Study on time-dependent stress-strain behaviors of clayey soils

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Study on time-dependent stress-strain behaviors of clayey soils

 

Author: Tong, Fei
Title: Study on time-dependent stress-strain behaviors of clayey soils
Degree: Ph.D.
Year: 2012
Subject: Clay soils -- Testing.
Strains and stresses.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xiii, 235, xxvii leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2530094
URI: http://theses.lib.polyu.edu.hk/handle/200/6706
Abstract: The stress-strain behaviors of saturated clays are time-dependent. Thus it is essential to take into account the viscous properties of soils in geotechnical engineering practice. This thesis mainly focuses on both experimental and constitutive modeling studies of the time-dependent stress-strain behaviors of two clayey soils. In the experimental study, the time-dependent stress-strain characteristics including creep, swelling, strain rate effects, and relaxation behaviors of Hong Kong Marine Deposits (HKMD) and Sand Mixed Bentonite (SMB) samples are investigated. In this thesis, "creep" means viscous deformation under constant effective stress; while "swelling" means viscous expansion under constant effective stress and is a reverse behavior of "creep", for saturated expansive soils. Firstly, the time-dependent behaviors of the SMB soils are examined by Multi-Stage Loading (MSL) oedometer tests, Constant Rate of Strain (CRSN) oedometer tests, and step-changed undrained triaxial CRSN tests. From the test data, creep, swelling and strain rate effects are observed and discussed. In MSL oedometer tests, it is found that SMB soils exhibit considerable viscous behaviors which are stronger time-dependent under lower stress level, but less obvious under higher stress condition. The sand content exerts an indispensible impact on soil behaviors in terms of compression and time (or strain rate)-dependency. In CRSN tests, the strain rate effect is examined by step-changed strain rate, in both oedometer and undrained triaxial conditions. At a given rate in a CRSN oedometer test, the higher the strain rate, the higher the effective stress. In addition, results of step-changed undrained triaxial CRSN tests indicate both the deviator stress and excess porewater pressure are influenced by strain rate. The impacts of sand content on the effective stress and porewater pressure are also discussed. At a given axial strain and a given confining pressure, the CRSN curves of SMB specimens with higher sand content have higher deviator stress and porewater pressure.
Secondly, the time-dependent behaviors of the HKMD soils are examined using MSL oedometer tests, 1D relaxation tests (in both loading and unloading stages) and settling column tests. In MSL oedometer tests, both the creep and swelling behaviors of HKMDs are less obvious compared with SMBs. In 1D relaxation tests, the viscosities of HKMDs are observed in both creep and swelling regions. Different responses in effective stress under relaxation are found in loading and unloading process and these prove that both creep and swelling are related to the viscous behaviors of HKMD soils. The time-dependent large-strain behavior is studied by settling column tests of HKMDs. By normalizing parameter in settling stage, a unique complete relationship between effective stress and strain is established. Test results reveal that the settling curve and settling rate are significantly affected by the solid phase in self-weight consolidation. After the primary self-weight consolidation, the settling rate is reduced in the "secondary self-weight consolidation". In the constitutive modeling of the time-dependent stress-strain behaviors, based on the concept of "reference time line" and "equivalent time", the previous Elastic Visco-Plastic (EVP) model by Yin and Graham (1989, 1994 and 1999) is extended to a new Elastic Visco-Plastic model considering swelling (EVPS model). The swelling here denotes the volume expansion under constant load for saturated soils. The model considers the contribution of swelling strain rate to the total strain rate, so that the unloading/reloading loop can be simulated. The 1D EVPS model is calibrated by the oedometer test in 1D straining, and then the model is verified by simulating the stress-strain behaviors of SMBs in CRSN oedometer tests and 1D relaxation tests. Simulated results agree well with the test data. The 1D EVPS model is also valid in predicting all the time-dependent (or strain-dependent) behaviors of a soil element under any loading condition in 1D straining, such as constant rate of stress (CRSS) loading, unloading/reloading, swelling pressure, etc. Based on the classical plasticity and the framework proposed in Yin and Graham (1999), a 3D EVPS model is developed. Eight parameters are needed to construct the model for a specific soil. Once all the parameters are determined by isotropic consolidation tests, this model can predict the time-dependent soil behaviors. Proper estimations of viscous parameters for both creep and swelling are essential to the 3D EVPS model. The 3D EVPS model has been adopted to simulate triaxial undrained CRSN tests on different SMB specimens. The simulated results generally agree well with the test data. The unloading/reloading loop can be reproduced. This 3D EVPS model is valid in describing all the viscous soil behaviors in 3D stress condition.

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