UNIVERSITY OF NAIROBI

Transcription

1 UNIVERSITY OF NAIROBI Department of Civil and Construction Engineering FCE 590: PROJECT The stabilization of slopes using vegetation as reinforcement By Gatiaga Evalyne Njeri, F16/39765/2011 A project submitted as a partial fulfillment for the requirement for the award of the degree of BACHELOR OF SCIENCE IN CIVIL ENGINEERING Year of Submission 2016

2 Declaration I, GATIAGA EVALYNE NJERI, declare that this work is my original work and has not been submitted to any other university. Signature: Date: This project has been submitted with the approval of supervisor, DR. SIMPSON OSANO Signature: Date: i

3 Abstract The role of plant roots in slope stabilization is important however the effect is varied in different species. For this study, both pull-out and tensile strength tests were conducted on some local shrub species namely Lantana camara (Wild Sage), Triumfetta tomentosa (Burrbark) and Tagetes lemmonii (Marigold) found in the central region of Kenya. From the pull out test, a single peak value was observed for all the species. Overall results showed that T. tomentosa offered the highest pull out resistance. The pull out was sudden however the irregular sound of root snapping was heard just before failure. This could be a warning sign. The pull out resistance however reduces with increase in soil moisture content. For the tensile strength test, the shrub with the highest tensile force is the wild sage (136N) followed by burrbark (119N) and finally marigold (37N). The tensile strength however b reduces with increase in root diameter, following a power law in the form of f ( x) ax. Root Area Ratio (RAR) was also calculated for all species, and the highest values were observed within a depth of 0.15m. The root reinforcement, in terms of cohesion, decreases with increase in depth. The maximum reinforcement is from the wild sage and burrbark species at 125Kpa while the marigold species has a maximum of 70Kpa, all at depths of 0.2m. ii

4 Dedication I would like to dedicate this project to God who has seen me through this course, Thank you. I also dedicate it to my family and friends that have supported me throughout this journey. iii

5 Acknowledgement I wish to express my sincere gratitude to the following: I would like to thank the Almighty God for giving me strength, endurance, encouragement and success in this project. I would also like to thank my family members my father Mr. Alfred Thuku, my mother Mrs. Catherine Thuku, my brother Mark Ben Thuku and grandmother Margaret Thuku for their continuous support, prayers and encouragement. I would also like to appreciate my supervisor, Dr. Simpson Osano who has guided me through this project and devoted his time to helping me achieve it through positive criticism and adequate guidance. To my friends, Angela Mwende,Victor Kebabe, Caleb Mathuva and Collins Amenya, thank you for your support in this project. To the laboratory technicians, Mr. Mucina and Mr. Oyier, thank you for your support, time and contribution to this project. I also appreciate Mr. John Miano for his time and dedication. And to all others who contributed in one way or another to the completion of this project thank you and God bless you. iv

6 TABLE OF CONTENTS Declaration... i Abstract... ii Dedication... iii Acknowledgement... iv LIST OF FIGURES... vii 1. INTRODUCTION Background Problem Statement Objective of the study LITERATURE REVIEW Introduction to slope instability Areas Prone to Landslides Types of Landslides Slides Falls Flows Causes of Landslides Shear Strength Methods Of Determining Shear Strength Parameters Effect of rain water and excess pore pressure Effect of vegetation in slope stabilization Root morphology and structure Root Types Root Depth Root Area Ratio Root as Reinforcement Root failure mechanism Alternative soil reinforcement materials: Geosynthetics METHODOLOGY Introduction v

7 3.2 Description of study area Sampling Laboratory apparatus and testing procedure Root tensile test Root Pull-out Test RESULTS AND ANALYSIS Pull-out test Root Tensile Test Root Area Ratio Root cohesion CONCLUSION AND RECOMMENDATIONS Conclusion Pull out test Tensile strength and root distribution Final remarks Recommendations REFERENCES APPENDIX vi

8 LIST OF FIGURES Figure 1.1: Landslide prone areas... 5 Figure 2.1: Major Geophysical hazards in Kenya... 8 Figure 2.2: Mudslide in Murang a County, Kenya... 9 Figure 2.3: Landslide in Salvador, Brazil... 9 Figure 2.4: Landslide-prone areas in Kenya Figure 2.5: Types of Landslides Figure 2.6: Example of Rockslide and landslide Figure 2.7: Example of rock fall Figure 2.8: Example of mud flows Figure 2.9: Map showing rainfall distribution in Kenya Figure 2.10: Root morphology Figure 3.1: Map of Central Kenya Figure 3.2: Examples of Triumfetta tomentosa, Tagetes lemmonii and Lantana camara respectively Figure 3.3: Hounsfield Tensometer Figure 3.4: Root pull out apparatus Figure 4.1: Pull out resistance against displacement for Lantana camara (Wild Sage) Figure 4.2: Pull out resistance against displacement for Triumfetta tomentosa (Burrbark) Figure 4.3: Pull out resistance against displacement for Tagetes lemmonii (Marigold) Figure 4.4: Pull out resistance against moisture content for all the species Figure 4.5: Root tensile force against root diameter for all species Figure 4.6: Root tensile strength against root diameter following the power law vii

9 Figure 4.7: Root area ratio against depth for all 3 species Figure 4.8: Root cohesion against root depth for all species Figure 7.1: Triumfetta tomentosa (Burrbark) Figure 7.2: Lantana camara (Wild Sage) Figure 7.3: Tagetes lemmonii (Marigold) Figure 7.4: Pull out test at the University of Nairobi Lab Figure 7.5: Laboratory tests viii

10 CHAPTER ONE 1. INTRODUCTION 1.1 Background Slope instability has been an issue of great concern for many years now in many different parts of the world. It is a slope failure that results in downward movement of soil mass with gravity as its driving force. It is of great concern because of its catastrophic nature and sudden occurrence in most cases and sometimes resulting in deaths of multitudes of people and livestock and destruction of property, making it a natural calamity. It is known to occur in mountainous regions and areas with steep slopes and is attributed to many factors such as soil erosion, heavy rainstorms, volcanic activities and earthquakes which reduce the shear strength of soil. Its attributes to nature gave rise to the discovery of methods and measures of mitigation and prediction of landslides in order to prepare adequately and reduce the impact. Population growth and industrialization has contributed to slope instability because there has been settlement in the slope areas leading to deforestation of natural forests in order to create land for settlement and cultivation or development. Studies have shown that vegetation is vital in stabilizing the soil in slopes because the roots act as anchorage and reinforcement to the soil thus increasing its shear strength. Kenya is no exception when it comes to landslides. It has been faced with many landslides over the years during the 2 rainy seasons between March and May and between October and November. Unfortunately the landslides have caused deaths of several people living in prone areas, destruction of their properties, displacement of people and a set back to the economic growth of the area. According to the Kenya Red Cross society the areas at high risk of experiencing landslides and mud slides are Kakamega, Kisii, Murang a, Rift Valley region and Nandi Hills. However, the Government of Kenya has made efforts to create awareness to the public on ways of being prepared by moving to safer grounds during the heavy rain season and also urging those living in the area to integrate their cultivation with tree planting and incorporate other methods of slope stabilization such as using gabions and cut off drains. 1

11 It has been noted that slopes covered with vegetation have a higher resistance to landslide occurrence than deforested slopes. Vegetation reduce soil erosion of water and land movements by holding the soil particles together and intercepting rain water. The roots affect the properties of soil such as infiltration rate, shear strength, organic matter content, moisture content and aggregate stability (Dr. S. Osano). The roots also act as reinforcement to the soil. The magnitude of root reinforcement depends on; morphological characteristics of the root system (e.g. root distribution with depth, root distribution over different root diameter classes), root tensile strengths, root tensile modulus values, the interface friction between roots; and the soil and the orientation of roots to the principal direction of strain (Greenway, 1987, Osano and Mwea, 2011). The use of vegetation in mitigating landslides is effective environmentally friendly, beneficial and economic and can be used in several slopes. However it is important to note that tall vegetation act as surcharge to the soil increasing the weight acting on the soil leading to slippage in the fault. 1.2 Problem Statement For many years, Kenya has been faced with the landslide challenge. In May 1997, Kenya experienced unusually heavy rainfall that lasted until February 1998 due to the El-Nino weather phenomenon. This period of about 10 months of heavy rainfall caused widespread landslides and floods in different parts of the country. The landslides were attributed to 4 major factors: High relief Steep slopes with poor anchorage for stabilization Heavy precipitation Oversaturation (Ngecu and Mathu, 1999) These landslides resulted in loss of lives of many, swept away bridges and some road sections, swept away homes leaving many homeless and cultivated crops were destroyed. This has created 2

12 the need for development of methods of mitigating and reducing the occurrence of landslides and being able to predict weather patterns and act accordingly before a disaster happens. From various studies and surveys, it has been said that vegetation is a key element in slope stabilization. Its roots act as anchorage to the top soil thus increasing the shear strength of the soil. Therefore there is a need to protect forested areas and encourage and educate people on the importance of conservation and planting of trees. Kenya has a growing population and its people are beginning to encroach forested areas such as the Mau Forest in search of land for cultivation. This has led to the cutting down of many trees to facilitate this and also to develop these areas. As a result the soil is left bare and loose thus making it susceptible to soil erosion and, in severe weather conditions or seismic movements, landslides. Some civil works use the cut and fill methods of making embankments thus creating manmade slopes which are also susceptible to failure in severe weather conditions if not reinforced. Most local communities view landslides as an act of God and therefore are not keen in knowing the main reason why they occur. Therefore there is need of creating awareness to the general public especially those living in the prone areas and improve their quality of life. Mitigation measures are important and lasting solution to this problem need to be devised. Landslides tend to occur during heavy rainy season as the soil is saturated beyond its holding capacity hence making it weak and susceptible to slipping. Drainage and channeling of excess water needs to be addressed. Vegetation roots uptake water and reduce the rate of infiltration and help in regulating the amount of water in the soil. 3

13 Table 1: Socioeconomic impacts of landslides in Kenya Date Landslide Location Impact April 2013 Kijabe Landslide Blocked railway line, disrupted services Maua Landslide 11 People died Kitheu Landslide 4 People died, destroyed water pipes Muranga Landslides 11 People died, 7 houses November destroyed, 3 cows died Child died Timboiwo Houses, crops, pasture destroyed Yabsoi s Farm, Kericho whole farm destroyed 2003 Kokwet, Kipkelion 4 People died, pasture lost, arable land rendered infertile November Chesegon Landslide, Pokot Central 11 People died, pasture lost, 2008 arable land rendered infertile 2002 Kocholwa Livestock killed 2010 Mt. Elgon, Pokot, Kessup, Subukia, Lives lost and Timobo (continuous rock falls) Houses destroyed Roads blocked Crops destroyed April 2013 Yatta Landslide 3 people died, houses destroyed Source: Overview of Landslide Occurrences in Kenya 4

14 Figure 1.1: Landslide prone areas Source: OCHA (United Nations Office for the coordination of humanitarian affairs) 5

15 1.3 Objective of the study The main objectives of this study are To investigate the relevance of using vegetation roots as reinforcement in soil in order to stabilize slopes. To determine the pull out resistance or strength of different plant roots embedded in the soil under different moisture contents of the soil. To determine the tensile strength of different vegetation roots when they have been subjected to a tensile force. To determine root cohesion properties To determine whether vegetation roots are the least cost solution to slope stabilization In order to achieve these objectives the following steps were taken: Samples of roots were taken and tested in the lab using the root pullout test done in a specially designed equipment to determine the roots pullout resistance. The Hounsfield tensometer was also required to test the root tensile strength. The root morphology and architecture was studied carefully. The soil was also tested to obtain its optimum moisture content (OMC), Maximum Dry Density (MDD) and the natural moisture content (NMC) Research was also conducted on slope stability, its causes, outcomes and mitigation measures Research on the role of vegetation in stabilization of slopes was also conducted 6

16 CHAPTER TWO 2. LITERATURE REVIEW 2.1 Introduction to slope instability A slope is a surface inclined at a certain angle. The steepness depends on the angle of elevation of the surface. Slopes are categorized into two, namely, natural slopes which exist in nature and are formed due to natural causes and manmade slopes constructed for embankments on roads, rivers and even dams. However, sometimes these slopes experience failure that results in downward movement of rock debris or soil mass and it is commonly referred to as landslide or slope instability. The downward movement of rock or soil mass occurs when the equilibrium is disturbed along a certain plane and shear stresses along it exceed the available shearing resistance. Loss of lives and destruction of property and built up environment are the aftermath of landslides and hence the need to find mitigations to minimize and avert such situations from occurring in future. Therefore, geotechnical analysis is done to establish the shear stress developed along the most likely rupture surface with the shear strength of the soil. (Das 1985) Slope stability has been there throughout history and its causes of failure ranging from natural calamities such as earthquakes and heavy rainfall to human activities such as poor construction methods on slope areas. Geologists and geotechnical engineers have carried out research and studies on slope stabilization methods, mitigations, soil and rock mechanisms and soil excavation in order to reduce the effects of landslides and come up with safe options of construction on elevated lands. Slope instability has affected many different parts of the world such as Nepal, Brazil, Philippines and Kenya is not an exception. The most affected areas are the highlands because they have steep natural slopes. These areas are inhabited by quite a number of people whose main economic activity is cultivation. This has led to deforestation in order to create room for settlement but vegetation is considered as one of the best methods of mitigating slope instability. According to the Kenya Meteorological Department, landslides contribute to 7% of the geophysical hazards in Kenya. 7

17 Figure 2.1: Major Geophysical hazards in Kenya Source: Kenya Meteorological Department 8

18 Figure 2.2: Mudslide in Murang a County, Kenya Figure 2.3: Landslide in Salvador, Brazil 9

19 2.2 Areas Prone to Landslides Highland areas and slopes in earthquake prone areas are at a risk of experiencing landslides. In Kenya, landslides and mudslides occur mostly during the rainy season and are accelerated by flooding. Usually they affect parts of the country like western, Nyanza and north Rift Valley provinces, however the most vulnerable areas have been the following: - Murang a district in central province, Kirinyaga, Nyeri, parts of Meru, which are areas around the mount Kenya region, Kisii and Mombasa Island. These are areas with annual rainfall of over 1200 mm and steep slopes. (UNDP) Figure 2.4: Landslide-prone areas in Kenya. Source: Mines and Geology Department (2012). 10

20 2.3 Types of Landslides Figure 2.5: Types of Landslides Source: U.S. Geological Survey Landslides are categorized according to the type of movement or material involved. They include: i. Slides 11

21 ii. iii. iv. Falls Topples Lateral spreads and mud flows Slides Slopes are generally translational in nature and tend to occur where the adjacent stratum is at a relatively shallow depth below the surface of the slope with the failure surface tending to be plane and roughly parallel to the slope. (Craig, 2004) A block may slide down intersecting joints or move down a steeply inclined joint or bedding plane due to lateral thrust from water filling some of the joints. (Bromhead, 1986) In weaker soils and in soils shearing can take place through the rock mass as well as joints and other discontinuities in the rock. The shearing tends to follow along curved shear surfaces and all or part of the slide may rotate. In soft uniform soils, the sliding surface may be nearly an arc of the circle in cross section, but the presence of different lithologies in a stratified deposit causes a slide to adopt a flat-soled shape. Where slopes are retrogressive the slides are often in multiple form. (Bromhead, 1986) Where water contents in the soil are raised by infiltration of surface water or concentration of overland flow, slide activity may be increased locally to mudslides. They exhibit high mobility however they should not be confused for flows because of the existence of discrete boundary shear surfaces at their base and sides. (Bromhead, 1986) Figure 2.6: Example of Rockslide and landslide Source: U.S. Geological Survey 12

22 2.3.2 Falls A fall of material, rock or soil is a characteristic of extremely steep slopes. Some shear surfaces may develop in response to gravitational stresses causing the material to be projected out. (Bromhead, 1986) Falls are confined to surface zones in soil or rock and are preceded by formation of enlargement of cracks and removal of base support of individual blocks or masses. Rock falls may be caused by frost shattering, chemical decomposition, temperature variations, the wedging effect of roots and water pressure. (Chowdhury, 1978) Figure 2.7: Example of rock fall Source: U.S. Geological Survey Flows A flow is a mass movement which involves a much greater internal deformation than a slide. It is characterized by movements taking place on a large number of discrete shear surfaces or by the water content of the moving mass being so high that it behaves like a fluid. (Bromhead, 1986) The distribution of velocities in the displacing mass resembles that of a viscous liquid. Slides may turn gradually into flows with changes in water content, mobility and evolution of movement. (Abramson, 2002) 13

23 Figure 2.8: Example of mud flows Source: U.S. Geological Survey 2.4 Causes of Landslides Main factors that cause slope failures are: Gravitational forces Force due to seepage of water Erosion of the surface of slopes due to flowing water The sudden lowering of water adjacent to a slope Forces due to earthquakes (Murthy, 2000) The primary cause of slope instability due to possible shearing is the inadequate mobilization of shear strength to meet the shear stresses induced on any impending failure plane by the loading on the slope. (Gunaratne, 2013) The effect of all the above forces is to cause movements of soil from high points to low points. The most important of such forces is the component of gravity that acts in the direction of probable motion. The various effects of flowing or seeping water are generally recognized as very important in stability problems, but often these effects have not been properly identified. It is a fact that the seepage occurring within a soil mass causes seepage forces, which have much greater effect than is commonly realized. Erosion on the surface of a slope may be the cause of the removal of a certain weight of soil, and may thus lead to an increased stability as far as mass movement is concerned. On the other hand, 14

24 erosion in the form of undercutting at the toe may increase the height of the slope, or decrease the length of the incipient failure surface, thus decreasing the stability. When there is a lowering of the ground water or of a free water surface adjacent to the slope, for example in a sudden drawdown of the water surface in a reservoir there is a decrease in the buoyancy of the soil which is in effect an increase in the weight. Increase in weight causes increase in shear stresses. (Murthy, 2002) 2.5 Shear Strength Shear may be defined as the tendency of one soil mass to slide with respect to another and occurs on all planes throughout the soil mass. The singular plane of interest however, is the plane of potential failure called the plane of rupture. Shear strength is the ability of the soil to resist occurrence of shear failure between the soil above and below the potential failure plane. All soils have the ability to develop shear strength in their own ways. (Duncan, 1998) 1. In sands and gravels, the resistance is due to the physical interlocking of soil particles and is commonly referred to as intergranular friction. Because this resistance is one of friction, its magnitude is a function of the particular details of the interlocking of particles and on the pressure of contact acting normal to the plane upon which shear is being considered. 2. In cohesive soils the shear strength is developed due to cohesion, the molecular force of attraction between particles. 3. In mixed grained soil the resistance is equal to the combined action of friction, provided by granular fraction of the soil, and cohesion, provided by the cohesive fraction of the soil. (Duncan, 1998) The shear strength of soils is assumed to originate from the strength properties of cohesion (c) and internal friction Using Coulomb s principle of friction, the shear strength of a soil, can be expressed as τ f = c + σ n tan Φ 15

25 Where σ n is the effective normal stress on the failure plane. c_ cohesion Φ _ angle of internal friction τ f _ shear stress on the failure plane In saturated soil, the total normal stress at a point is the sum of the effective stress and the pore water pressure, or σ = σ + u The effective stress, σ, is carried by the soil solids. So τ f = c + σ tan Φ (Das, 1985) Methods Of Determining Shear Strength Parameters The shear strength parameters c and Φ of soils either in the undisturbed or remolded states may be determined by any of the following methods: 1. Laboratory methods Direct or box shear test The direct shear test is rather simple to perform, but it has some inherent shortcomings. The reliability of the results may be questioned. This is due to the fact that in this test the soil is not allowed to fail along the weakest plane but is forced to fail along the plane of split of the shear box. Also, the shear stress distribution over the shear surface of the specimen is not uniform. In spite of these shortcomings, the direct shear test is the simplest and most economical for a dry or saturated sandy soil. (Das, 1985) Triaxial compression test 16

26 The Triaxial shear test is one of the most reliable methods available for determining the shear strength parameters. It is widely used for both research and conventional testing. The test is considered reliable for the following reasons: 1. It provides information on the stress strain behavior of the soil that the direct shear test does not. 2. It provides more uniform stress conditions than the direct shear test does with its stress concentration along the failure plane. 3. It provides more flexibility in terms of loading path. (Das, 1985) 2. Field method: Vane shear test or by any other indirect methods (Murthy, 2000) 2.6 Effect of rain water and excess pore pressure Soils have interconnected voids through which water can flow from points of high energy to points of low energy. The study of the flow of water through porous soil media is important in soil mechanics. It is necessary for estimating the quantity of underground seepage under various hydraulic conditions, for investigating problems involving the pumping of water for underground construction, and for making stability analyses of earth dams and earth-retaining structures that are subject to seepage forces.(das, ) Water can influence the strength of soil forming materials by Chemical and hydrothermal alteration and solution Increase in pore water pressure and subsequent decrease in shear strength Reduction of apparent cohesion due to capillary forces in saturation Softening of stiff fissures and shale (Abramson et al, 2002) 17

27 Figure 2.9: Map showing rainfall distribution in Kenya 18

28 2.7 Effect of vegetation in slope stabilization Vegetation is an important aspect of slope stabilization. Its effectiveness depends on the age of the tree, the species and type of root of the vegetation. It contains mechanisms that improve the stability of slopes each playing a different role. For example: The leaves intercept rainfall and absorb it and also causes it to evaporate thus reducing the amount of rainfall available for infiltration Roots and stems increase the roughness of the ground surface and the permeability of the soil leading to increased infiltration capacity Roots extract moisture from the soil which is lost to the atmosphere through transpiration leading to lower pore water pressure Roots reinforce the soil, increasing the soil shear strength Roots bind soil particles at the ground surface reducing their susceptibility to soil erosion (Greenway, 1987) Plant roots are the most significant mechanism. They can help stabilize slopes by anchoring a weak soil mass to fractures in bedrock, by crossing zones of weakness to more stable soil, and by providing long fibrous binders within a weak soil mass. In deep soil, anchoring to bedrock becomes negligible and the other two conditions predominate. The reinforcement effect of plant roots intermixed with soil resembles soil cohesion (Endo and Tsuruta, 1969). In heavy rainfall, forests have a high interception rate and reduce the amount of rain water reaching the ground, hence being a better alternative to any other type of vegetation. They also increase secondary permeability in the soil by forming preferential drainage paths and networks through the soil and substrate. Soils, on the other hand, are strong in compression and weak in tension. A combined effect of soil and roots, producing a composite material in which the roots are fibers of relatively high tensile strength and adhesion embedded in a matrix of lower tensile strength soil mass, resulting in a reinforced soil. Therefore, it is the tensile of the roots which contribute to the overall strength of the soil-root composite. (Faisal Ali, 2010) Vegetation with deep roots is considered to be the best in soil stabilization and anchorage compared to shallow roots. A good example of such a root system is the taproot. Shear strength increases when the roots are able to grow beyond the potential failure plane. 19

29 To prove the strength of roots in soil reinforcement, a root pullout test and root tensile strength is done. In root pull out test a tensile force is applied at the end of the root. Failure may occur either by tension in the main root or progressive tension failure in the branch roots or by slippage between root and soil. The controlling failure mode depends on the root geometry and the tensile strength of the root relative to the shear strength of the soil. (Tien Wu, 2007) For the root tensile strength, the root is clamped and then pulled to create tensile stress until failure occurs Root morphology and structure Root morphology is the study of the structure of roots. Different plants have different types of roots making certain types of vegetation better suited for slope stabilization and soil erosion control than others. Traits such as root distribution, length, orientation and diameter are considered when determining the roots better suited for stabilization and soil erosion control. Figure 2.10: Root morphology Root distribution can be extensive but many factors including the type of soil, tree species, age, health, environmental stresses, planting density and silvicultural management all impact upon the final root structure. Woody vegetation especially trees, help in preventing shallow landslides by modifying soil moisture through evapotranspiration and providing root reinforcement. 20

30 2.7.2 Root Types Roots are of different types and are adapted differently to suit certain needs hence each type of root has unique characteristics that distinguish it from other types. These types are 1. Fibrous roots They have very many fine hair-like roots and are concentrated near the soil surface. They are good in controlling soil erosion as the system is effective in water and mineral absorption. 2. Taproot system This system has a large vertical root with many smaller horizontal root structures. The root penetrates deep into the soil providing anchorage to the plant and to the soil thus stabilizing the soil 21

31 3. Adventitious root system These are roots that develop from stems, branches, leaves, or old woody roots. They are commonly found in grasses and other monocots with shallow roots. 4. Contractile Roots These are roots that contract to pull the shoot, corm, or bulb down deeper into the soil. Roots extend through the soil and become firmly anchored. The uppermost parts begin to contract and the stem, etc. is pulled downward so it buries deeper. This is caused by changes in the shape of cortical cells as they expand radially and shorten, and the vascular tissue buckles but does not lose its function. They have a wrinkled surface. 22

32 ` 5. Aerial Roots These are adventitious roots produced from above ground structures (a good example is the ivy Hedera), the roots cling to the surface of objects, trees or walls to support the climbing stem. There are many specialized types of aerial roots such as Prop Roots, which are roots that serve for support, as in corn. They can branch down from lower nodes of the stem or drop down from branches as in some trees. They can enter the soil and absorb water and nutrients. Examples: Mangroves, Banyan tree, palms. There are also Pneumatophores. These are roots that grow upward from submerged roots in mud/water; found in trees that grow in swamps, such as mangrove Root Depth The depth at which plants are able to grow roots has important implications for the whole ecosystem hydrological balance. Some plant species have roots that grow to great depths of up to 68m, however it has been assumed that a good understanding of the role of roots system regarding structure and function can be achieved by studying only the first 0.5m of the soil (Canadell et al, 1996). Rooting depth may be physically restricted by rock or compacted soil close below the surface, or by anaerobic soil conditions (Phillips, 1963). For slope stabilization the main focus is on the top 1 or 2 meters of soil because that is where failure occurs. 23

33 2.7.4 Root Area Ratio Root area ratio is the ratio between the area occupied by roots in a unit area of soil. Vegetation increases stability of slopes by reinforcing the soil and influencing hydrological conditions. Root strength in tension normally tends to decrease with diameter Root as Reinforcement The effect of root reinforcement depends on the morphological characteristics of the root system, the tensile strength of individual roots, the soil-root cohesive strength, and the distribution of the root system in the soil. (Osano and Mwea, 2011) In mountain regions, trees are usually affected by wind loadings and self loadings. Self loading is the mechanical stimulus due to the weight of a plant growing on a slope. To achieve anchorage, the tree transfers the loading forces experienced by the stem into the ground via roots (Chiatante et al., 2003a). Forces that trees and soil have to resist to maintain stability are mainly bending stress (within roots and stem), tension (within roots), compression (within and between roots and soil), shearing forces (between root and soil and within soil), gravity, (which acts in the direction of the probable motion) and force of seeping water. These forces produce shear stresses throughout the soil mass and a movement will occur unless the shearing resistance on every possible failure surface throughout the mass is sufficiently larger than the shearing stress. (Faisal Ali, 2010) Hydrological effects involve the removal of soil water by evapo-transpiration through vegetation, which can lead to an increase in soil suction or reduction in pore water pressure, hence an increase in the shear strength. Apart from increasing the strength of soil by reducing its moisture content, evaporation by plant reduces the weight of the soil mass. (Farshchi et al, 2012) The mass of vegetation is only likely to have an influence on slope stability when larger trees are growing on the slope. A tree of 30-50m height is likely to have a loading of approximately kN/m 2. The larger trees should be planted at the toe of the slope with a potential rotational failure as this could increase the factor of safety by 10%. However if the tree is planted at the top of the slope this could reduce the factor of safety by 10%. (Dr S. Osano, 2012) Roots also have the ability to grow beyond the potential failure plane and also beyond the bedrock thus increasing the root anchorage and the soil shear strength. The tensile force found in 24

34 roots is then transferred to the soil through cohesion thus providing a greater resistance to pull out Root failure mechanism The root length and the type of root branching affects the way root failure occurs (Greenwood et al. 2004; Norris 2005). The roots generally fail in tension when the aiding forces are greater than the resisting forces causing the root to slip out of the soil mass. Once the pull out occurs there is no more soil-root interaction through adhesion hence there is no longer any increase in the strength of the soil. Some reach maximum pull out resistance then rapidly fail at a weak point, others reach their maximum peak resistance then sustain a high resistance that slowly reduces as the branches of the roots fail after significant strain and others break with increasingly applied force in stages corresponding to the progressive breaking of roots of greater diameters. (Dr. S. Osano) 2.8 Alternative soil reinforcement materials: Geosynthetics Geosynthetics are man-made materials used to improve soil conditions. They are made from petrochemical-based polymers (plastics) that are biologically inert and will not decompose from bacterial or fungal action. Most of them are chemical inert but some may be damaged by petrochemicals and ultraviolet light. Their main functions are: separation/confinement/distribute loads - improve level-grade soil situations such as roads, alleys, lane ways - improve sloped-grade situations such as banks, hillsides, stream access points reinforce soil- soil walls, bridge abutments, box culverts/bridges, and soil arches prevent soil movement (piping) while letting water move through the material - such as in drainage systems and back fill around water intakes controlling water pressure allowing flow (drainage) in the plane of the material - such as on foundation walls to allow water to move down to perimeter drains. Common types of geosynthetic materials used are 1. Geogrids These are open grid-like materials of integrally connected polymers and they are used primarily for soil reinforcement such as for sub grade stabilization, slope reinforcement, 25

35 erosion control, mechanically stabilized earth retaining walls, and to strengthen the junction between the top of soft clays and overlying embankments. Geogrids are strong in tension but weak in compression. However, they degrade when exposed to ultra violet rays for a long period and can also be susceptible to creep. 2. Geotextiles Geotextiles are permeable textile or fabric used with foundation soil, rock, earth, or any other geotechnical engineering-related material. It is used for soil reinforcement, asphalt laying, sediment control, erosion control, filtration and drainage. They are also affected by ultra violet rays and are at risk of damage during compaction of soil. 3. Geomembrane Geomembranes are polymer sheets used to control fluid movement. These materials have very low permeability and would be used for lining ponds and pits to control leachate. They may be used over top of geotextiles. 26

36 CHAPTER THREE 3. METHODOLOGY 3.1 Introduction In order to determine the strength and resistance properties of the plant roots and soil and their interaction, a laboratory test was done. This chapter shows the test procedures undertaken to determine these characteristics and properties of soil. For this tests samples of plant roots commonly found in Murang a County were used. Three specimens were acquired for use in this experiment. The experiment was conducted in the University of Nairobi Geotechnical Lab. 3.2 Description of study area For this project analysis the area of study was the slopes in Central part of Kenya as it has had several cases of landslides. Figure 3.1: Map of Central Kenya 27

37 3.3 Sampling Roots were sampled for conducting pull out tests and tensile strength test in the laboratory. Three root specimens were identified for testing. Selection was random, and roots species having root penetration into soil of more than 0.2m but less than a metre were excavated. This is because the maximum depths of shallow landslides in which the sliding surface is located within the soil mantle or weathered bedrock which typically ranges depth from few decimetres to 1 meter. The species were Lantana camara (Wild Sage) locally known as mukigi (Kikuyu), Triumfetta tomentosa (Burrbark) and Tagetes lemmonii (Marigold). Figure 3.2: Examples of Triumfetta tomentosa, Tagetes lemmonii and Lantana camara respectively The samples were identified and collected a day prior to the experiment and preserved in a plastic bag and kept in a cold room for preservation of moisture content in the University s laboratory. A random soil sample was used to conduct the experiment. 28

38 3.4 Laboratory apparatus and testing procedure Root tensile test For the root tensile strength test, the Hounsfield Tensometer apparatus was used. Below is a picture of the apparatus. Figure 3.3: Hounsfield Tensometer This is a universal machine and is motor driven to achieve a constant rate of extension for the large extensions expected of plant roots. The samples to be tested were placed between two wedge grips that were easy to use and provided the sufficient grip to resist slippage. Once the roots had been clamped, initial tension was applied manually then the mercury scale was set to zero before the apparatus was turned on. The force and extensions were recorded at failure. T = F D 4 max 2 Where; 29

39 Fmax is the maximum force (N) needed to break the root, and D is the mean root diameter (mm) before stretching Root Pull-out Test Figure 3.4: Root pull out apparatus The above apparatus was specifically designed for the pull out test. The main features of the apparatus are: 1. A box measuring 1m x 1m x 1m filled with soil material from the site under investigation, where roots are embedded for pull-out. 2. A pulley mechanism with loading cap to apply a horizontal pull-out force on the root. The wire-rope to be inelastic and to accommodate large pull-out forces of up to 100N, which is the estimated maximum pull-out force for small-rooted vegetation. 3. A steel table where the box is fixed high enough to allow the pulley movement to take place during the pull-out displacement 30

40 4. Loading weights measuring 1Kg each The random soil sample was remolded in the box and the root sample placed halfway before filling the box with the required weight. The root was then gripped between 2 plates and then mounted on a pulley. Weights were then added at intervals of 10kg and displacement was recorded for every added weight until root sudden pull-out failure took place. The force just before failure was recorded for each root system. The test was then repeated with different root types. In a different set-up, moisture contents of soil samples were varied, and the maximum pull-out resistance was determined. 31

41 PULL-OUT RESISTANCE (KN) CHAPTER FOUR 4. RESULTS AND ANALYSIS 4.1 Pull-out test a) Wild Sage (Lantana camara) shrub The maximum stem diameter ranged from 15mm-18mm. At the early stages of the test, the pull out resistance increased drastically with little displacement before reaching a maximum of 0.7, 0.6 and 0.5 KN at 10%, 20% and 40% moisture contents respectively. Then the gradient decreases gradually and pull out resistance begins to drop as the displacement increases until finally the snapping sound of the root is heard when the root is pulled out of the soil. The roots failed in tension as they behave as flexible fibers resisting forces only on tension. This is shown in the graph below Lantana camara (Wild Sage) DISPLACEMENT (mm) at 10% MC at 20% MC at 40% MC Figure 4.1: Pull out resistance against displacement for Lantana camara (Wild Sage) b) Triumfetta tomentosa (Burrbark) The stem diameters of the samples ranged from mm. Just as in the previous one, the pull out resistance reached maximum values of 1, 0.9 and 0.8 KN at 10%, 20% and 40% moisture 32

42 PULL-OUT RESISTANCE (KN) contents respectively before decreasing gradually as the displacement continued increasing. Finally the snapping sound was heard just before the root was completely pulled out of the soil. Triumfetta tomentosa (Burrbark) DISPLACEMENT (mm) at 10% MC at 20% MC at 40% MC Figure 4.2: Pull out resistance against displacement for Triumfetta tomentosa (Burrbark) c) Tagetes lemmonii (Marigold) The stem diameter of the samples ranged between 6.5 and 9 mm. Its pull out resistance began rising gradually and reached maximum values of 0.7, 0.6 and 0.4 KN at 10%, 20% and 40% moisture contents respectively beyond which it began dropping gradually until failure occurred. Just before failure the snapping sound of the root was heard as the root was being pulled out of the soil. 33

43 PULL-OUT RESISTANCE (KN) Tagetes lemmonii (Marigold) at 10%MC at 20% MC at 40% MC DISPLACEMENT (mm) Figure 4.3: Pull out resistance against displacement for Tagetes lemmonii (Marigold) DISCUSSION From these results it can be seen that these shrubs indeed do provide resistance to pull out forces with some providing more resistance than others. Triumfetta tomentosa (Burrbark) shrub provides the highest resistance followed by Lantana camara (wild sage) then Tagetes lemmonii (marigold). They all have a similar trend with a single maximum value of pull out resistance at all moisture content values. The maximum pull out strength is mainly acquired from their lateral roots. Burrbark and Wild sage have larger diameters compared to the Marigold shrub and it can be observed that they offer better resistance as well. This indicates that larger diameters provide more resistance than smaller ones. Moisture content also affects the resistance of the roots to pull out. From the graphs above it can be observed that the general trend is that the peak values of the pull out resistance decrease with an increase in moisture content and the curve is less steep. Thus the resistance reduces with an increase in moisture content. At around 50% moisture content the roots resistance is completely compromised. 34

44 PULL OUT RESISTANCE (KN) PULL OUT RESISTANCE AGAINST MOISTURE CONTENT wild sage-(16mm diameter) burrbark- (19mm diameter) marigold- (9mm diameter) MOISTURE CONTENT ADDED (%) Figure 4.4: Pull out resistance against moisture content for all the species From compaction test of the soil sample the following information was obtained to determine the amount of rainfall that would result in the percentage of moisture content in the soil. 35

45 MDD (kg/m 3 ) y = x x x R² = 1 Moisture Content (%) Optimum Moisture Content (%) 30.2 Maximum Dry Density (kg/m 3 ) 1370 Volume of soil mass = 1 x 1 x 1 = 1m 3 Weight of soil = MDD x (1+OMC) x volume x 95% = 1370 x ( ) x 1 x 0.95 = 1694kg Bulk density = 1694kg / 1m 3 Take bulk density as 1700kg/m 3 36

46 root failure force(n) Table 2: Moisture content in terms of rainfall (mm) % of moisture content Volumetric soil water (cm 3 ) Rainfall (mm) Volumetric soil water = % MC x bulk density From 50% MC that is at 850mm inhabitants of the slope should relocate as slope failure is imminent. 4.2 Root Tensile Test The root tensile strength is compared with the root diameter in the graph below Chart Title root diameter (mm) wild sage Burrbark Marigold Figure 4.5: Root tensile force against root diameter for all species From the graph the general trend is that the root tensile force increases with an increase in the diameter. This means that large diameters provide greater tensile resistance. Root elongation, slippage and breakage are the common failure mechanisms when the root is subjected to tensile 37

47 tensile strength (N/mm 2 ) stress. The root failure is abrupt. The shrub with the highest tensile force is the wild sage (136N) followed by burrbark (119N) and finally marigold (37N). These results imply that the wild sage and burrbark species have prominent root mechanical properties and can be outstanding slope reinforcement plants compared to the marigold species. The tensile strength-diameter relationship follows a power law in the form of f ( x) b ax Where a and b are regression coefficients. From the graph below it is evident that the root tensile strength decreases with an increase in the diameter. Root moisture content and root length do not affect the tensile strength. Generally the burrbark species has the highest tensile strength followed by wild sage and finally the marigold species. Tensile strength against root diameter y = 32.43x R² = y = x R² = y = x R² = root diameter (mm) wild sage burrbark marigold Power (wild sage) Power (burrbark) Power (marigold) Figure 4.6: Root tensile strength against root diameter following the power law 38

48 Depth (m) 4.3 Root Area Ratio The obtained results generally show that the root area ratio decreases with the increase in depth for all plant species however they show great variability with depth. It can be noted that the highest values of the root area ratio are found within a depth of 0.15m. Root Area Ratio Against Depth 0 RAR wild sage burrbark marigold Figure 4.7: Root area ratio against depth for all 3 species 4.4 Root cohesion The root reinforcement decreases with depth. The maximum reinforcement is from the wild sage and burrbark species at 125Kpa while the marigold species has a maximum of 70Kpa, all at depths of 0.2m. These roots are not sufficient to reinforce soils of depths from 1m. 39

49 Depth (m) Root cohesion against depth 0 Cr (Kpa) wild sage burrbark marigold Figure 4.8: Root cohesion against root depth for all species 40

50 CHAPTER FIVE 5. CONCLUSION AND RECOMMENDATIONS 5.1 Conclusion The role of vegetation roots in soil and slope reinforcement is now hard to ignore. Its significance is slowly being recognized and the tests conducted as described in the previous chapters, including other tests, are being carried out on various species. So far, fibrous roots with an extensive lateral system are considered to be the better option to achieve stabilization Pull out test From the pullout test, the overall results show that the Lantana camara (Wild Sage) and the Triumfetta tomentosa (Burrbark) species show greater reinforcing properties compared to the Tagetes lemmonii (Marigold) species. All these shrubs however show a similar trend in that they have a single peak value for the pull out resistance force. Their pullout resistance is evidence of soil and slope reinforcement. However this force reduces with an increase in moisture content. At around 50% moisture content (850mm rainfall) the roots resistance is completely compromised. Also bigger plants can resist pull-out force better than the smaller plants. This is evident as marigold plant is the smallest and gives the least resistance. The fabricated equipment served its purpose and pull out test was conducted successfully Tensile strength and root distribution The root tensile strength was found to decrease with increasing root diameter in accordance with the power law however larger diameters require more force for failure. The tensile strength of Triumfetta tomentosa was significantly higher than that of Lantana camara and Tagetes lemmonii. Root moisture content does not influence the tensile strength. Root area ratio was also obtained to determine the distribution of roots of particular species in the soil. The maximum values were found to range within a depth of 0.2m implying that these roots are only suitable for reinforcing slopes that experience shallow landslides of less than 1m. Root cohesion or reinforcement effect was found to decrease with increasing soil depth. maximum reinforcement is from the wild sage and burrbark species at 125Kpa while the marigold species has a maximum of 70Kpa, all at depths of 0.2m. 41

51 Final remarks All these shrubs take a short time to fully mature and therefore suitable for reinforcement. They also have other benefits however the Lantana camara is considered to be invasive. From the experiments conducted, it can be concluded that Triumfetta tomentosa (Burrbark) has the most superior properties of all the species tested. Though there are other methods used for slope stabilization such as use of geomembranes, planting vegetation is cheaper and more economical and it enhances the aesthetic value of the area in addition to its reinforcing properties. Vegetation is readily available and can be easily implemented even by the locals. All objectives of this study were achieved, however, further studies and experiments are required on different species as well before the most appropriate species is chosen for stabilization of slopes. 5.2 Recommendations Based on the above findings, the following recommendations may be considered: a. Vegetation that has roots deeper than 1m are most suitable as shallow landslides sometimes occur up to 2m deep. Shrub species are most preferred due to their superior properties in tensile strength, pull out strength and root cohesion with the soil. b. Vegetation that are likely to add surcharge to the ground should be avoided as their weight contributes to the aiding forces leading to slope failure. These include tall trees and trees with very large girth. c. In order to avoid over reinforcing or under reinforcing the soil, a distance of about 1m apart between species is sufficient. Over reinforced soil reduces cohesion between plant roots and soil while under reinforced soil still puts the slope at a risk of failure. d. Cut off drains should also be established to facilitate drainage of excess run off to keep soils from being saturated. They are also suitable for temporary measures before the plants are fully matured. As seen from results, moisture content contributes to failure even with root reinforcement. e. Further research should be carried to determine other suitable species other than those carried out in this study. This also includes carrying out other tests that may require enhanced equipment and technology to determine other root properties. 42

52 CHAPTER SIX 6. REFERENCES 1. Braja M. Das, 1985, Fundamentals of Geotechnical Engineering 3 rd Edition 2. Bromhead, E. N., 1992, The Stability of Slopes, St. Edmundsbury press, Bury St. Edmunds, Suffolk. 3. Bromhead, E.N., 1986, The Stability of Slopes 2 nd Edition, St. Edmundsbury press, Bury St. Edmunds, Suffolk. 4. Capper, P.L Cassie W.F. (1976), The Mechanics of Engineering Soils, John Wiley and Sons Inc., Newyork. 5. Chowdhury, R. N., 1978, Slope Analysis, Elsevier Scientific Publishing Company. 6. Coppin, N. J. and Richards, I. G. (1990) Use of vegetation in civil engineering. Butterworth, London 7. Craig R. F Soil Mechanics Van Nostrand Reinhold (UK) Co. Ltd 8. Faisal Ali, May 2010, Use of vegetation for slope protection: Root mechanical properties of some tropical plants, International Journal of Physical Sciences Vol. 5(5), pp ,, IJPS,ISSN Academic Journals 9. Greenwood, J.; Norris, J. & Wint, J. 2004, Assessing the contribution of vegetation to slope stability, Proceedings of the Institution of Civil Engineers, vol. 157, no. 4, pp J. Michael Duncan, Stephen G. Wright, Thomas L. Brandon, 1998, Soil Strength and Slope Stability, 2 nd Edition 11. Lee W. Abramson, Thomas S. Lee, Sunil Sharma, Glenn M. Boyce, 2002, Slope Stability and Stabilization Methods, 2 nd Edition, John Wiley and Sons Inc., Newyork. 12. Manjriker Gunaratne, 2013, The Foundation Engineering Handbook, Second Edition (2nd Edition) 13. Ngecu, W.M. & Mathu, E.M., 1999, The El-Nino triggered landslides and their socialeconomic impact on Kenya, Environmental Geology 38 (4). 14. Norris, J. 2005, Root reinforcement by hawthorn and oak roots on a highway cut-slope in Southern England, Plant and Soil, vol. 278, no. 1, pp

53 15. Ocha Kenya, 2009, Kenya Humanitarian Update, United Nation Office for the Coordination of Humanitarian Affairs, Volume Osano SN & Mwea SK, 2008 The Effects of Vegetation Roots on Stability of Slopes, Conference Proceedings of the 2 nd International Civil Engineering Conference on Civil Engineering and Sustainable Development, Page Osano SN & Mwea SK, 2011 Root tensile strength of 3 typical plant species and their contribution to soil shear strength: a case study: Sasumua Backslope, Nyandarua District, Kenya, Journal of Civil Engineering and Practice, Vol. 8 No. 1, April 2011, pp V. N. S. Murthy, 2002,Geotechnical Engineering Series, Advanced Foundation Engineering. 19. Wu T.H., McKinnel W.P. & Swanston D.N. (1979), Strength of tree roots and landslides on Prince of Wales Island, Alaska, Canadian Geotechnical Journal 16: Wu, T. H., (2007). Reliability Analysis of Slopes, In K. K. Phoon (ed.). Reliability based 21. Wu, T.H., McOmber, R. M., Erb, R. T. and Beal, B. E. (1988) A study of soil root interaction. Journal of Geotechnical Engineering, ASCE, 114(12): Internet references strength and root area ratio of forest species in Lombardy (Northern Italy) Springer)

54 CHAPTER SEVEN 1. APPENDIX Figure 1.1: Triumfetta tomentosa (Burrbark) 45

55 Figure 1.2: Lantana camara (Wild Sage) 46

56 Figure 1.3: Tagetes lemmonii (Marigold) 47

57 Figure 1.4: Pull out test at the University of Nairobi Lab 48

58 Figure 1.5: Laboratory tests 49