MASTER'S THESIS Natural Dams and Internal Erosion Field Study - Lake Hennan Wuhib Zewde Tamrat Master of Science Civil Engineering Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Master’s Thesis in Mining and Geotechnical Engineering Department of Civil, Environmental and Natural Resources Engineering NATURAL DAMS AND INTERNAL EROSION FIELD STUDY - LAKE HENNAN WUHIB ZEWDE TAMRAT 31st May 2011 Preface First and for most, I would like to show my deepest gratitude to my supervisor Dr.Tommy Edeskär for the continuous support and guidance he has given me, and for being there full of support at times when I required motivation and encouragement the most. I would like to thank my professor Sven Knutsson for facilitating me in acquiring a suitable master’s thesis project that best suited my area of interest and generally for being a dominating presence in the time I was carrying out the study. I would like to thank research engineers Thomas Forsberg and Kerstin Pousette for giving me assistance during the laboratory investigation stage of the study and for their invaluable experience, practical ideas and help. Finally, I would like to dedicate all my life achievements, including this masters study, to my father (God rest his sole). I can only see it in my mind, the pride you would have felt to see me standing at this point in my life. And Tet, this is to you too. May you live long enough to see me achieve more, for that is truly the only way I think I can repay you back. i Abstract In dam design, and especially for tailings dams, long term stability is of importance. One process that could endanger a dam construction is internal erosion, i.e. particle transportation by groundwater flow with in the dam that may result in a failure process. This is a known problem in dam construction but is poorly understood for long time perspective. In Sweden, glacial deposits have acted as natural dams for about 20,000 years. This implies that the composition of these deposits in combination with the present hydraulic gradient has been stable in a long term perspective. In this study such a formation, Lake Hennan, has been studied and compared with general dam design guidelines. The study has included a geotechnical field survey and subsequent laboratory investigations. The aim of the field survey was to explore the stratigraphy of the geological formation, collect samples for laboratory testing and collect ground water level observations. The laboratory investigations targeted on classification of the collected samples mainly by sieving. The final conclusion from this study is that the geological formation at Lake Hennan is a natural dam. And it is an esker formation surrounded by lacustrine deposits of finer materials at the base adjacent to the upper and lower lakes. The groundwater observations indicate a maximum hydraulic gradient of 2.4%. Compared to the reviewed dam design guidelines based on gradation curves, the filter and suffusion criteria requirements are met. The permeability criteria based on gradation curves is not fulfilled since the amount of fines is too high. ii Table of Contents Preface ............................................................................................................................................................................. i Abstract .......................................................................................................................................................................... ii Table of Contents ...................................................................................................................................................... iii 1. Introduction ......................................................................................................................................................... 1 2. Scope of study .................................................................................................................................................... 2 2.1 Aim ................................................................................................................................................................ 2 2.2 Goal ............................................................................................................................................................... 2 2.3 Delimitation ................................................................................................................................................ 2 3 Geology ................................................................................................................................................................ 4 3.1 The Quaternary period ............................................................................................................................ 4 3.2 Glacio-fluvial deposits ............................................................................................................................ 4 3.3 Lacustrine deposits ................................................................................................................................... 7 4 Dams ...................................................................................................................................................................... 7 4.1 Purpose ......................................................................................................................................................... 7 4.2 Construction material .............................................................................................................................. 8 4.3 Impounded material ................................................................................................................................. 9 5 Internal Erosion ............................................................................................................................................... 13 5.1 Processes of internal erosion ............................................................................................................. 13 5.2 Criteria for internal erosion ................................................................................................................ 14 5.2.1 Geometric criteria ......................................................................................................................... 15 5.2.2 Hydraulic criteria .......................................................................................................................... 15 5.3 Important factors related to internal erosion ................................................................................ 16 5.3.1 Particle density .............................................................................................................................. 16 5.3.2 Relative density ............................................................................................................................. 16 5.3.3 Hydraulic conductivity ............................................................................................................... 17 5.3.4 Grain size distribution……………………………………………………………………….......…..20 iii 6 Dam Design Criteria ...................................................................................................................................... 21 6.1 Filter criteria ............................................................................................................................................ 21 6.2 Recommended design criteria ........................................................................................................... 23 7 Natural Dam Analogies ................................................................................................................................ 25 7.1 Natural dam formations in Sweden ................................................................................................. 25 7.1.1 Lake Ragunda ................................................................................................................................ 26 7.1.2 Lake Mången ................................................................................................................................. 26 7.1.3 Lake Styggtjärn ............................................................................................................................. 26 7.2 Lake Hennan case study ...................................................................................................................... 26 7.2.1 Site description .............................................................................................................................. 27 7.2.2 Field investigation ........................................................................................................................ 28 7.2.3 Laboratory investigations .......................................................................................................... 32 7.2.4 Analysis ........................................................................................................................................... 37 8 Investigation Results ..................................................................................................................................... 39 8.1 Field investigation results ................................................................................................................... 39 8.1.1 Densometer tests/Field density analysis