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Slope Stability Assessment Through Field Monitoring

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Slope Stability Assessment Through Field Monitoring DEGREE PROJECT IN THE BUILT ENVIRONMENT, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2018 Slope stability assessment through field monitoring YUKUN WEI KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT Slope stability assessment through field monitoring YUKUN WEI Master Thesis, 2018 KTH Royal Institute of Technology School of Architecture and the Built Environment Department of Civil and Architectural Engineering Division of Soil and Rock Mechanics SE-100 44, Stockholm, Sweden TRITA KRV Report 14/06 [TRITA-ABE-MBT-18477] ISSN 1100-7990 ISRN KTH/KRV/14/06-SE ISBN 978-91-7595-367-0 © Yukun Wei, 2018 Abstract Deterministic methods have been used in geotechnical engineering for a long period, such as slope stability calculations. However, only applying deterministic methods is subjective and imperfect. There is a demand to develop a systematic methodology to link the assessed slope stability and field measurement data, which is also known as inverse analysis and forward calculation. Based on the Nya Slussen project, this thesis includes the development of a methodology, deterministic calculation for 4 cross sections using finite element program Plaxis 2D and probabilistic calculation for one section. Deterministic analyses showed satisfying results for all the studied cross sections since their factors of safety exceeded the minimum requirement. In probabilistic design, three parameters were found to have the most uncertainties through sensitivity analysis (undrained shear strength of clay, Young’s modulus of clay and friction angle of fill). Inverse analysis was done by testing different values of them in Plaxis and to try to match the displacement components provided by field measurement. After finding the best optimization for all the parameters, forward calculation gave a final factor of safety. It is suggested that both of the methods should be utilized together for better assessment. Keywords Slope stability, inclinometer, displacement, FEM, inverse analysis. Sammanfattning Deterministiska metoder har använts inom geotekniken under lång tid, såsom släntstabilitetberäkningar. Dessa beräkningar är dock behäftade med subjektiva bedömningar och stor osäkerhet. Det finns ett behov av att utveckla en systematisk metodik för att koppla utvärderingen av släntstabilitet med fältmätningar. Detta kan göras med ’inverse analysis’ och ’forward calculation’. I samband med den nya Slussen projektet, innefattar detta examensarbete utvecklingen av metodik, deterministiska beräkningar för 4 sektioner med FEM programmet Plaxis 2D och probabilistisk beräkning för en sektion. Deterministiska analyser visade tillfredsställande resultat för alla sektioner, eftersom de utvärderade säkerhetsfaktorerna översteg gränsvärdet. I de probabilistiska analyserna hittades tre parametrar genom känslighetsanalys som hade störst osäkerheter (odränerad skjuvhållfasthet för lera, Youngs E-modulus för lera och friktionsvinkel för fyllning). Omvända analyser utfördes genom att testa olika värden av dem i numerisk modellering och försöka finna värdena som kan koppla dem med uppmätta faktiska rörelser. Efter att den bästa optimeringen hittats, gav de främre beräkningarna den slutliga uppdaterade säkerhetsfaktorn. Det föreslås att både deterministiska och probabilistiska metoder används parallellt för en bättre utvärdering av släntstabiliteten. Nyckelord Släntstabilitet, inklinometer, rörelse, FEM, omvänd analys. Preface / Acknowledgements This thesis is considered as the last part of my master’s study at Civil and Architectural Engineering at KTH. The idea of this thesis was proposed by ELU Konsult AB. It has been almost six months since I started my thesis work at ELU Konsult AB. I would like to thank all the talented and nice people from geotechnical department for your kind help during my thesis period, especially my supervisor Anders Beijer-Lundberg for your teaching, guidance and encouragement. I would also like to thank my supervisor Stefan Larsson at KTH for your valuable suggestions and comments. Finally, thanks very much to all the people who have been supporting me at KTH. Stockholm, June 2018 Yukun Wei Nomenclature Abbreviations LOAD Load and resistance factor design CS Cross section FS Factor of safety PEM Point estimate method FOSM First Order Second Moment Method FORM First Order Reliability Method SORM Second order reliability method MCS/MCM Monte Carli simulation/method FEM Finite element method RFEM Random finite element method TDR Time domain reflectometry FBG Fiber Bragg Grating EBM Extended Bayesian method LSM Least square method MLM Maximum likehood method WLSM Weighted least square method Latin Symbols Resistance [ ] Load effect [ ] � Variance �� � Coefficient of variation �� ���[� ] Autocovariance ���[� ] Weight factor � � (�) Young’s Modulus [ ] ±±± � Factor for parameter 1 � Factor for parameter 2 ��� � Factor for parameter 3 � Soil cohesion [ ] � Secant stiffness from triaxial [ ] �′ ��� ��� Tangent stiffness from oedometer [ ] �50 ��� ��� Unloading reloading stiffness [ ] ��� ���� Power ��� ��� Initial void ratio ��� � ���� 4 | INTRODUCTION Permeability [ ] Undrained shear strength [ ] � � � � Distance between soil nailings �[/�] � � Maximum compression/tension force ���[ ] �������� � ���� Greek � Symbols ��� Standard deviation/stress [none/ ] Mean value � Skewness ��� � Pearson correlation coefficient 1 � Reliability index �,� � Strain � Weight factor for inclinometer � Measured displacement [ ] �� ���� Calculated displacement [��] Difference between and [ ] ���� Model parameter �� �� Unsaturated unit weight���� ���� [ ��] � Saturated unit weight [ 3] ��� � Friction angle ��/�[3] ���� dilatancy angle ��/�[ ] � Poisson’s ratio ° � Vertical displacement [ °] � Horizontal displacement [ ] � � Angle between Slope R1 and North ��[ ] �� Friction angle of fill ��[ ] � ° ����� ° Contents Abstract ................................................................................. i Sammanfattning .................................................................... i Preface / Acknowledgements ................................................. i Nomenclature ....................................................................... 3 Contents ............................................................................... 5 1. Introduction ...................................................................... 1 1.1. Aims ................................................................................... 2 1.2. Scope .................................................................................. 2 1.3. Content ............................................................................... 2 2. Literature review ............................................................... 2 2.1. Slope stability ..................................................................... 2 2.2. Calculation and design ......................................................... 5 2.3. Field measurement............................................................ 16 2.4. Risk analysis ...................................................................... 19 3. Methodology .................................................................... 1 3.1. General inverse analysis ...................................................... 1 3.2. Simplified inverse method ................................................... 3 4. Case study: the Nya Slussen project ................................... 2 4.1. Background ......................................................................... 2 4.2. Location and geometry ........................................................ 4 4.3. Geotechnical/Geological conditions ..................................... 8 4.4. Design load and factor of safety ......................................... 16 4.5. Field monitoring ................................................................ 17 4.6. Deterministic design .......................................................... 19 4.7. Uncertainties and comments ............................................. 23 6 | INTRODUCTION 5. Case study: application of the methodology in the Slussen project .......................................................................................1 5.1. Uncertainties remaining in the deterministic method ........... 1 5.2. Flowchart of the case study ................................................. 1 5.3. Measurement data .............................................................. 2 5.4. Systematic analysis.............................................................. 7 6. Discussion and conclusion ..................................................1 6.1. Discussion ........................................................................... 1 6.2. Conclusion .......................................................................... 3 6.3. Future scope ....................................................................... 4 7. References .........................................................................1 8. Appendix ...........................................................................1 INTRODUCTION | 1 1. Introduction Slope stability is normally an important geotechnical task during construction, which includes excavations and changes in load or groundwater conditions. It is therefore a significant feature of practical geotechnical design. Regarding the safety requirements, extensive monitoring of soil deformation and pore pressure is sometimes carried out throughout the construction procedure. Traditionally, those measurement data
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