The Role of in Slope Stability

Hamed Niroumand1, Khairul Anuar Kassim1, Amin Ghafooripour2, Ramli Nazir1 1 Department of , Faculty of , Universiti

Teknologi Malaysia, E-mail: [email protected] 2 Department of Structural Engineering & Vibrations, School of the Built Environment, Heriot – Watt University, Dubai, UAE

ABSTRACT Geosynthetics are fibrous materials made of elements such as individuals fibers, filaments, yarns, tapes, etc. that are long, small in cross section and strong in tension. It must be sufficiently durable to last a reasonable length of time in the hostile environment. Use of in civil engineering structures are rapidly expanding in terms of volume, types of products and range of applications. The largest area of application of these materials in Civil Engineering is Geotechnical Engineering. Based on a few laboratory work and numerical analysis, few investigators reported geosynthetics in slope reinforcement, a review of related last works shows that not much research has been done to define performance of geosynthetics in slopes, a problem that is often encountered in field. The paper observed the performance of geosynthetics in slope reinforcement. KEYWORDS: Geosynthetics, Slope, Geotextile, Reinforcement

INTRODUCTION Geotextile are fibrous materials, which made of elements such as individual fibers, filaments, yarns, tapes, etc. that are long, small in cross section and strong in tension. One of important characteristics of geotextile is flexibility. Flexibility is useful both for good contact conditions and for avoiding stress concentration in the fibers. Besides, hydraulic functions of geotextile due to its fibrous nature allows geotextile to have a high (high permeability) and at a same time, a small filtration diameter. The tensile strength of the geotextile is also important. From scientific research, it appears that to obtain the highest tensile resistance from a material, the best way is to use it in the form of fibers, which have a high degree of molecular orientation. Therefore, basically the concept of geotextile is strongly related to fibers. The importance of the fiber concept is the strong reason for using the word “geotextile”, because the word “” implies the concept of fiber.

History of Geotextile Development of the geotextile revolution will be discussed in this chapter. Forms of geotextile have existed for almost thousands of years. The first application of soil reinforcement or ground improvement techniques was adopted by Babylonians to construct Ziggurats more than three thousand years ago. One famous Ziggurat, Tower of Babel, collapsed perhaps because it was not reinforced. The Tower of Babel was constructed by foreign laborers. According to the writer of the Bible, it was all too easy to blame the failure on them since they could not defend themselves because of language barrier.

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Probably, the writer may have vested interest in finding a scapegoat since that monk writer and the consulting engineer were one and the same trade. (as some modern consulting engineers would agree.) The Chinese have used , bamboo and straw to strengthen soil for thousands of years. The importance of soil reinforcement in ancient China is demonstrated by the fact that the Chinese symbol for Civil Engineering simply means “earth and wood”. At that time, portions of the Great Wall of China was constructed using the soil reinforcement concept. The concept of soil reinforcement method was brought to Japan and the use of natural materials for stabilization purposes continued to this decade. The Romans used reed, wood and animal hides for soil reinforcement during the Middle Ages. The Dutch, below of the low land and in their age old battle with the sea, made extensive use of willow fascines to reinforce dikes and to protect themselves against wave action. of to shorten the coastline was carried out and this action is still on going till this century, culminating with Delta Project. In 1926, the South Carolina Department of Highways used special types of vehicles to lay down the rolls of fabrics in the construction of . It was only during last two decades that these materials made of synthetic polymers have been increasingly adopted ranging widely from construction of roads over poor to reinforcement of slope for stabilization.

Classification of Slopes Hill site development is often related to , and safety of building at the hill site is often a topic of discussion among government officers in local authorities, engineers and public. This matter has become increasingly serious. With the recent awareness of risks involved in hill site development, a more proper and systematic control and precaution is taking shape through the private and public sectors. According to the Institute Engineering of Malaysia (IEM), slope for hill site development can be classified into 3 classes and the necessary requirements and characteristics are as follows:

(a) Class 1 (Low Risk) Application of existing Legislation Procedures can still go on.

(b) Class 2 (Medium risk) It is mandatory for professional engineer to submit geotechnical report to the relevant local authority. The professional engineer must posses relevant expertise and experience in analysis, design and supervision of construction of slopes, retaining structures and on hill site.

(c) Class 3 (High Risk) Besides submission of geotechnical report, the developer shall engage an “Accredited Checker” (AC) in the consulting team. With reference to the original proposal by the workforce, AC shall have at least 10 years working experience at hill site and have published at least five technical papers on geotechnical works in local or international conferences, seminars or journals.

The general risk of classification is actually based on the geometry of the slopes, for instance the height and angle. There are other factors that contribute to the stability of slopes, for instance geological features, engineering properties of soil/rock, groundwater level, etc. However, for the simplicity of implementation by non-technical personnel in our local authorities, simple geometry has been selected as the basis of risk classification.

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Figure 1: Geometry of Slope (after IEM, 2000)

Table 1: Classification of Risk of Landslide on Hill-Site Development (after IEM, 2000)

L= Hkgp

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Comparisons between Biodegradable and Non-biodegradable Geotextile For the past few years, geotextile has played a major and significant role in geo-environmental engineering applications. Woven and non-woven geotextile are widely used in applications such as , turf reinforcement, control, separation, filtration and . Geotextile can be classified into two types, biodegradable and non-biodegradable geotextile. Biodegradable geotextile are made of natural fibers, for example penduculata fiber, or Raffia Vinifera, was obtained by drying raffia palm fronds in the sun and then beating the raffia fronds with a piece of wood to create the fibre. Non-biodegradable geotextile are made of synthetic materials, for example and polypropylene. Because of the advances in technology, non-biodegradable geotextile are preferred compared to biodegradable geotextile. The use of naturally occurring fibre products for similar applications has not received significant consideration despite their potential. Only limited amount of scientific literature research has been published with regard to the use of biodegradable geotextile as a practical solution to geo-environmental engineering problems. Experiment was conducted to compare the effectiveness of biodegradable (penduculata) and non-biodegradable (polypropylene) geotextile in geo-environmental engineering problems. The experiment consists of a rainfall simulation apparatus, used on slopes (protected or unprotected) that were inclined at different angles to the horizontal. From the experiment, penduculata geotextile shows high water absorbency characteristics which can influence the initial run-off velocity values at the beginning of a rainfall event. On the other hand, the polypropylene geotextile shows zero water absorbency characteristic. Because of this, lower run-off velocities were measured for natural fiber geotextile which is likely due to the higher water absorbency values. However, in terms of better slope protection, the polypropylene geotextile was more effective (lower cover factor values) compared to penduculata geotextile although the run-off velocity measured for polyprolene geotextile at slope was high. The performance difference may be attributed to differences in Percentage Open Area (POA) values between polyprolene and penduculata geotextile. Despite of this, natural fiber geotextile has potential and has a role in geotechnical engineering. The potential use was shown in the Manchester, United Kingdom, airport rail link construction project (Ellis 1993) where a naturally occurring biodegradable erosion and soil stabilization mat was successfully installed. Basic Concept and Function of Geotextile

A geotextile can perform several functions. The need for identifying and describing geotextile functions appeared when geotextile began to be used in a variety of applications. Before design can take its place, it is very important to identify the functions required of the geotextile in the considered application. A geotextile function is a specialized action of the geotextile which is required to achieve a design purpose and results from a unique combination of geotextile properties. Generally, has six main functions: a) The Drainage Function or Fluid Transmission. The geotextile is placed in contact with a material of low permeability through which water is seeping slowly, its fuction is to gather water and conveys it, within its own plane towards an outlet. In order to function as a drain, a geotextile must exhibit transmissivity. The flow of water into the plane of a geotextile is governed by Darcy’s formula:

where Q = rate of flow (m3/s) L = length of the cross section of geotextile perpendicular to the flow direction (m) kp = coefficient of permeability of the geotextile in its plane (m/s) Hg = thickness of the geotextile (m)

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Δh = hydraulic head loss (m) L’ = length of geotextile parallel to the flow direction (m) b.) Filtration. A geotextile acts as a filter when it allows liquid to pass normal to its own plane, while preventing most soil particles from being carried away by the liquid current. There are three cases to be considered: filter for particles suspended in a liquid, filter for removing water from a granular soil and filter associated with armor. Filter for particles suspended in a liquid: the geotextile is placed across a flow of liquid carrying fines particles in suspension; the function of the geotextile is to stop the fine particles while allowing water to go through it. Filter for removing water from granular soil: the geotextile is placed between the soil, from which water is removed (by drainage or pumping), and the open material (aggregate, perforated pipe, porous mat) the function of which is to collect and convey the water; the function of the geotextile is to prevent movement of soil particles while allowing the water to go thorugh it. Filter associated with armor: the geotextile is placed between the soil which has to be protected from the wave action and the coarse material which constitutes the armor; the function of the geotextile is to minimize movement and loss of soil particles while allowing the water to go through it. The difference between the case of water removal and the case of armor is related to the flow: in the case of water removal, the flow of water is in one direction and partially steady; in the case of armor exposed to waves, the direction of flow alternates and the flow is unsteady and dynamic. c.) Separation. A geotextile is placed between two materials which have a tendency to mix when they are squeezed together under the applied loads; the function of the geotextile is to separate these materials. A separator must retain the soil particles and must have sufficient strength to withstand the stresses induced by the applied loads. Consequently, designing a geotextile separator involves retention analysis and strength analysis. d.) Protection. A geotextile protects a material when it alleviates or distributes stresses and strains transmitted to the protected material. There are two cases to be considered: surface protection and interface protection. Surface Protection: A geotextile, placed on the soil prevents its surface from being damaged by such actions as weather, light traffic, etc. Interface Protection: A geotextile, placed between two materials, prevents one of the materials from being damaged by concentrated stresses applied by the other materials. e.) Tension Membrane. A geotextile function as a tensioned membrane when it is placed between two materials having different pressures, and its tension balances the pressure difference between the two materials, thus strengthening the structure. f.) Tensile member. A geotextile acts as a tensile member when it provides tensile modulus and strength to a soil with which it is interacting through interface , for instance the interlocking, , and adhesion.

Geotextile as Slope Protection in the residual or weathered rocks in Malaysia are generally induced. These slopes when dry or partially saturated, they are normally stable at inclinations exceeding the effective angle of internal friction, Φ of the soil. When the soil is partially saturated, the negative pore water pressures impart to the soil as an which is higher than the corresponding total stress. The shear strength of the soil is thereby increased, enabling the slopes to remain in stable condition even though when the inclination exceeds the effective friction angle, Φ of the soil. After heavy rainfall, the soil will become saturated because of the of the rainwater into the ground. The

Vol. 17 [2012], Bund. R 2744 original negative existed in the soil are therefore eliminated or drastically reduced, causing a large reduction in the effective stress and the shear strength. The slope will become unstable and eventually fail. Geotextile has been used successfully in numerous occasions to stabilized steep slope in residual soil and weathered rock. Geotextile was used as tensile reinforcement and filter to stabilized slopes or embankments. The geotextile are usually placed in horizontal layers within the slope. It is placed along the slope cutting across potential sliding surfaces in the soil. The geotextile will reduce the pore water pressure within the slopes during the rainy season, thereby increased the shear strength. The geotextile also acts as a filter which prevents the migration of soil or sometimes called the internal erosion within the slope. Last but not least, the geotextile reinforces the soil along potential sliding zones or planes. All these will increase the stability of the slope.

Factors Attributing Towards Selection of Geotextile There are many factors attributing towards the selection of geotextile in geotechnical engineering. The first fundamental reason is that there is need for membrane-like materials because geotechnical structures are built with granular materials; the integrity of layers of granular soils can be disrupted by erosion, settlements and while a geotextile layer remains continuous. Besides, geotextile are bi-dimensional and flexible materials and is -suited to geotechnical structures subjected to different movements. Geotextile are also useful, either as interface between layers or as a liner or a protection at the surface of the mass geotechnical structures. In addition to the factors mentioned above, geotextile have been successful because manufacturers have aggressively developed and marketed them and because contractors, designers and owners have elected to use them. Reasons attributing to the selection of geotextile application in geotechnical engineering by contractors, designers and owners are discussed below. a) Contractors: Contractors have adopted geotextile very rapidly because it brings instant benefits to them. For example, easier installation of geotextile compared to granular fill will reduced construction time. Using geotextile in construction is recommended because geotextile are less weather dependent and truck are less likely to get bogged down when a geotextile is used. Geotextile can reduce the amount of earthwork as geotextile drains and filters are less bulky than their granular counterparts. The cost of earthwork is reduced if geotextile reinforcement permits the usage of lower quality fill materials, which are less expensive. Besides earthwork, transportation costs can be reduced by replacing granular fills with geotextile. It will do the environment better than harm since the noise and dust associated with transportation of construction materials are reduced. b) Designers: With the emphasis now placed on “value engineering”, designers are required to produce less expensive design to remain competitive. Designer find that geotextile may increase the reliability of a structure because the quality control of their placement is relatively easy, their installation is not weather dependant, their properties are more uniform than soil particles and they mitigate soil defects by bridging weak spots and separate layers which tends to mix. Geotextile open new possibilities for innovative design instead of using the same, old and dull design. Especially in coastal protection application, geotextile present solutions to problems which designers have long been struggling, for instance, filters wash away and difficult to construct under water while geotextile are secure and easy to place. c) Owners: Owners also plays a major role to the success of geotextile because they dare to use them in the early days. Motivations of owners are a combination of contractors and designers motivations. Owners and contractors are most interested in low cost and designers are interested in stability, reliability and sometimes experimentation. For owners, by adopting geotextile, maintenance work can be reduced which in turn save cost.

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Long Term Resistance of Geotextile Geotextile have been widely used in geotechnical engineering for several decades. Along with polymers such as (PET) or (PE), polypropylene (PP) is the polymer most commonly used for these applications. When engineers began to use these materials, the first investigations on the long term performance for instance, UV resistance, chemical resistance, biological resistance and etc under practical environmental conditions started.

UV Resistance Any polymer used for the manufacture of geotextile will degrade when exposed to the ultraviolet radiation of natural sunlight overtime. Therefore, it is essential to consider resistance of geotextile to the effects of sunlight when designing geotextile. Particular care is necessary when geotextile is to be installed in regions of the world whereby the UV radiation levels are high or when geotextile will remained exposed over period of weeks or even months on large scale projects. It is wiser to protect the geotextile from degradation. This can be done by using stabilizers, in order to match the aging process with the long term requirements of the application. High quality geotextile comes equipped with high performance stabilizers, therefore the required life time of the geotextile is guaranteed. However, prediction based on laboratory testing is not possible to determine the degradation of geotextile caused by UV sunlight due to the large number of parameters influencing the product life time. For instance: a) The degradation process within the polymer of the geotextile takes place extremely slow under ambient temperatures. b) There is no proven correlation between laboratory tests and practical application, as these products have only been in use for 30 years. However, a design lifetime of 120 years is required. c) Products installed 30 years ago cannot be compared to today’s product, as structure and chemical composition have changed because of constant ongoing product development. d) The chemical reaction of oxidative process is very well known, but in practical applications other stress factors, such as installation damage, chemical attack and many others, may be superimposed on it.

Chemical Resistance Polypropylene is characterized by an excellent resistance to chemicals. It is proven in the course of CE certification programme as a number of investigations were carried out in accordance to ISO 14030. For polypropylene geotextile, no strength loss was observed, even in acidic or alkaline conditions, in contrast to polyester products. The fiber surface of polyester yarns is particularly susceptible to degradation when exposed to alkaline condition (pH >10), external hydrolysis will take place. But even when it is exposed to acidic condition, the material is gradually degraded by internal hydrolysis. In this case, the polymer chains are split by the presence of water, thereby reducing the molecular weight. Last but not least, it will lead to a drastic reduction of mechanical properties. Therefore, it is essential to protect polyesters material by providing extra coating.

Biological Resistance Investigations according to EN ISO 12225 have shown that polypropylene geotextile are 100% resistant to micro-organisms. At the moment, no organisms are known to be harmful to polypropylene. It is important to know the biological resistance of the material in long term applications, as the influence of the organisms cannot be estimated.

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General Installation Guidelines of Geotextile It is very important to adopt the right approach for geotextile installation to prevent geotextile from further damage. It requires pre-tensioning of the geotextile layers before covering them with fill material. After that, compaction of fill materials needs to be carried out appropriately, especially near the slope and the facing. Compaction increases the shear strength of the material and thus the stability of the structure with respect to settlement and possible failures in shear. It also contributes to the pre- tensioning of the reinforcement, as a precondition of good mechanical interlock between soil and the geotextile. It is also essential to use special construction arrangements to ensure that the geometry of the slopes is as specified by the working drawings and to protect visible parts of the geotextile layers on slopes, by any appropriate means, for instance concrete or wooden facing systems. For the construction of slopes (inclination 45o to 70o), we can use either sacrificial formwork or removable rigid formwork. Types of sacrificial formwork are sand or filled , concrete elements and welded wire mesh folded to the desired angle. The advantages for using concrete elements are the placement of geotextile is relatively simple and no parts of geotextile (reinforced sheeting) can be visible. The negative aspects are it requires accurate leveling for each layer of geotextile laid and it is not compatible with large deformations. On the other hand, for welded wire mesh, the benefits are solely due to easy of placement (light weight), unnecessary to protect the visible parts of the reinforcing sheets, its facing is very flexible and a planar slope can be produced. In removable rigid formwork, the formwork consists of panels assembled into a chair. The horizontal part rests on the end of the layer just placed. The chair is held in position during the placement of the layer by a suitable system. The investment for using removable rigid formwork is relatively low. The disadvantages for adopting this approach are protection of the exposed parts of the reinforcing sheets should be provided and it iss difficult to control the geometry of each layer, due to the deformability of the underlaying layer.

o o Figure 2: Possible Construction Methods for Slopes (Inclination 45 to 70 )

For the construction of slopes (inclination 70o to 90o), for low structures (< 2m), a fixed formwork may be utilized. The advantages are the ease of construction and the exact geometry of the slope can be obtained. The disadvantages are the high costs and are space constraint as large space is required for the base of the formwork. For high structures, the method described for low structures is no longer appropriiate, because the amplitude of the thrusts on the formwork requires strong and expensive systems of props.

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Figure 3: Fixed Formwork for Low Figure 4: Possible Construction Methods Structures fo r High Walls and Steep Slopes

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