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Clay Slopes and Their Stability: an Evaluation of Different Methods

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Clay Slopes and Their Stability: an Evaluation of Different Methods Clay slopes and their stability: An evaluation of different methods Master’s thesis in the Master’s Programme Infrastructure and Environmental Engineering Hannes Hernvall Department of Civil and Environmental Engineering Division of Geology and Geotechnics CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2017 Master’s thesis: BOMX02-17-74 Master’s thesis: BOMX02-17-74 Clay slopes and their stability: An evaluation of different methods Hannes Hernvall Department of Civil and Environmental Engineering Division of Geology and Geotechnics Research group Engineering Geology and Geotechnic Chalmers University of Technology Gothenburg, Sweden 2017 Clay slopes and their stability: An evaluation of different mthods Hannes Hernvall © Hannes Hernvall, 2017. Supervisor: Jelke Dijkstra, Department of Architecture and Civil Engineering Examiner: Jelke Dijkstra, Department of Architecture and Civil Engineering Master’s Thesis 2017:BOMX02-17-74 Department of Civil and Environmental Engineering Division of Geology and Geotechnics Research group Engineering Geology and Geotechnics Chalmers University of Technology SE-412 96 Gothenburg Telephone +46 31 772 1000 Cover: The shear dissipation along the failure surface in a model of a slope. Typeset in LATEX Printed by [Name of printing company] Gothenburg, Sweden 2017 iv Clay slopes and their stability: An evaluation of different methods Hannes Hernvall Department of Architecture and Civil Engineering Chalmers University of Technology Abstract This thesis compare and evaluate slope stability and the methods used to determine it. Slope stability is already an important aspect when it comes to infrastructure projects and with the climate changes and increasing amounts of precipitation will this become even more important in the future. Slope stability is today evaluated with a Factor of Safety (FOS) that is determined with numerical computer code in Geotechnical programs. How these programs work will be looked at in detail. The thesis is limited to clay slopes, 2D models and only the ultimate limit state of the slopes is analysed. A literature study was first conducted to see which methods exist to determine the FOS and to understand the theory behind them. Three geotechnical programs that employs three different methods (Limit Equilibrium Method LEM, Discontinuity Layout Optimisation DLO and Finite Element Limit Analysis FELA) to determine the FOS was then evaluated and compared to each other. First was the methods used to calculate a FOS for one case with a closed-form solution and one case where true solution has been narrowed done to a very small bracket to determine which program had the best accuracy. Then was the result from the method with the most accurate result used as the benchmark to compare the results from the other two methods against for cases that are more complex and realistic. As a final step was the method used as the benchmark used to determine the FOS for a slope with data and parameters from a real case. Of the three methods was the results from the FELA closest to the closed-form solution and was therefore used as the benchmark method. In the more complex cases was it found that the DLO results overall had a smaller discrepancy compared to FELA solutions than the results from LEM. The program using FELA was also able to model seepage and suction and show how the suction could raise the sta- bility of a slope. In the case based on real data and topography was anisotropy also implemented into the model to show how the FOS could be higher than first expected. All three methods was able to determine FOS in a satisfying manner and produced results that were comparable to each other. For the more complex models did the FELA combined with the upper and lower bound meshes it uses show that it holds great potential for geotechnical modelling, since it gives the possibility to bracket the solution and thereby give a margin of error for the model. There are some parts in the program using FELA that should be analysed further in more detail, especially when it comes to including the anisotropy of soft clays into models. Keywords: Slope stability, Slopes, Clay, Limit Equilibrium Method (LEM), Finite Element Limit Analysis (FELA), Factor of Safety (FOS), Slope/w, Optum G2, Limistate:GEO, Discontinuity Layout Optimisation (DLO). v Acknowledgements I want to thank my supervisor and examiner Dr. Jelke Dijkstra for his assistance, comments and support during the work of this thesis. The thesis is to a large part based on his ideas so it could not been done without him. I would also like to thank Prof. Minna Karstunen for her input and Dr. Mats Karlsson who provided both data used for parts of the thesis and suggestion on how to interpret it. Finally i would also like to thank my friends at Chalmers and my family for their support during this thesis. Hannes Hernvall, Gothenburg, June 2017 vii Contents List of Figures xi List of Tables xiii List of Notations xv 1 Introduction1 1.1 Background................................1 1.2 Aim & Objectives.............................2 1.3 Limitations................................2 2 Limit analysis3 2.1 Limit equilibrium.............................3 2.1.1 Slice methods...........................3 2.1.2 Slip surface............................5 2.2 Limit Analysis...............................6 2.2.1 Lower-bound formulation.....................6 2.2.2 Upper-bound formulation....................6 2.2.3 Non-linear optimisation problem.................7 2.3 Discontinuity Layout Optimisation...................7 2.4 Strength Reduction............................8 2.5 Software description...........................8 2.5.1 Slope/W..............................8 2.5.2 Limitstate:GEO..........................9 2.5.3 Optum G2.............................9 3 Basic models 11 3.1 Introduction................................ 11 3.2 Closed-form solution........................... 11 3.2.1 Results closed-form solution................... 11 3.3 Vertical cut................................ 12 3.3.1 Results vertical cut........................ 13 3.4 Discussion................................. 14 4 Complex models 15 4.1 Slope description............................. 15 4.2 Soil parameters.............................. 16 ix Contents 4.3 Results for Case 1 to 4.......................... 17 4.3.1 Case 1............................... 17 4.3.2 Case 2............................... 17 4.3.3 Case 3............................... 18 4.3.4 Case 4............................... 19 4.4 Long term drained............................ 19 4.5 Discussion................................. 20 5 Suction and Anisotropy 21 5.1 Suction and Seepage........................... 21 5.1.1 Introduction............................ 21 5.1.2 Parameters and layout...................... 21 5.1.3 Results seepage and water.................... 22 5.2 Anisotropy................................. 22 5.2.1 Introduction............................ 22 5.2.2 Parameters............................ 23 5.2.3 Results anisotropy........................ 23 5.3 Discussion Suction and Anisotropy................... 24 6 Real case: E45 and Railway Expansion 25 6.1 Background................................ 25 6.2 Soil parameters and lab results..................... 26 6.3 Optum model............................... 28 6.4 Results................................... 30 6.4.1 Ordinary results.......................... 30 6.4.2 Results Monte Carlo simulations................. 31 6.5 Discussion................................. 33 7 Conclusions & Recommendations 35 7.1 Conclusion................................. 35 7.2 Recommendations............................. 35 A AppendixI x List of Figures 2.1 A: Slope with circular slip surface divided into several slices. B: The forces acting on a slice that are taken into account with the Ordinary method...................................4 2.2 A: The forces taken into account for a slice with Fellenius method. B: The forces taken into account for a slice with Spencer’s method..4 2.3 The four steps in the Discontinuity Layout Optimisation process. Fig- ure is based on Figure in Gilbert, Smith, Haslam, and Pritchard (2010).8 3.1 A picture of a lower-bound mesh and the failure surface for a case in Optum G2................................. 14 4.1 Figure of the basic slope at the inclination of 1/3 and the dimensions used for the models............................ 15 5.1 Layout of the slope with 1:1 inclination with water table and pressure head included................................ 22 6.1 Map of the section of E45 motorway and the railway, map taken from maps.google.com.............................. 25 6.2 Diagram of the clays overall undrained shear strength together with the results from triaxial and DSS tests.................. 27 6.3 The cross section of the east bank of Göta river that is to be evaluated. 29 6.4 Failure surface for slope in the real case................. 30 6.5 Top picture shows how the undrained strength normally is distributed while the bottom pictures shows the distribution of the undrained strength in one of the stochastic simulations............... 31 6.6 Results from the Monte Carlo simulations displaying the Load mul- tiplier/ FOS and the Probability Distribution Function for the real case both with and without anisotropy included............. 32 xi List of Figures xii List of Tables 3.1 The parameters for the soil strength and the solutions for the different cases..................................... 11 3.2 Results and how many percent the cases differ from the
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