Xiong Zhang | University of Alaska Fairbanks (original) (raw)
Uploads
Papers by Xiong Zhang
When characterizing an unsaturated soil using the triaxial test apparatus, it is required to meas... more When characterizing an unsaturated soil using the triaxial test apparatus, it is required to
measure the soil deformation during loading. Recently, a photogrammetry-based method has
been developed for total and localized volume change measurements on unsaturated soils during
triaxial testing. In this study, more in-depth discussions on the photogrammetry-based method
are addressed such as system setup, measurement procedure, accuracy self-check, data postprocessing,
and differences from the conventional image-based methods. Also, an application of
the photogrammetry-based method on unsaturated soil deformation measurements is presented
through a series of undrained triaxial tests with different loading paths. After testing, threedimensional
(3D) models of the tested soils at different loading steps were constructed based on
the 3D coordinates of measurement targets on the soil surface. Clear barreling processes for soils
during deviatoric loading were observed through the constructed 3D models at different axial
strain levels. Soil volume changes and volumetric strain non-uniformities during isotropic and
deviatoric loadings were extracted based upon detailed analyses on different soil layers. Through
a full-field strain distribution analysis, a shear band evolution process was captured for the soil
during deviatoric loading at a low confining stress. The photogrammetry-based method proved to
be very powerful for in-depth soil deformation characteristics investigation.
Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutiv... more Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutive modeling of unsaturated soils. However, this type of tests requires sophisticated equipment and is time-consuming. This has been one of the major obstacles to the implementation and dissemination of unsaturated soil mechanics beyond the research context.
In contrast to suction-controlled triaxial tests, the suction-controlled oedometer test requires simpler equipment and a shorter testing period. Oedometer tests represent the at-rest earth pressure (K0) condition, which is an important stress state in any simulation. The major disadvantage of the oedometer test is that its lateral stress is controlled by the condition of zero lateral strain and remains unknown during the testing process. At present, no well-established, simple, and objective methods are available that take advantage of oedometer test results for constitutive modeling purposes.
This paper derives an explicit formulation of the at-rest coefficient for unsaturated soils and develops an optimization approach for simple and objective identification of material parameters in elasto-plastic models for unsaturated soils using the results from suction-controlled oedometer tests. This is achieved by combining a modified state surface approach (MSSA), recently proposed to model the elasto-plastic behavior of unsaturated soils, with the quasi-Newton method to simultaneously calibrate all parameters governing virgin behavior in elasto-plastic models. The Barcelona Basic Model (BBM) is used to demonstrate the application of the proposed explicit formulation and calibration method. Results predicted using obtained parameters are compared with laboratory test results for the same stress paths in order to evaluate the simplicity and objectivity of the proposed method.
International Journal of Geomechanics, May 2011
When characterizing an unsaturated soil using the triaxial test apparatus, it is required to meas... more When characterizing an unsaturated soil using the triaxial test apparatus, it is required to
measure the soil deformation during loading. Recently, a photogrammetry-based method has
been developed for total and localized volume change measurements on unsaturated soils during
triaxial testing. In this study, more in-depth discussions on the photogrammetry-based method
are addressed such as system setup, measurement procedure, accuracy self-check, data postprocessing,
and differences from the conventional image-based methods. Also, an application of
the photogrammetry-based method on unsaturated soil deformation measurements is presented
through a series of undrained triaxial tests with different loading paths. After testing, threedimensional
(3D) models of the tested soils at different loading steps were constructed based on
the 3D coordinates of measurement targets on the soil surface. Clear barreling processes for soils
during deviatoric loading were observed through the constructed 3D models at different axial
strain levels. Soil volume changes and volumetric strain non-uniformities during isotropic and
deviatoric loadings were extracted based upon detailed analyses on different soil layers. Through
a full-field strain distribution analysis, a shear band evolution process was captured for the soil
during deviatoric loading at a low confining stress. The photogrammetry-based method proved to
be very powerful for in-depth soil deformation characteristics investigation.
Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutiv... more Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutive modeling of unsaturated soils. However, this type of tests requires sophisticated equipment and is time-consuming. This has been one of the major obstacles to the implementation and dissemination of unsaturated soil mechanics beyond the research context.
In contrast to suction-controlled triaxial tests, the suction-controlled oedometer test requires simpler equipment and a shorter testing period. Oedometer tests represent the at-rest earth pressure (K0) condition, which is an important stress state in any simulation. The major disadvantage of the oedometer test is that its lateral stress is controlled by the condition of zero lateral strain and remains unknown during the testing process. At present, no well-established, simple, and objective methods are available that take advantage of oedometer test results for constitutive modeling purposes.
This paper derives an explicit formulation of the at-rest coefficient for unsaturated soils and develops an optimization approach for simple and objective identification of material parameters in elasto-plastic models for unsaturated soils using the results from suction-controlled oedometer tests. This is achieved by combining a modified state surface approach (MSSA), recently proposed to model the elasto-plastic behavior of unsaturated soils, with the quasi-Newton method to simultaneously calibrate all parameters governing virgin behavior in elasto-plastic models. The Barcelona Basic Model (BBM) is used to demonstrate the application of the proposed explicit formulation and calibration method. Results predicted using obtained parameters are compared with laboratory test results for the same stress paths in order to evaluate the simplicity and objectivity of the proposed method.
International Journal of Geomechanics, May 2011