Addis Ababa Science and Technology University College of Architecture and Civil Engineering Department of Civil Engineering Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis Yared Beyene PROJECT SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING IN GEO-TECHNICAL ENGINEERING September, 2017 Addis Ababa, Ethiopia Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 ADDIS ABABA SCIENCE & TECHNOLOGY UNIVERSITY COLLEGE OF ARCHITECTURE AND CIVIL ENGINEERING COMPARISON OF FINITE ELEMENT AND LIMIT EQUILIBRIUM METHODS FOR SLOPE STABILITY ANALYSIS BY: YARED BEYENE Approved by Board of Examiners: Chair Person ________________________ Signature __________________ Internal Examiner _________________________ Signature __________________ Head, Civil Eng’g _________________________ Signature __________________ Department Dean, College of _________________________ Signature__________________ Architecture and Civil Engineering YARED B. Page II Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 DECLARATION I hereby declare that all information in this document has been obtained and presented in accordance with academic rules and ethical conduct. I also declare that, as required by these rules and conduct, I have fully cited and referenced all material and results that are not original to this work. Name: Yared Beyene Signature: ______________________ YARED B. Page III Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 Abstract Slope stability analysis is one of the most prominent subject matter in geotechnical engineering. Slope may be natural and man-made in its nature, and those slopes can be finite and infinite. It can also be soil mass slope or rock slopes. There are different kinds of slope failures and causes for those slope failures. For those failures, since 1922 by different scholar’s various types of slope stability analysis methods has been discovered. The stability methods can broadly be classified as the limit equilibrium method, the finite element method, the finite difference method and limit analysis method. Limit equilibrium method is the most commonly used methods for analysis. Even though some literatures states that using the finite element method slopes can be analyzed with better accuracy. Therefore, it is better to identify the difference between the two methods more clearly and to identify when to use which method in different circumstances. Thus, the main objective of this project is to compare the limit equilibrium method and the finite element method, to assess their differences and to study when to use which method. Plaxis 2D program has been used for the finite element analysis whereas Slope/W program has been used for the limit equilibrium analysis. Three condition of soil mass was considered, the first one is homogeneous silty sand soil mass, the second one is stratified soil mass and the third one is homogenous clay soil with free surface. The factor of safety value is used as an output for the comparison of the two methods. The analysis result for the homogenous silty sand soil mass shows a Factor of safety value for finite element method is 2.9. whereas for limit equilibrium methods, 3.0 is obtained for ordinary method of slice and 3.19 is obtained for the other types. And similar results are obtained in the other cases. This leads to a conclusion that finite element method is more efficient method for analysis of slope failure than the limit equilibrium methods without a predetermined assumption. Key words: Slope Stability; Finite Element method; Limit equilibrium method; Factor of Safety. YARED B. Page IV Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 ACKNOWLEDGEMENTS I would like to express my most sincerely gratitude to my advisor Dr. Yared Worku for his guidance and suggestions he provided throughout the project. And I would like to thank Ethiopian road authority and Addis Ababa Science & Technology University for granting me this opportunity. And also Stadia Engineering Works consultants Plc for providing me the resources needed for this project. Finally, my family who has been there in this journey supporting and motivating me in every way showing me endless compassion, I am beyond words to thank you. YARED B. Page V Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 Table of Contents DECLARATION ...................................................................................................................... III Abstract ................................................................................................................................... IV ACKNOWLEDGEMENTS ....................................................................................................... V Table of Contents ..................................................................................................................... VI Abbreviations ......................................................................................................................... VIII List of Figures .......................................................................................................................... IX List of Tables ........................................................................................................................... XI 1. Introduction............................................................................................................................. 1 1.1 Background and Justification ............................................................................................. 1 1.2 Statement of the Problem ................................................................................................... 3 1.3 Objectives of the Study ...................................................................................................... 3 1.3.1 General Objective ........................................................................................................3 1.3.2 Specific Objective .......................................................................................................3 1.4 Scope ................................................................................................................................. 4 1.5 Organization of the Project ................................................................................................ 4 2. Literature Review .................................................................................................................... 5 2.1 Introduction ....................................................................................................................... 5 2.2 Types of Slopes ................................................................................................................. 6 2.3 Types of Slope Failure ....................................................................................................... 7 2.4 Causes of Slope Failure ..................................................................................................... 9 2.5 Slope Stability Analysis ................................................................................................... 14 2.5.1 Limit Equilibrium Method ......................................................................................... 14 2.5.2 Finite Element Method .............................................................................................. 26 3. Materials and Methods .......................................................................................................... 29 YARED B. Page VI Comparison of Finite Element and Limit Equilibrium Methods for Slope Stability Analysis 2017 3.1 General ............................................................................................................................ 29 3.2 Study Area ....................................................................................................................... 29 3.3 Demography .................................................................................................................... 30 3.4 Traffic Study ................................................................................................................... 30 3.5 Geology of the study area ................................................................................................ 31 3.6 Soil Characteristics .......................................................................................................... 32 3.7 Selected slope sections for Analysis ................................................................................. 32 3.8 Data Collection ................................................................................................................ 32 3.9 Analysis Methods ............................................................................................................ 34 3.10 Statistical Comparison ................................................................................................... 38 4. Results and Discussions ........................................................................................................ 39 4.1 Plaxis Analysis ................................................................................................................ 39 4.2 Slope/W Analysis ...........................................................................................................