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Measuring Pore-Water Pressures in Partially Frozen Soils

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Measuring Pore-Water Pressures in Partially Frozen Soils University of Alberta Measuring Pore-water Pressures in Partially Frozen Soils by Mohammadali Kia A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geotechnical Engineering Department of Civil and Environmental Engineering © Mohammadali Kia Fall 2012 Edmonton, Alberta Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission. Dedication To my ever kind, loving parents & To all those who endeavor for the humanity of humans ii Abstract Knowledge of pore-water pressure is essential to predict the ‘effective stress’ that controls the ‘resistance and deformation’ of a soil and to assess the ‘flow’ of water through it. Flow of water towards the freezing fringe controls the amount of frost heave in freezing soils. Drainage of this excess water controls subsequent thaw settlements as the frozen soil thaws. Further, the rate of dissipation of pore-water pressures control the thaw-instability in warming permafrost slopes. Not all of the water in a soil is ice at subzero temperatures; therefore, these soils are ‘partially frozen’. Hence, the challenges associated with measuring pore-water pressure distribution in freezing, thawing, and frozen soils can all be considered as one category: measuring pore-water pressures within a ‘partially frozen soil’. Therefore, measurement of pore-water pressures in partially frozen soils and having methods for estimating the pore-water pressure response to the applied loads are desirable. In this research, first a new instrument was developed to accurately conduct these measurements. Then, the measured pore-water pressures were used to study stress transmission within a partially frozen soil under applied loads, as well as under warming conditions. It was shown that if a sufficient amount of unfrozen water exists in a soil at subfreezing temperatures, it provides a continuous liquid phase that transfers pressures independent from the solid phase. Therefore, effective stress material properties and analysis should be used to evaluate the resistance and deformation of these partially frozen soils. This is of practical significance in analyzing stability and in modeling constitutive behavior of soil masses in warm and warming permafrost, especially for assessing geohazards associated with climate change. It is also of practical significance in analyzing and designing foundations, retaining structures, underground facilities, and frozen-core dams in cold regions. For the first time, measurements of Skempton’s B-bar coefficient in a partially frozen soil at various initial void ratios were presented and compared to that of the unfrozen soil. Thus, by assuming superposition, stress distribution between the soil matrix, pore-ice, and pore-water was evaluated. Further, decrease in load bearing of the ice matrix with increasing temperature was evaluated via measuring pore-water pressure distribution within a partially frozen soil during undrained warming. It was also found that in both the partially frozen and unfrozen states, the pore-water pressure response of overconsolidated sand is bilinear with a well-defined inflection point. Further, it was shown that the change in effective stress under undrained loading is not zero in overconsolidated sand specimens; hence, frictional resistance is not zero under undrained loading due to the stiffer solid phase. Keywords: Skempton’s pore-pressure parameter, effective stress, permafrost, slope stability, cemented soils, rapid response piezometer, rigidity of a piezometer iv Acknowledgement I am most thankful to my Creator, The Merciful, The Beneficent, who gives purpose to the completion of another task. I wish to extend my sincere gratitude to my supervisors, colleagues, and friends, who have always been supporting me in my conquest to make this research possible. Their friendship is an invaluable treasure for the rest of my life. It has been a tremendous honorable privilege and a pleasant experience for me to work with my supervisors Dr. David C. Sego and Dr. Norbert R. Morgenstern. Intellectually stimulant discussions we had, led to great findings as well as to my personal growth. I also extend my special appreciation to Dr. Lukas Arenson for the initial technical discussions at the beginning of this research and to Mr. Stephen Gamble for his priceless technical assistance during the laboratory experiments. I will always remember Mr. Jerry Kowalyk and other staff at the University of Alberta’s Cameron Library for their substantial help in accessing large amounts of multi-discipline literature pertaining to my research. Further, I acknowledge FISO Technologies Inc. for technical assistance with the fiber optic sensors, NSERC for financial support via the NSERC Discovery Grant, and the University of Alberta for financial support through the Provost Doctoral Entrance Award. I am greatly thankful to my defense committee for meticulously examining this research and providing comments that improved the final text of the thesis. Lastly, I wish to extend my most heartfelt thanks to my family, especially to my parents, for their prayers, unwavering support, encouragement, and patience, without whom this research would have never been accomplished. v Table of contents Dedication .............................................................................................................. ii Abstract ................................................................................................................. iii Acknowledgement ..................................................................................................v Table of contents .................................................................................................. vi List of tables......................................................................................................... xii List of figures ...................................................................................................... xiii List of symbols ........................................................................................................1 1. Introduction .....................................................................................................6 1.1 Definition of the problems ......................................................................................................... 6 1.1.1 Background: what triggered this research? 6 1.1.2 Outline 7 1.1.3 Practical demands for research on the mechanics of frozen, freezing, and thawing soils 8 1.1.4 Need for measuring pore-water pressure in partially frozen soils 11 1.1.5 Need for conducting experiments at temperatures close to 0oC and low applied pressures 14 1.2 Description of the study ........................................................................................................... 15 1.3 References ................................................................................................................................ 16 2. Measuring pore-water pressure in partially frozen soils: theory and technical requirements .................................................................................25 2.1 Introduction .............................................................................................................................. 25 vi 2.2 Lessons from measuring pore-water pressure in unfrozen soils .............................................. 27 2.2.1 Measurement of pore-water pressure in unfrozen soils 27 2.2.2 Transmission of pressure states in unfrozen soils 28 2.2.3 Significance of rigidity of piezometer in correctly measuring pore-water pressures 29 2.2.4 Rigidity of the piezometer fluid and pore-water 34 2.2.5 The role of a filter in a piezometer 37 2.3 Measuring pore-pressures in partially frozen soils .................................................................. 42 2.3.1 Significance of continuity of unfrozen water phase 42 2.3.2 Significance of experiments at subfreezing temperatures close to 0 o C and at low confining pressures 43 2.3.3 Previous research on measuring pore-water pressure in soils at subfreezing temperatures 44 2.4 Need for a miniature piezometer for measuring pore-water pressures in partially frozen soils .......................................................................................................................................... 46 2.5 Design of the filter-less rigid piezometer (FRP) ...................................................................... 47 2.5.1 Miniature fiber-optic pressure sensors 48 2.5.2 Mineral oil in FRPs 48 2.5.3 Assembling of the filter-less rigid piezometer (FRP) 52 2.6 Comparison of the pore-water pressure measurements using FRP and ordinary piezometers . 54 2.6.1 Triaxial testing (comparison with electrical pressuremeter) 54 2.6.2 Suction in an unsaturated soil (comparison with tensiometer) 55 2.7 Summary and conclusions ......................................................................................................
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