Using Modified State Surface Approach to Study the Hydro-Mechanical Behavior of Unsaturated Soils


Studying the hydro-mechanical behavior of the unsaturated soils is one of the critical challenges in the geotechnical engineering field. Many researchers developed elastoplastic models for unsaturated soils in which special attention was paid to the water phase modeling. However, most of these models have limitations in terms of compatibility of the elastoplastic behavior of the soil skeleton, water phase, and air phase. Neglecting the compatibility between these three phases led to conflicting predictions for the behavior of the unsaturated soil. The coupling effect between the mechanical and hydraulic behavior of unsaturated soils can be clearly seen under undrained loading conditions since the soil suction variations is not externally controlled, but internally determined by the coupling requirement that water content is constant. Therefore, a constitutive model is required to predict the coupled hydro-mechanical behavior of unsaturated soils under undrained loading conditions in a consistent way. The modified state surface approach (MSSA) was first developed to study the unsaturated soils' mechanical behavior only. Later, it was extended to consider the hydro-mechanical behavior. In this paper, the ability of the MSSA to predict the soil behavior under undrained loading condition was demonstrated. Therefore, the coupled MSSA is evaluated by comparing its predictions against the experimental results for unsaturated soils collected from the literature. A very good agreement (coefficient of determination is greater than 99%) was reached between the measured and predicted void ratios and degree of saturation.

Meeting Name

Geo-Congress 2020: Modeling, Geomaterials, and Site Characterization (2020: Feb. 25-28, Minneapolis, MN)


Civil, Architectural and Environmental Engineering

Research Center/Lab(s)

Center for Research in Energy and Environment (CREE)

Keywords and Phrases

BBM; Constitutive Modeling; Hydro-Mechanical Behavior; Modified State Surface Approach (MSSA); Unsaturated Soils

International Standard Serial Number (ISSN)


Document Type

Article - Conference proceedings

Document Version


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© 2020 American Society of Civil Engineers (ASCE), All rights reserved.

Publication Date

01 Feb 2020