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A STUDY OF STABILITY OF SLOPE WITH IRREGULAR PARTICLES

Author

Student Name:

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Supervisors

Supervisor’s Name

Academic Year

[2020]

Submitted in support of: [MA Ingineering]

DECLARATION

I declare that this research project is truly my original work, it has not been submitted at …………………(Your University) for degree award or any other learning institute.

Student:

Signed:

Date: 27/06/2020.

This research project has been submitted for examination with my approval as university supervisor

Professor:

Date; 28/06/2020.

ACKNOWLEDGEMENT

Gratitude and glory be to Almighty God, for free gift of life and keeping me health, giving me guidance and wisdom during the period of the proposal writing.

The successful completion of this proposal would not have been possible without the assistance and input of many people, in various ways. I am most sincerely grateful to my supervisor, Professor …………. for his dedication and valuable pieces of advice throughout the process of preparing this proposal. I am also greatly Indebted to other lecturers in the School of Engineering who offered valuable suggestions at the inception of the project. Special mention goes to the librarians and staffs of department of Finance and investment for their support by providing material for performing my research.

Many thanks also to my family for their support in completing the project. I can never repay the generosity you displayed throughout my MA degree providing that safety net on the numerous occasion when college fees was had to come by.

DEDICATION

I dedicate the dissertation project to my family. Your faith and heartfelt support in me has shaped my character and has made me to have faith in myself. Thanks to our almighty GOD for his blessings in our family.

Table of Contents

DECLARATION	2

ACKNOWLEDGEMENT	3

Background of the research study	5

Irregular particle numerical model	11

Purpose of the research study	13

Research Problem	14

Research Objectives	16

Study Methodology	16

Study Region - Yumokjeong Landslide, South Korea	16

Data analysis	18

Finite Element Model	20

Appendix 1: Research Time Schedule	21

References	22

Background of the research study

A slope is an inclined boundary surface between air and the body of an earthwork such as highways, cut or fill, railway cut or fill, earth dams, levees, and river training work. The slope stability analysis is crucial in engineering practice to ensure the stability of structures and prevent loss of human life and money. Instabilities of natural and cut slopes cause enormous loss of life, injuries, and damage to the property every year. Researchers have carried out a large number of evaluation studies for slope stability to reduce the loss caused by slope sliding and save the cost of disaster prevention and reduction. Generally, the stability of slope is affected not only by quantifiable indicators (mechanics parameters, rainfall, slope height, etc.) but also by more qualitative indicators (geotechnical properties, slope morphology, fault development degree, etc.). Thereby, it is necessary to comprehensively consider the influence of quantitative and qualitative indexes on slope stability.

To compare and analyze the influence of the irregular shape of particles on a stable slope angle, a calculation simulation of the process of accumulation and landslide of the circular particle system is also established. The research results show that the stable slope angle formed by the accumulation of the two particles increases with the increase of the friction factor, and the upward curve of the curve gradually slows down, which can be fitted by an exponential function; under the same conditions, the stable slope angle of the circular granular material It is smaller than the stable slope angle of irregular granular materials, and the difference between the stable slope angles of the two granular materials in the friction coefficient range of 0.3-0.7 is significant. Studies have shown that the irregularity of particles has a certain effect on the slope angle of material accumulation and stability. The research conclusion can provide a reference for the understanding of the macroscopic mechanical properties of irregular granular materials and the theoretical study of slope stability.

Granular media materials are widely used in engineering, such as site fill, slope, rock fill dam, and other projects. Especially in the gravel-filling site and slope engineering, the particle size and irregularity of the gravel and the contact characteristics have a significant effect on the stability of the site and slope. The mechanical properties of granular media materials often show similar characteristics to solids and fluids, but their mechanical behaviors are far more complex than solids and fluids. Scholars at home and abroad have extensively studied the mechanical properties of particulate materials. Through physical mechanics experiments and numerical simulations, they have a deeper understanding of the internal structure and dynamics of particulate materials, but the mechanical theory of particulate materials is far from mature.

The mechanical stability of granular materials is significantly affected by the structure of the particles. The particle size, shape, and contact characteristics of the particles are closely related to the particle movement and the macroscopic deformation behavior of the material. Al-Hussaini 2013), found through experiments that the internal friction angle of crushed stone increased with the increase of its average particle size. Hamidi et al. (2010) also obtained similar conclusions. Dai and Beibing, (2012), discovered the shear of the material through the glass bead material shear test. The swell ability increases as the particle size increases.

However, Marachi et al. (2015) found that the internal friction angle of gravel soil decreased with the increase of particle size through the triaxial drainage test. Sitharam and Nimbkar (2016) found through discrete element numerical simulation that when the gradation curves of the samples are parallel to each other, the internal friction angle increases with the increase of the average particle size, but when the minimum particle size of the sample remains the same, the internal friction The angle decreases as the average particle size increases. Marachi et al. (2015) used the discrete element method to simulate the accumulation of spherical particles of equal diameter. It was found that the shape of the stable slope of the pile is not affected by the size of the particle, and the change of the lateral rolling resistance parameter between the particles has a significant effect on the shape of the particle accumulation.

The study of Bagherzadeh-Khalkhali et al. (2018) showed that the critical state friction angle (that is, the friction angle when the sample is continuously deformed and destroyed under constant volume) does not change with the particle size. It can be seen that the mechanical characteristics of particulate materials obtained by different researchers relying on particle size parameters are inconsistent, and even there are contradictions. It is difficult to effectively characterize the mechanical stability of particulate materials based on particle size parameters alone.

The mechanical properties of granular materials are also closely related to the shape of the particles. The embedding and biting between particles can increase the deformation resistance of the material. Many scholars have obtained the conclusion that the irregular properties of the particles significantly increase the shear strength of the material through numerical simulation. However, Azema et al. (2019) also found that the shear coefficient of granular materials with different shapes under the maximum compression state is consistent with the shear coefficient under the critical state. Analyze the reason. It may be that under the maximum compression state, the contact pressure between the particles increases, and the relative sliding resistance of the particles increases. The occlusal embedding of irregular particles no longer improves the shear strength of the material. The sliding resistance between particles and irregular particles. The effect of the property on the macroscopic properties of the material requires in-depth understanding.

In the past decades, earthquake-induced slope failures have caused enormous losses of properties and lives (Rodrı́guez et al.1999; Bommer and Rodriguez2002; Uzuoka et al.2005; Wang et al.2009; Keefer2002). How to predict the earthquake-induced slope failure is essential to rational risk management (Dai et al.2002; Corominas et al.2014). Several approaches have been developed for the stability analysis of slopes under the seismic loading, including the pseudo-static analysis (e.g., Kramer1996; Stewart et al.2003), the permanent displacement analysis (e.g., Newmark1965), and the stress-deformation analysis (e.g., Griffiths andPrevost1988; Elgamal et al.1990). A detailed review and discussion of these methods can be found in Jibson (2011). The availability of these methods greatly enhances the capability of the profession to assess the slope stability under seismic loading. Nevertheless, during the practical application of these methods, many uncertainties may be involved, such as the uncertainty in the soil properties and uncertainty in the seismic loading, which may significantly affect the results from a slope stability analysis and hence the relevant decision-making.

In recent years, probabilistic methods have been increasingly recognized as a useful tool to explicitly consider the effect of uncertainties on slope stability assessment. Many methods have been developed for reliability analysis of slopes under the static condition (e.g., Christian et al. 1994; Hassan and Wolff1999; El-Ramly et al.2011; Griffiths and Fenton2004; Juang et al.2015). It is now widely recognized that as a slope may have numerous potential slip surfaces, the failure probability of a slope is usually greater than that along any single slip surface and that the reliability of a slope can be better analyzed in the framework of system reliability (e.g., Ching et al.2009; Li et al.2013; Realeet al.2016). If the system effect is not considered, the risk of slope failure may be underestimated (Huang et al.2013; Zhang andHuang2016). The potential of predicting earthquake-induced slope failure using probabilistic methods has also been explored by several researchers. For example, Al-Homoud and Tahtamoni (2002) performed a sensitivity analysis to study the effect of seismic loading on the reliability of earth slopes using the first-order, second-moment method. Hata et al. (2012) suggested a method to predict the size of earthquake-induced slope failure considering the uncertainties in soil strength parameters. Babu and Murthy (2003) illustrated the advantage of evaluating the seismic stability of slopes through reliability analysis. Wu (2015) studied how to derive fragility curves for analysis of earthquake-induced slope instability. These studies have increased our understanding of the effect of seismic loading on the stability of slopes in an uncertain environment. During the assessment of earthquake-induced slope failure, the chance to experience a greater seismic loading will increase with the exposure time. As a result, the failure probability of a slope may also increase with the exposure time. Nevertheless, the relationship between the slope stability and the exposure time is not considered in these studies.

In geotechnical earthquake engineering, the usefulness of the performance-based design concept is increasingly being appreciated (e.g., Juang et al.2008; Kramer2014; Franke and Kramer2014), and how to calculate the failure probability of a slope under the seismic condition during a given exposure time is an important consideration in this framework. In recent years, several studies have also been conducted to address this problem. Christian and Urzua (1998) suggested a method to assess the annual failure probability of a slope assuming the slip surface is plane. Rathje and Saygili (2008) developed scalar and vector approaches for probabilistic seismic hazard analysis of the sliding displacement of slopes. Rodríguez-Ochoa et al. (2015) studied the annual failure probability of a submarine slope based on the infinite slope stability model. Xiao et al. (2016) suggested a versa-tile method to assess the failure probability of a slope during a given exposure time based on the shear strength reduction method. For simplicity, the uncertainty in the site-specific amplification factor is not considered in their study. When the failure probability is small, the method in Xiao et al. (2016) is likely to be computationally intensive.

Currently, most probabilistic studies on slope stability mainly focus on the reliability of the slope under the static condition. If the probabilistic methods for static slope stability can be extended for seismic slope stability analysis, it will greatly facilitate the application of probabilistic methods to the assessment of earthquake-induced slope failure. The pseudo-static method is a widely used approach for seismic slope stability analysis (Jibson, 2011), and is a natural extension of the limit equilibrium method for static slope stability analysis. The objective of this paper is thus to suggest a two-stage method to assess the failure probability of a slope within a given exposure time based on the pseudo-static stability analysis considering all possible earthquakes that may occur during the exposure time. With the two-stage method suggested, the methods for reliability analysis of the slope under the static condition can be conveniently extended to slopes under the seismic condition, thus providing a consistent framework for assessing the reliability of slopes in the static and seismic conditions. The structure of this paper is as follows. First, the key uncertain parameters in the pseudo-static slope stability model are analyzed. Then, a two-stage method is suggested to calculate the failure probability of a slope during a given exposure time. Finally, the proposed method is illustrated with two examples. The method described in this paper will provide a practical tool to assess the failure probability of a slope under the seismic condition during a given exposure time based on the commonly used pseudo-static stability analysis.

Irregular particle numerical model

In this paper, the random aggregate model method will be used to generate and drop three-dimensional irregular particles. The generated numerical model particles are three-dimensional irregular convex polyhedra, which are grown from the space octahedron. Select the particle characteristic parameters: the particle's longest radius the ratio of the shortest radius to the longest radius γ, and set other control parameters to randomly generate particles, which can simulate gravel particles with different roundness in the project. Two typical random particles are shown in the figure.

The longest radius rs=6cm, the ratio of the shortest radius to the longest radius.

Factors responsible to cause instability of slopes

The stable slope angle of irregular particles is affected by many factors. Following are the factors which causes the instability of slopes-

Rise of the slope.

High frequency vibrations.

Creep and shrinkage on the slope.

Rapid drawdown.

Change in Ground Water Table Level.

Large scale deformation of the earth crus.

Here the research paper will mainly analyze the influence of particle friction coefficient and irregularity of particle shape on the accumulation of stable slope angle. According to the numerical experiment design, the typical particle accumulation and landslide shape are calculated as shown in figure below

Purpose of the research study

The research project will mainly research the accumulation stability of irregular granular materials. The finite element method will be used to numerically simulate the accumulation of granular materials and the dynamic process of landslides. The influence of particle shape and contact characteristics on the stability of granular materials will be first studied. First, three-dimensional irregular particles will be generated based on the random aggregate model algorithm, and an irregular particle model library with a random distribution of surface shape and particle size was established. Then, the process of putting and stacking granular materials and slope sliding will be designed, using dynamic nonlinear finite element Methods Stable slope angle (rest angle) of irregular particle system will be simulated, and the collision contact and friction between particle units were considered. Finally, the variation law of the stable slope angle of granular material affected by the friction coefficient between particles will be analyzed using the abacus software.

Research Problem

Landslides are natural hazards that have devastating economic and social costs, sometimes with tragic impacts, including loss of human life. These slope failures, ubiquitous in both the natural and engineered slopes and earthen structures, are often associated with destabilizing events such as excavation, erosion, seismic activity, or heavy and/or sustained precipitation. For unsaturated soils, variations in suction stress due to changes in soil moisture often govern the mechanical behavior in natural and man-made slopes. The effects of negative pore-water pressures and associated loss of suction due to rainfall infiltration on slope failures have long been an area of interest, prompting several investigations on the behavior of soil under unsaturated flow conditions (Guzzetti et al., 2012).

Guzzetti et al., (2012) postulates that landslides occur when large amounts of earth, rock, sand, or mudflow swiftly downhill and mountain slopes. The incidence of this phenomenon, usually triggered by natural hazards such as earthquakes, volcanic eruptions, heavy rainstorms, or cyclones, is increasing due to modern land-use practices, climate change, and deforestation. The impact of a landslide can be extensive, including loss of life, destruction of infrastructure, damage to land, and loss of natural resources. Landslide material can also block rivers and increase the risk of floods (Guzzetti et al., 2012. Deep landslides, triggered by major earthquakes or volcanic activity can destroy thousands of square kilometers of land and kill thousands of people. Landslides have a devastating effect on farmers' livelihoods as they can prevent access to land for years, destroy seed and food stocks, and will commonly result in the loss of livestock and standing crops (Guzzetti et al., 2012).

Landslides occur over a wide range of velocities and are recognized as the third most crucial natural disaster worldwide (Zillman, 1999). Landslides are usually triggered without warning, giving people less time to evacuate. Therefore, the direct impact of landslides on the socio-economic system is crucial (Christopher, 2016). Landslides are responsible for significant loss of life and injury to people and their livestock as well as damage to infrastructure, agricultural lands, and housing (Schuster and Fleming, 1986; JRC, 2003; Blöchl and Braun, 2005; Guzzetti et al., 2012). Economic losses from landslides have been increasing over recent decades (Petley et al., 2005; Guha-sapir et al., 2011; Guzzetti, 2012), mainly due to increasing development and investment in landslide-prone areas (Bandara et al., 2013; Petley et al., 2005).

Besides, the potential effects of a changing climate include increased intensity of extreme precipitation, as has been documented in various research projects. Similarly, annual rain and rain intensity have significantly increased in most regions over the world, especially Asian parts within the past few decades (Yoo, 2014). These climate trends have noticeably increased the number of catastrophic disasters such as typhoons and precipitation-induced landslides in Asia. For instance, due to such disasters, over 1300 people were killed, 280,000 people were displaced and over 2 billion dollars in damages have been reported in South Korea from 1993 to 2012 (Yoo, 2014). Among others, precipitation-induced landslides have been the major cause of casualties and economic loss. Severe rainfall events result in insubstantial and unprecedented changes in the degree of saturation within the unsaturated zone of man-made or natural slopes, which can lead to failure of these slopes. The occurrence of many precipitation-induced landslides in unsaturated soils highlights the need for explicit consideration of the negative pore pressure or matric suction above the water table in modeling and monitoring of theses slopes (Yoo, 2014).

Few studies have attempted to investigate scope stability analysis with irregular parties, and hence a gap in the existing literature review. For effective mitigation of landslides, erosions, and other hazards emanating from loose slopes, there is a need for thorough research on scope stability associated with irregular particles.

Research Objectives

Landslides are natural hazards that have devastating economic and social costs, sometimes with tragic impacts, including loss of human life. These slope failures, ubiquitous in both the natural and engineered slopes and earthen structures, are often associated with destabilizing events such as excavation, erosion, seismic activity, or heavy and/or sustained precipitation. The research project aims to study slope stability associated with irregular particles adopting the Yumokjeong Landslide region in South Korea as an area of study. Hence, the objectives of the research project will be;

To determine slope stability associated with irregular particles.

To determine endangered areas and investigate potential failure mechanisms.

To determine the slope sensitivity to different triggering mechanisms,

To design optimal slopes concerning safety, reliability, and economics, designing possible remedial measures.

Study Methodology

Study Region - Yumokjeong Landslide, South Korea

Lying within the central Mountainous region of South Korea, approximately 100 km east of Seoul, an excavation of a road cut and subsequent years of above-average rainfall triggered the progressive movement of a shallow, translational slide. The slope located at coordinates N (37° 35′20.8′′), E (127° 45′34.4′′) (terrain model shown in Fig.1) will be instrumented with wire tension meters for post-construction quality control(Fig.2), along with a series wire tension meters on adjacent slopes. These instruments, which transmitted data wirelessly, collected slope movement data at once per day.

The slope consisted of a shallow soil layer (5–8.5 m deep) of lightly cemented, colluvium soil classified as well-graded sand (SW) overlying weathered bedrock (Fig.3). A trace presence of fines (clay) contributed to light cementation. Based on laboratory testing, the soil’s saturated hydraulic conductivity will be measured directly shear apparatus. Saturated hydraulic conductivity will also be measures. Precipitation will be measured at a weather station near the construction site. Beginning at the date of excavation (September, 2020), progressive displacement has already occurred (Fig.4a) with above-average rainfall (Fig.4b). In the last 5-10 years, almost 350 mm of movement has occurred at the measured displacement monitoring point, with a total measured rainfall of approximately 8000 mm. Visible erosion along the slope has been noted after subsequent rainy seasons, likely due to extremely heavy rainfall and saturated soil conditions. This progressive slope failure is colloquially known as the Yumokjeong landslide based on its location.

Figure 4: Location of slope failure along highway corridor—center of topographical map, coordinates N(37° 35′20.8′′), E (127° 45′34.4′′).

Data analysis

A FE analysis will be performed to capture coupled behavior of the slope under transient, unsaturated seepage conditions. When using FE to model a coupled pore pressure-stress formulation, the soil is treated as a porous medium in a solid phase (i.e., no action as a colloid or liquid). To implement this coupled formulation, the SWCC as well as HCF will be directly implemented into Abaqus Version 12 (Hibbitt et al.2001), as has been done in previous studies (e.g., Yoo2013, 2014). The implementation of these properties will allow application of Bishop’s (1959) model for applying matric suction directly within the framework of Mohr-Coulomb failure criteria. The SWCC will be implemented into Abaqus by adding sorption material properties to the soil, implementing a range of saturation values and associated suction stress values, (ua−uw) representative of the SWCC fitted to then andαparameters necessitated by the van Genuchten (1980)-Mualem (1976) approach. The hydraulic conductivity will be assigned as a function of the degree of saturation, implemented to represent the HCF from Gardner (1958), also dependent on theαparameter.

To perform the stress-pore pressure-displacement analysis, a mesh consisting of 2259 eight-node pore stress, reduced-integration quadrilateral elements (CPE8RP) will be used to discretize the rock and the soil materials. A more refined mesh will be used to model the soil layer due to higher expected movement and plastic behavior, comprised of 677elements. The remaining 1582 elements will be used to mesh the rock, which will be treated as an elastic material due to its competence in comparison to the overlying soil. Although weathered rock may demonstrate plastic behavior, the relative strength of the overlying colluvium will be considered notably weaker, manifested by the demonstrated, progressive slope failure.

The weathered rock will be modeled with the same permeability as the soil as its weathered and fractured nature lends itself to a higher permeability due to the presence of fractures, fissures and macro-pores (George1992; Terzaghietal.1996; Hsieh1998; Jiaoetal.2005). It will be difficult to capture this phenomenon discretely with continuum methods like FEM, but the higher permeability will be homogenized throughout the rock elements to simplify computations. The model geometry will be modeled with basic soil properties representative of Mohr-Coulomb plasticity, coupled with potential matric suction resulting from partial saturation from the transient seepage analysis. As it will be established from laboratory direct shear testing, approximate strength values of φ′=20° and c′=15 kPa will be to define Mohr-Coulomb criteria.

Finite Element Model

The concept of the Finite Element Method (FEM) was coined by Clough in the early 1960s in his infamous book entitled “The finite element method in plane stress analysis”. The finite element method (FEM) is the most widely used method for solving problems of engineering and mathematical models. It was originally developed for solving problems in solid-state mechanics, but it has since found wide application in all areas of computational physics and engineering, as well as in CFD. FEM is by far the most flexible method of all methods we have studied so far, and it can be adapted to a wide range of numerical problems. This makes FEM a universal tool for solving differential equations numerically.

The Finite Element Method to be used in the project will be based closely on Program 6.2 in the text by Smith &Griffiths (1998), the main difference will be the ability to model more general geometries and slope property variations, including variable water levels and pore pressures. Further graphical output cap-abilities have been added. The programs will be for two-dimensional plane strain analysis of elastic perfectly plastic soils with a Mohr-Coulomb failure criterion utilizing eight-node quadrilateral elements with reduced integration in the gravity loads generation, the stiff-ness matrix generation and the stress redistribution phases of the algorithm. These stresses will then be compared with the Mohr-Coulomb failure criterion. If the stresses at a particular Gauss point lie within the Mohr-Coulomb failure envelope, then that location will be assumed to remain elastic. If the stresses lie on or outside the failure envelope, then that location will be assumed to be yielding. Yielding stresses will be redistributed throughout the mesh utilizing the viscos plastic algorithm. Overall shear failure occurs when a sufficient number of Gauss points have yielded to allowing a mechanism to develop.

Appendix 1: Research Time Schedule

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