Microstructure-Based Effective Stress Formulation for Unsaturated Granular Soils
Publication: International Journal of Geomechanics
Volume 16, Issue 6
Abstract
The principle of effective stress states that the strength and volume change behaviors of soil are governed by intergranular forces expressed in terms of a continuum quantity called effective stress. Although the principle of effective stress is regarded as one of the most fundamental concepts in soil mechanics, its applicability to unsaturated soil has been questioned. The central issue is whether a measure can be developed for three-phase soils that plays an equivalent role as the effective stress does for two-phase soils. Combining the techniques of microstructural analysis and image processing, this study formulated the effective stress for unsaturated granular soils. A novel suction-controlled experimental setup was integrated with an X-ray computed tomography (CT) scanning system and used to image and model microstructural features. A tensorial quantity, called the fabric tensor of the liquid phase, that characterized the complex fabric resulting from saturated pockets and networks of liquid bridges was identified and introduced in the proposed formulation. The trend in the variation of the fabric tensor of the liquid phase as a function of suction (or saturation) was captured for both the wetting and drying paths of the partial saturation. It was observed that the fabric tensor of the liquid phase had an intrinsic association with the evolution of the effective stress tensor. It is concluded that, for unsaturated granular soils, consideration of the fabric tensor of the liquid phase is imperative in effective stress formulations.
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Acknowledgments
The work presented here was supported by the National Science Foundation (NSF) under grant CMMI-0856793 to Washington State University and grant CMMI-1308110 to the University of Wisconsin–Madison. This support is gratefully acknowledged. Any opinions, findings, conclusions, and recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.
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International Journal of Geomechanics
Volume 16 • Issue 6 • December 2016
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© 2016 American Society of Civil Engineers.
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Received: Oct 3, 2014
Accepted: Oct 1, 2015
Published online: Feb 10, 2016
Discussion open until: Jul 10, 2016
Published in print: Dec 1, 2016
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