AN EXPERIMENTAL AND ANALYTICAL STUDY OF VARIOUS SOIL SLOPES IN LABORATORY CONDITIONS A Thesis Submitted to the Graduate School of Engineering and Science of İzmir Institute of Technology in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE in Civil Engineering by Hasan Fırat PULAT November 2009 İZMİR We approve the thesis of Hasan Fırat PULAT Assoc. Prof. İsfendiyar EGELİ Supervisor Assist. Prof. Yusuf ERZİN Committee Member Assist. Prof. Nurhan ECEMİŞ Committee Member 9 November 2009 Prof. Gökmen TAYFUR Assoc. Prof. Talat YALÇIN Head of the Department of Civil Engineering Dean of the Graduate School of Engineering and Sciences ACKNOWLEDGMENTS I would like to express my great gratitude and sincere thanks to my supervisor, Assoc. Prof. Dr. Đsfendiyar EGEL Đ, for his continued inspiration, support and invaluable guidance during my study. Without his endless efforts and experienced guidance, it would not have been possible to complete this work. I thank the members of the thesis defense committee Assist. Prof. Dr. Yusuf ERZ ĐN and Assist. Prof. Dr. Nurhan ECEM ĐŞ for their assistance in reviewing and editing my manuscript. My special thanks go to Assist. Prof. Dr. Tahir Kemal ERDEM for his inspiring discussions and comments. I also would like to thank to my research assistant colleagues Can Ali GÜVEN and Ba şak DO ĞAN for their dedicated support during my experimental studies. I am grateful to my parents and my sister for their constant support and endless encouragement. Lastly, my gratitude also due to my fiancee Pelin TUAÇ, whose love, patience and continuous support helped me to finish this thesis. Thanks to all. iii ABSTRACT AN EXPERIMENTAL AND ANALYTICAL STUDY OF VARIOUS SOIL SLOPES IN LABORATORY CONDITIONS Slope stability is a significant subject of geotechnical engineering. Slope failures triggered by rainfall are causing considerable damage and loss of life every year throughout the world. Especially at dry seasons when the rainfall is scarce, the ground can develop considerable amount of suction and this improves the shear strength of the soil. In rainy season, when rainfall infiltrates into ground, suction decreases, while the shear strength also reduces, which may lead to slope instability. One of the principle objectives of this study is to represent the development of soil-water interaction modeling system (SWIMS) at IYTE. Using this system; effects of 3 different parameters, such as: initial water content, soil density, slope angle on modelling unsaturated slope stability were studied. Moreover, effects of infiltration on slope stability in shallow landslides, where it is assumed that the ground water tables are located at significant depths, were examined. In this thesis study, 12 main slope model experiments were completed in laboratory conditions, using Soil-Water Interaction Modelling System (SWIMS) by varying 3 different parameters. Result of studies shows that slope angle is the most important parameter affecting slope stability. Furthermore, parameters such as; soil density, degree of relative compaction of soil and initial water content affects slope stability, while these parameters also affect slope surface erosion and infiltration depths. In addition to experimental studies conducted in laboratory conditions with the 12 main slope model experiments, slope stability analyses to find FOS were performed by using Plaxis V9 (2D) finite element program (FEM), which uses shear strength reduction (SSR) technique and infiltration analyses using the Plaxflow module to model the rainwater infiltration into slope soil were performed. The FEM analyses show conforming results with the actual observations made using the tested soil model in laboratory conditions. iv ÖZET ÇEŞİTLİ ZEMİN ŞEVLERİNİN DURAYLILIĞININ LABORATUAR ŞARTLARINDA DENEYSEL İNCELENMESİ VE ANALİZİ Şev duraylılığı geoteknik mühendisliğinin önemli bir konusudur. Yağmur suyunun tetiklediği şev göçmeleri tüm dünyada her sene önemli miktarda can ve mal kaybına sebep olmaktadır. Özellikle yağmurun az olduğu kurak mevsimlerde, zemin yüksek emme gösterebilmekte ve bu zeminin kayma mukavemetini arttırmaktadır. Yağmur sezonunda ise zemine infiltre olan yağmur suyu, zeminin emmesini ve kayma mukavemetini azaltmakta, bu durum zeminin göçmesine sebep olabilmektedir. Bu çalışmanın temel amaçlarından biri İYTE’de geliştirilen Zemin-Su Etkileşim Modelleme Sistemi’nin (SWIMS) tanımlanmasıdır. Bu sistemi kullanarak; zeminin başlangıç su içeriği, zeminin sıkılığı ve şev açısı gibi üç farklı parametrenin doymamış şev zemininin duraylılığına etkisinin modellenmesidir. Ayrıca, yeraltı su seviyesinin derinde olduğu şevlerdeki sığ toprak kaymalarında yağmur suyu infiltrasyonunun şev duraylılığına etkisinin incelenmesidir. Bu tez çalışmasında 12 ana şev modeli deneyi, 3 değişik parametre ile Zemin-Su Etkileşim Modelleme Sistemi (SWIMS) kullanılarak, laboratuvar koşularında tamamlanmıştır. Yapılan çalışmalar sonucunda; şev açısının, duraylılığı etkileyen en önemli parametre olduğu belirlenmiştir. Ayrıca zeminin yoğunluğu, rölatif sıkılığı, başlangıçtaki su muhtevası gibi parametrelerin şev duraylılığını etkilemesinin yanısıra, bu parametreler şev yüzeyi erozyon ve infiltrasyon derinliklerini de etkilemektedir. Deneysel çalışmalara ek olarak; laboratuvar koşullarında gerçekleştirilen 12 şev modeline ait güvenlik katsayısı duraylılık analizleri için kayma mukavemetini azaltma (SSR) tekniğini kullanan Plaxis V9 (2D) ve yağmur suyunun şev zeminine infiltrasyon koşullarını modelleyen Plaxflow modüllü sonlu elemanlar programı kullanılmıştır. Yapılan analizler ve laboratuvarda gerçekleştirilen deneysel şev modeli neticeleri birbirine uyumlu sonuçlar vermiştir. v TABLE OF CONTENTS LIST OF FIGURES ......................................................................................................... ix LIST OF TABLES......................................................................................................... xiv LIST OF SYMBOLS...................................................................................................... xv CHAPTER 1. INTRODUCTION..................................................................................... 1 1.1. Overview.................................................................................................. 1 1.2. Objective and Scope of Study.................................................................. 3 1.3. Organization of the Thesis....................................................................... 4 CHAPTER 2. BACKGROUND OF SLOPE STABILITY ANALYSES METHODS AND MASS MOVEMENTS TYPES ....................................................... 6 2.1. Introduction.............................................................................................. 6 2.2. Factors Causing Instability ...................................................................... 8 2.3. Slope Stability Analysis Methods............................................................ 8 2.3.1. Limit Equilibrium Methods ................................................................ 9 2.3.1.1 . Infinite Slope Stability.............................................................. 11 2.3.1.2 . Method of Slices ....................................................................... 12 2.3.1.2.1 . Ordinary Method of Slices or the Fellenius Method ......... 14 2.3.1.2.2. Bishop’s Simplified Method............................................... 15 2.3.1.2.3. Janbu’s Simplified Method................................................ 17 2.3.1.2.4. Spencer Method .................................................................. 18 2.3.1.2.5. Morgenstern-Price Method................................................. 19 2.3.1.3 . Limit Analysis Method ............................................................. 21 2.3.1.4 . Variational Calculus Method.................................................... 22 2.3.1.5 . Strength Reduction Method..................................................... 22 2.4. Classification of Mass Movements........................................................ 23 2.4.1. Topples.............................................................................................. 24 2.4.2. Falls................................................................................................... 24 vi 2.4.3. Slides................................................................................................. 24 2.4.4. Flows................................................................................................. 26 2.4.5. Creeping............................................................................................ 27 CHAPTER 3. LITERATURE REVIEW ON RAINFALL INFILTRATION PARAMETERS AFFECTING SLOPE STABILITY ............................. 28 3.1. Introduction............................................................................................ 28 3.2. Infiltration .............................................................................................. 29 3.2.1. Infiltration Scenarios......................................................................... 30 3.2.2. Factors Affecting Infiltration............................................................ 30 3.3. Influence of Seepage on Slope Instability ............................................. 31 3.4. Rainfall Infiltration and Shallow Landslides ......................................... 32 3.5. Uncertainties in Slope Stability under Rainfall Conditions................... 33 3.6. Saturated and Unsaturated Soils ...........................................................