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Assessment of the Hydro-mechanical Behavior of Undisturbed Collapsible Soils by Conducting Unsaturated Triaxial Tests

Akbari Garakani, Amir | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44934 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Haeri, Mohsen; Habib Agahi, Ghassem
  7. Abstract:
  8. Wide lands in IRAN are covered by the “Hydro – Collapsible” soils. These types of soils possess high apparent shear strength in their natural condition but are highly susceptible to volume collapse upon wetting. This behavior is generally attributed to their open, unstable soil fabric and their weak inter-particle bonding forces, which together yield a soil void structure that is susceptible to collapse.
    Previous studies rarely investigated the roles of stress state level, matric suction and stress path on the hydro-mechanical nature of the collapse phenomenon. Also most studies dealt with conducting laboratory tests on remolded soil specimens through which the role of natural soil fabric was missed in assessments.
    Current study is furnished in the format of a Ph.D. thesis in Civil Engineering Department at Sharif University of Technology, through which the “Hydro – Collapsible” behavior of undisturbed Loessial specimens is investigated using a modified unsaturated triaxial device. For this purpose, an existing triaxial apparatus has been upgraded to a full digital matric suction controlled triaxial device which is capable to control and record the applied pressures as well as the specimen deformations or the degree of saturation. Two types of isotropic loading and shear tests have been performed by applying two different stress paths namely, the “Staged increasing net mean stress under applying the constant matric suction” and the “Staged decreasing the matric suction under applying the constant net mean stresses”. The volume changing behavior of the specimens was steadily monitored as well as the degree of saturation. Test results revealed the “Steplike” collapsible volume change of the soil through loading, which stands as a novel achievement in the filed of unsaturated soil mechanics. Generally, results show reduction in soil stiffness, reduction in shear strength and decrease in soil volume by decreasing the matric suction under applying the constant mean net stress. The influence of stress level on SWRC of the soil has been demonstrated as well as the influence of the applying stress path on the hydro-mechanical collapsible behavior. Test results have been analyzed based on the effective stress approach. In this regard, the effective stress parameter was calculated for the tested collapsible soil and a hardening model was developed to interpret the collapsible behavior. Also the uniqueness existence of the critical state line in shear behavior was proved for both saturated and unsaturated conditions.
  9. Keywords:
  10. Loess Soil ; Collapsible Soils ; Unsaturated Soil ; Soil Mechanics ; Unsaturated Triaxial Device ; Undisturbed Soil Sample ; Triaxial Test ; Shear Load ; Isotropic Loading ; Effective Stress Analysis ; Uniqueness of Critical State Line

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