Fig. 7 A neem tree [7] Fig. 5 Vetiver grass and its roots [5] improvement point of view, while using vegetation, it is very important to understand the morphology and spatial distribution of the tree canopy and that of the root system. Canopy affects lateral forces such as the wind forces and the root system provides mechanical strengthening. Neem (Azadirachta indica) (Fig.7), is the most versatile tropical tree with immense potential. It is native to India and many Southeast Asian countries, but grows well in a variety of tropical environment. Neem is a fastgrowing tree that can reach a height of 15-20m, rarely to 35-40m.The branches are wide spread. The dense crown is round or oval shape and may reach the diameter of 4.5-6.0 m in old, free standing specimens. The root system consists of a strong tap root and well-developed lateral roots. NUMERICAL MODELING Fig.6 Vetiver grass on slope for slope stability and erosion control [6] Finite Element Program PLAXIS [10] a finite element program for geotechnical applications is used in the present investigations. Factor of Safety (FOS) is computed by using the c-φ reduction procedure. This approach involves in successively reducing the soil strength parameters c and tanφ until failure occurs. The strength parameters are automatically reduced until the final calculation
S.Anaswara, R.Shivashankar step results in a fully developed failure mechanism. The Factor of Safety (FOS) is calculated as the ratio of the available shear strength to the strength at failure by summing up the incremental multiplier (Msf) as defined by: FOS = = value of ΣMsf at failure (1) Different combinations of parameters c and φ are considered for embankment heights of 5m,10m and 15m. Input and geometry model The slope is analyzed as a plane strain model [11]. Fifteen node triangular elements are selected for model soil layers and volume clusters. The stressstrain behavior of the soil is modeled using the Mohr-Coulomb model. The material is taken in drained condition. Effect of turf vegetation is included as a different layer of soil, 3 m thick, with increased cohesion. Cohesion is a parameter that is varied to represent different densities of the vetiver root system in the upper layers of the slope. Tree root element is modeled using plate elements. The geometry contains a non-horizontal soil surface. Geometric profile of the slope used in the study is shown in Fig. 8 and root pattern is shown in Fig 9. The initial stresses were calculated by means of gravity loading. There are three phases of calculation. The first two phases are the plastic calculations and the last phase is the phi/c reduction calculation. From the phi/c reduction calculation, the factor of safety is obtained. Factor of safety is obtained from the output chart or calculation info. The slope consists of a soil with apparent cohesion underlain by a same soil without apparent cohesion. The thickness of the sloping top soil layer is considered as 3 m. Standard fixities were used as boundary conditions (Fig.10). The ground condition is free in all directions. Four different soil types are used for the analysis (Table 1). An additional cohesion contributed by the root system was added to the root reinforced soil block. Two loads are considered in this analysis: (1) self-weight of tree and (2) wind load. Thus, in addition to the weight of the trees, the effects of both uphill and downhill wind loads on the trees and their impact on the stability of slopes are studied. A typical tree weight of 15kN is assumed. Weight of the tree acting downward at the centre of gravity. Height of tree is taken as 20m with a round crown of 6m diameter. Roots are taken as 3 m deep. Wind load is calculated from IS: 875 (Part3) - 1987 [12]. Vetiver grass (Figs. 5, 6) and Neem tree (Fig.8) are considered and their combined effects are also noted. Neem is commonly called 'Indian lilac' or 'Margosa'. Mangalore region which is considered for this study receives copious amount of rainfall. The rainfall is not only heavy but also happens for long durations. Such rainfalls will reduce the factors of safety of slopes. Vegetation on slopes in such heavy rainfall areas will add to stability and control the reduction in factors of safety due to saturation of soil, seepage and erosion effects. Tree positioned at three different locations such as at crest, mid slope and at toe of the slope are considered (Figs. 2 to 4). Fig 8 The geometric profile of the slope Fig 9 The pattern of the root system
50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India RESULTS AND DISCUSSIONS Figure 11 shows factor of safety values obtained for different cases without considering the wind effect. Tables 2-7 show FOS values for slopes with the four different soil types with wind load effect. The results are presented for 1H: 1 V slope angle [11]. Fig.10 Slope showing the turfed zone, tree at toe and the boundary conditions Table 1 Soil types and their properties used in the study Parameter Soil1 Soil2 Soil3 Soil4 Material Model M-C # M-C # M-C # M-C # Type of Drained Drained Drained Drained Material Dry Unit Weight 20.00 20.00 20.00 20.00 kn/m 3 Sat. Unit Weight 23.00 23.00 23.00 23.00 kn/m 3 Young's Modulii 100.00 450.00 2200.00 7500.00 kn/m 2 Poisson's 0.40 0.35 0.30 0.30 Ratio Cohesion 10.00 20.00 20.00 25.00 kn/m 2 Angle of Internal 22.50 26.25 30.00 33.75 Friction (degrees) Dilatancy Angle 0.00 0.00 0.00 0.00 (degrees) # M-C is Mohr-Coulomb model Fig.11 FOS values for 1H: 1V slopes, with the four different soil types, without wind effect, for 10 m high embankments Table 2 FOS values for 1H: 1V slopes, with the four different soil types, for 5 m high and wind blowing uphill Details Wind blowing uphill Tree at toe 1.722 2.718 3.092 3.497 Tree at mid failed 2.536 2.986 2.547 Tree at top 1.628 2.372 2.830 2.749 Grass only 2.223 2.966 3.398 3.671 Bare slope 1.396 2.161 2.499 2.802
S.Anaswara, R.Shivashankar Table 3 FOS values for 1H: 1V slopes, with the four different soil types, for 5 m high and wind blowing downhill Details Wind blowing downhill Tree at toe 1.730 2.712 3.092 3.495 Tree at mid failed 1.936 2.100 1.433 Tree at top 1.318 1.960 2.180 2.289 Grass only 2.223 2.966 3.398 3.671 Bare slope 1.396 2.161 2.499 2.802 Table 6 FOS values for 1H: 1V slopes, with the four different soil types, for 15 m high and wind blowing uphill Details Wind blowing uphill Tree at toe failed failed 1.075 1.216 Tree at mid failed 1.188 1.394 1.551 Tree at top failed 1.185 1.419 1.545 Grass only failed 1.303 1.555 1.678 Bare slope failed 1.217 1.477 1.600 Table 4 FOS values for 1H: 1V slopes, with the four different soil types, for 10 m high and wind blowing uphill Wind blowing uphill Details Tree at toe 2.022 2.018 2.015 2.013 Tree at mid failed 1.552 1.830 2.035 Tree at top failed 1.480 1.786 1.944 Grass only 1.210 1.655 1.934 2.097 Bare slope 1.014 1.521 1.807 1.985 Table 7 FOS values for 1H: 1V slopes, with the four different soil types, for 15 m high and wind blowing downhill Details Wind blowing downhill Tree at toe failed failed 1.073 1.214 Tree at mid failed 1.115 1.305 1.458 Tree at top failed 1.168 1.369 1.527 Grass only failed 1.303 1.555 1.678 Bare slope failed 1.217 1.477 1.600 Table 5 FOS values for 1H: 1V slopes, with the four different soil types, for 10 m high and wind blowing downhill Details Wind blowing downhill Tree at toe failed 1.549 1.815 2.014 Tree at mid failed 1.354 1.554 1.586 Tree at top failed 1.316 1.530 1.702 Grass only 1.201 1.655 1.934 2.097 Bare slope 1.014 1.521 1.804 1.985 Fig. 12 FOS values for 1H: 1V slopes, with the four different soil types, without wind effect, for 10 m high embankments (combined effect of grass and tree on slopes)
50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 2015, Pune, Maharashtra, India Venue: College of Engineering (Estd. 1854), Pune, India Combined effect of grass and tree on slope stability Factor of safety (FOS) values obtained for combined effect of grass and tree on slopes are shown in Fig 12. Therein, wind loads are not taken, only self-weight of tree are considered. Grass was assumed to spread uniformly over the slope surface. Comparing the FOS values in Fig. 12 with those values in Fig. 11, it can be seen that the gravity load of the tree (without wind loads) certainly has marginally increased the FOS values of turfed slopes. CONCLUSIONS AND SCOPE FOR FUTURE WORK In this study, work is carried out to investigate the effect of vegetated slope on stability of slopes by using the two-dimensional finite element modelling. Vegetation, in general, is very advantageous on slopes since it increases the FOS values of slopes. This study has looked into only the mechanical stabilization aspect of slopes, but considering effects of wind in a simple manner. The Earlier study [13] without considering the wind had also concluded the same. Factor of safety due to vegetation is also further enhanced due to its ability to effectively control erosion and impact of rainfall on slopes, due to matric suction and evapotranspiration etc. These various other factors are also very important and a detailed study is required to estimate the overall contribution of vegetation to slope stability. The factor of safety of vegetated slope decreases with increase in height of slope. A single tree on slope (without grass on slope), irrespective of its location, with no wind reduces FOS marginally as compared to bare slope. Gravity load of the tree (without considering the wind loads) marginally increases the FOS values of turfed slopes. A single tree on slope, irrespective of its location, with wind forces in either direction, reduces FOS further as compared to the case of tree with no wind and also bare slope, except for tree at toe and wind blowing uphill. Wind acting at downhill decreases the factor of safety more than that of wind acting uphill. In the case of weak soils such as S1 considered in this study, the wind loads - both downhill or uphill will cause failure of the slopes. Bare slope of same height with S1 was somewhat just stable and it was quite stable with just the grass on it. Tree at top with wind load gives lesser FOS as compared to tree at mid slope. With tree at toe and the wind blowing downhill or uphill, the FOS is nearly the same at for tree at toe with no wind load. For tree at toe and wind blowing uphill, Maximum stability of slope (maximum FOS) is achieved. The factor of safety of vegetated slope consisting of both grass and tree at toe is higher than the FOS of slope with only tree at toe position.. The present work can be extended with use of other plant species (i.e. with different spatial distributions of canopy and roots of the vegetation) to study the suitability of it in slope protection. This study looked into a single tree on slope. Such study can be extended for several trees on the slopes. Also many other factors such as rainfall data, matric suction, evapotranspiration, seepage, erosion etc can all the be considered. Three dimensional finite element modeling can be used for a better spatial layout of vegetation. REFERENCES 1. Coppin, N.J. and Richards,I.G. (1990), Use of Vegetation in Civil Engineering, 1st Edn., Construction Industry Research and Information Association, Butterworths, London, pp: 292-292. 2. Coder Kim D, (2010), Root Strength & Tree Anchorage, Warnell School of Forestry & Natural Resources, University of Georgia, https://www.warnell.uga.edu/outreach/pubs/pd f/forestry/root%20strength%20pub%2010-19.pdf
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