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Title:Meaning, measurement, and field application of fully softened shear strength of stiff clays and clay shales
Author(s):Wang, Cai
Director of Research:Mesri, Gholamreza
Doctoral Committee Chair(s):Mesri, Gholamreza
Doctoral Committee Member(s):Stark, Timothy; Long, James; Marshak, Stephen
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):fully softened shear strength
slope stability
triaxial compression tests
stiff clays and clay shales
Abstract:The mobilized shear strength in the failure of natural and cut slopes is important for design and stability evaluation. The intact shear strength is rarely available at the time of first-time slope failures. It is generally accepted that only residual shear strength is available for the reactivated landslides. Although fully softened shear strength mobilized in first-time slope failures has been recognized for decades, a comprehensive investigation on fully softened shear strength has not been made, which consists of its meaning, measurement, and field application. This study aims to clarify the meaning of fully softened shear strength, to examine the best procedure for its measurement in laboratory tests, and to generalize its field application. In this study, an empirical correlation for secant fully softened secant friction angle has been developed at 7 effective normal stresses, for a wide range of clay compositions characterized in terms of plasticity index from 10% to 250%, based on consolidated triaxial compression tests on remolded, normally consolidated specimens of 15 stiff clay and clay shale compositions. The fully softened shear strength data interpreted from laboratory tests reported in the literature on reconstituted, normally consolidated specimens prepared from 71 materials, including soft to stiff clays, clay shales, mudstones, and clay minerals, were also collected and compared to the empirical correlation for fully softened shear strength developed in the present study. An examination was made for the potential dependence of laboratory measurements for fully softened shear strength on specimen preparation procedure, mode of shear, and drainage condition during shear. Fully softened shear strength mobilized in full-scale situations was also estimated by means of back-analyses of 63 first-time slope failures, in geologic materials including soft to stiff clays, clay shales, mudstones, marls, clayey silt, and glacial clays, using the observed slip surface or possible slip surface determined based on adequate field observations, together with the observed or assumed groundwater pressure conditions reported in the original references. In summary, fully softened shear strength is defined at a random, predominantly edge to face arrangement and interaction of clay particles in a destructed stiff clay or clay shale composition. In the field, though geological weakening may bring stiff clays and clay shales to the fully softened condition, it is not a prerequisite as fully softened condition results from progressive deformation along a global slip surface. In triaxial compression of normally consolidated specimens, reconstituted at a liquidity index near unity, a random arrangement of clay particles is mobilized at failure. For first-time slope failures in stratified deposits including stiff clays, clay shales, and mudstones, fully softened shear strength is mobilized on the back-scarp of the slip surface, while residual shear strength is available on the horizontal or subhorizontal segments. For first-time slope failures in unstratified deposits including 1) soft to firm clays of low to medium plasticity excluding soft clays of Eastern Canada, 2) stiff clays or clay shales with significant amount of calcium carbonate and quartz content, 3) clayey silts including loess, and 4) glacial clays, the mobilized shear strength on the entire slip surface is equal or greater than the fully softened shear strength.
Issue Date:2019-04-08
Rights Information:Copyright 2019 Cai Wang
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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