STABILITY ANALYSIS OF SLOPES ALONG ROADS IN BIO-REINFORCED SOIL CONDITIONS

: Root system has ability to stabilize slopes, improving physical and mechanical prop-erties of soil on which it develops. Morphology and tendency of root system to compose soil particles into one monolithic mass, which we call bio-reinforced soil, contribute to increasing the resistance of soil to shearing. In this paper, is presents a comparative analysis of slope stability along roads without and with the influence of root system. The analyzes were made for the needs of defense of roads, finding most optimal types of root system as an alternative solution for stabilization of the slopes along roads. Оn the slope model was simulated influence of four groups of vegetation, based on morphology of root system (plate, heart, tap and undefined). For each selected species, value of root cohesion (c r ) has been adopted. Software for geotechnical numerical modeling-GeoStudio 2007, was used for all slope stability analyzes. Analyzing stability of the slope model without influence of vegetation, slope is unstable. By calculating stability of slope model with vegetation groups, an increase in stability of the slope model is achieved. The greatest influence on stability of the slope model has group 2.- vegetation with a tap root system, followed by group 1. - with a heart root, while group 3, plate root, and group 4, undefined types of root system, gave at least the values. The results from this paper, represent a contribution to choice of solutions for stabilization of slopes along roads and the prevention of erosion processes.


INTRODUCTION
Building activity during the construction of roads, often violates the natural balance of slopes along roads, which manifests it self in creation of various contemporary exogenous processes (Figure 1). Slopes along roads, after construction, remain unprotected from effects of erosion, which causes gravitational movement of soil and collapse of technical objects. S w a n s o n and D y r n e s s . Resistance of soil, with a stable vegetation system, can be increased by 50-70% or significantly reduced with clean cutting on slopes (Norris et al., 2008). Vegetation acts as a protective barrier between soil and agents (biotic and abiotic) that stimulate degradation processes. Effectively protect against water and eolian erosion as well from wind gusts and snowdrifts. Root system improves physical and mechanical parameters of soil by its growth and development (increase cohesion, bulk densitie, internal friction angle) (Wu et al., 1979 , 2007). Root system has tendencies to compose soil particles into one monolithic mass and thus contribute to increasing resistance of soil, primarily cohesion. Plant species have three main types of the root system (Köstler 1968): plate, heart and tap. Plate root system is developed shallow in surface of soil, has strong lateral roots and fine vertical roots.

STABILITY ANALYSIS OF SLOPES ALONG ROADS IN BIO-REINFORCED SOIL CONDITIONS
Mladen Marković, PE"Roads of Serbia", Belgrade, Serbia, mladen.markovic@putevi-srbije.rs Nikola Živanović, University of Belgrade Faculty Forestry Grozdana Gajić, University of Belgrade Faculty Forestry Abstract: Root system has ability to stabilize slopes, improving physical and mechanical properties of soil on which it develops. Morphology and tendency of root system to compose soil particles into one monolithic mass, which we call bio-reinforced soil, contribute to increasing the resistance of soil to shearing. In this paper, is presents a comparative analysis of slope stability along roads without and with the influence of root system. The analyzes were made for the needs of defense of roads, finding most optimal types of root system as an alternative solution for stabilization of the slopes along roads. Оn the slope model was simulated influence of four groups of vegetation, based on morphology of root system (plate, heart, tap and undefined). For each selected species, value of root cohesion (c r ) has been adopted. Software for geotechnical numerical modeling-GeoStudio 2007, was used for all slope stability analyzes. Analyzing stability of the slope model without influence of vegetation, slope is unstable. By calculating stability of slope model with vegetation groups, an increase in stability of the slope model is achieved. The greatest influence on stability of the slope model has group 2.-vegetation with a tap root system, followed by group 1. -with a heart root, while group 3, plate root, and group 4, undefined types of root system, gave at least the values. The results from this paper, represent a contribution to choice of solutions for stabilization of slopes along roads and the prevention of erosion processes. Keywords: root system, root cohesion, vegetation, slope stability, roads protection "Bio-reinforcement" of soil, the species with the heart root system, significantly influences on reduction and elimination process of inside erosion and filtration forces in loosely unconsolidated soil, creating a specific and coherent biologically reinforced mass, that is more resistant to tangential forces and formation of landslides (Га ј и ћ 2014). Tap root system has developed morphological development, in analysis of stability and resistance to sliding is introduced as anchorized group of "biopiles", provided root system is under shear plane in stable soil. Influence of root in calculating slope stability is expressed through root cohesion (c r ) (M a o еt al., 2012). Research has shown that with planting and maintenance of vegetation on a potential sliding surface, factor of safety can be increased up to 10 % .
In this paper, are presents analyzes and different scenarios of stability of slopes along roads without and with the influence of the root system. The analyzes were made for needs of defense of roads, finding most optimal types of root system as an alternative solution for stabilization of slopes along the roads.

MATERIAL AND METHODS
Analysis of stability of slope with influence and without the influence of root system was carried out on the prepared model of terrain with given dimensions and physical-mechanical values of soil (Table 1). The model is formed from one layer of soil with the values of physical-mechanical parameters for saturated soil condition, when resistant of soil characteristics are at least. Formation of slopes along the roads is often done only by filling and planning without soil compaction, resulting in a significant reduction of resistant soil characteristics.
Software for geotechnical numerical modeling-GeoStudio 2007, was used for all slope model stability analyzes, by method Mo rgen stern and P r i c e (1965). The method is basically analytical with an arbitrary shear plane (circularly cylindrical), which analyzes the balance conditions for each lamella and entire main body. For soil layer in the given model, parameters of soil resistance to shear and bulk densitie are introduced. Software considers more potential shear planes in given model and provides a critical shear plane with the lowest factor of safety using following formula. Where is: Fs-factor of safety, c-cohesion, N-normal forces, u -pore pressure, φ-internal friction angle, W sinα-shear (tangential) force, l-length of slice, α-angle of shear plane Stability condition is fulfilled when value of factor of safety is Fs ≥ 1.5, if the value is lower, condition is not fulfilled, slope is unstable. In this paper, are presents shear planes with lowest factor of safety for adopted conditions of the slope model.
From aspect of vegetation influenced on stability of slope model, root system was analyzed as a slice with a single pile, respectively anchored "bio-   Figure 2). In slices with "bio-pile" influence of root system is introduced into calculation by increasing cohesion for c r , which is represented by Mohr-Coulomb's equation of shear stress (Wo 2013.) ...........

[2]
Where is: τ-tangential stresses, c-soil cohesion, c r -root cohesion, σ-normal stress, u-pore pressure, φ-internal friction angle Bio-reinforcement of soil can increase stability in shallow landslides 2-3 m deep, while in deeper landslides, which are outside root system zone, this influence is negligible (N o r r i s et al., 2008). Values of root cohesion (c r ) and root types for selected species are shown in Tables 2. Dimensions of root system, R -radius, Z -depth and P-surface with root system, which have been adopted for purposes of this paper, are shown in Table 3. The use of "bio-piles" in the slope model was carried out depending on the dimensions of geometric approximation of root system, in order to avoid overlapping and to comply with the minimum conditions of tree planting.   On the slope model, was simulated influence of four groups of vegetation, based on the shape of root system, and for each species is adopted value of root cohesion (c r ). The slope model was analyzed with the following vegetation groups: • Group 1. species with heart root system (Pseudotsuga menziesii , Alnus glutinosa) • Group 2. species with tap root system (Pinus contorta, Pinus nigra) • Group 3. species with plate root system (Picea sitchensis) • Group 4. species with undefined root system (Festuca pratensis, Festuca rubra, Poa pratensis)

RESULTS AND DISCUSSION
Construction of roads in hilly-mountainous area requires cutting of slopes whose stability is questionable after a change in geometry of slope and disturbance ratio of stress in soil. After road construction has been completed, slopes along road are formed of excess of excavated soil which is in a loose condition, often without any compac-tion, because of that values of shear resistance parameters are significantly reduced compared to the natural ones.
By analyzing stability of the slope model without influence of vegetation with adopted soil parameter values, factor of safety Fs = 1.32 was obtained, which did not fulfill stability condition Fs ≥ 1.5 (Figure 3), the slope model is unstable.
Vegetation with heart root system, group 1, it was used in calculation of slope stability, values of factors of safety for the two species are shown. Using species Pseudotsuga menziesii factor of sefety is Fs=1.70 (Figure 4, а.). The second used species is Alnus glutinosaса which gave factor of safety Fs=1.60 (Figure 4, б.). By using both species with heart root system, satisfactory stability is achieved. Considering types and dimensions of root system of both species are same, difference in value of root cohesion from 5,5 КРа (Table 2) has influenced the increase factor of safety for 0,10, in favor of species Pseudotsuga menziesii. Using species with tap root system from group 2, stability of the slope model was analyzed, by using species Pinus contorta value of factor of safety Fs = 2.13 was obtained ( Figure 5, а.), whereas with species Pinus halepensis, factor of safety is Fs=1.94 ( Figure 5, б.). For both used species factor of safety is filled, Fs ≥ 1.5, the slope model with "bio-piles" is stable. Difference in root cohesion for these two mentioned calculation is 2,8 КРа, while the difference in factor od safety is 0.19.
Group 3, species with plate root system, through calculation it was analyzed one species Picea sitchensis. Value of factor of safety is Fs=1.38 (Figure 6), stability condition is not fulfilled, Fs ≥ 1.5, the slope model is unstable.
Analysis stability of the slope model using grass mixture from group 4. (Festuca pratensis, Festuca rubra, Poa pratensis) obtained factor of safety is Fs=1.34 (Figure 7), stability condition is not fulfilled, Fs ≥ 1.5, the slope model is unstable.
Analysis stability of the slope model, using species with plate and undefined root system (grass mix) satisfactory stability has not been achieved.
Comparing obtained factors of safety, in conditions without vegetation and with vegetation, it can be determined that use of vegetation increases factor of safety. "Bio-piles" with heart root system increase factor of safety for 0.38 (Pseudotsuga menziesii) and 0.28 (Alnus glutinosa). Species with tap root system increase factor of safety for 0.81 (Pinus contorta) and 0.62 (Pinus halepensis). Lower increases factor of safety was obtained for 0.06, species with plate root system, and for 0.02, undefined root system in relation to factor of safety of the slope model without vegetation.
Analysis tapes of root system, greatest influence of vegetation is with tap root, follow heart root, plate and undefined root system have the least influence on stability (Figure 8). Obtain results indicated that the greater role in increasing factor of safety has tapes and depth of root system, from value root cohesion. Holsworth (2014) obtained similar results by analyzing stability of slope by influence of different types of root system.

CONCLUSION
Construction and reconstruction of roads, slopes along road remain unprotected from effects of erosion, which causes gravitational movement of soil, especially surface erosion and landslide formation.
Properly selected tree, shrubs and grass species can be used to prevent erosion processes and stabilize unstable terrain. The selection must be based on conditions of environment, characteristics of species and plan of planting. For rehabilitation and formation of landslides, depths up to 3 meters, the major role in selecting species have tapes and depth of root system.
Analysis stability of given slope model along road, without influence of vegetation, factor of safety Fs = 1.32 was obtained, which did not fulfill stability condition Fs ≥ 1.5. Using vegetation groups in calculations stability of the slope model, results were obtained in range from Fs = 1.34 to Fs = 2.13. It can be concluded that the use of all vegetation groups increases the stability of slope.
For needs of defening roads, finding the most optimal form of root system as an alternative solution for the stabilization of slopes along road, the greatest influence on stability has group 2.-vegetation with tap root (Fs=2.13, Fs=1.94 ), followed by group 1.-with heart root (Fs=1.7, Fs=1.6), while group 3, plate root, (Fs = 1.38) and group 4, undefined root, (Fs = 1.34) gave at least values.
Depth and tapes of root system, relative to value of root cohesion (c r ), have greatest influence on stability of the slope model.
Presented analyzes of slope stability can be concluded that use of vegetation on unstable and conditionally stable slopes leads to an increasing factor of safety.