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Effect of Initial Static Shear Stress on Liquefaction Resistance of Gravelly Soils Using Medium-Size Cyclic Triaxial and Simple Shear Tests

Nikoonejad, Khashayar | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 55908 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Haeri, Mohsen
  7. Abstract:
  8. There are several case history records of liquefaction in gravelly layers with more broadly-graded particles than uniform sands which have been typically examined. Liquified deposits, in many cases, were subjected to initial static shear stress associated with the sloping ground condition or the adjacent shallow foundations prior to liquefaction. There are several graphs and recommendations for practitioners to consider the effects of the initial static shear stress on liquefaction resistance in design processes, developed based on the results of experiments on uniform clean sands. However, data for the effect of the initial static shear stress on the liquefaction strength of gravelly soils to set up a large enough database, similar to that of sands, is limited. The loading mode effects have been widely studied on sands and some correlation ratios have been presented between the liquefaction resistance of triaxial and simple shear loading modes. However, no previous study has investigated the effects of the loading modes from the two common devices (triaxial and simple shear) on the cyclic behavior of a single gravelly soil. Liquefaction resistance and cyclic behavior of a well-graded gravelly soil with an average relative density of 37% (prior to cyclic loading), consolidated under confining pressure (or vertical stress) of 100 kPa, and subjected to different values of initial static shear stress are investigated in this research. Two sets of cyclic tests are conducted using medium-size cyclic triaxial and simple shear devices so as to assess loading mode effects. Membrane penetration has a high potential of occurrence during undrained triaxial tests on gravel-size particles. In addition, the tested gravelly soil with well-graded particles had a high potential for particle segregation during the specimen preparation process for triaxial and simple shear testing. Therefore, approaches are utilized in this study to minimize the negative effects of the phenomena on testing results. Results showed that two different cyclic behaviors, with features similar to cyclic liquefaction and cyclic mobility presented to identify sands behavior, can occur in triaxial and simple shear tests depending on the cyclic and static loading combination. Furthermore, a new cyclic behavior with transitional behavior between cyclic liquefaction and cyclic mobility is observed under specific loading combinations in triaxial tests. The results of the study implementing cyclic simple shear tests indicate that as the α parameter, the ratio of initial static shear stress to associated vertical stress (or confining pressure), increases, the cyclic resistance of the tested gravelly soil, within the range of applied α, decreases. However, the variation of the cyclic resistance extracted from cyclic triaxial experiments seems to be dependent on the level of α and its associated stress reversal condition. The variation of K_α, the ratio of the cyclic resistance of a specimen with initial static shear stress to that of a similar specimen without the initial static shear stress, with α for the tested gravelly soil with sub-rounded particles implementing cyclic triaxial tests is comparable with those observed by others for semi-angular to angular sands with similar relative densities and initial conditions. In addition, the observed values of excess pore water pressure ratio (r_u) at failure for the tested gravelly soil are about 0.85 under both triaxial and simple shear loadings, unlike those from existing data on sands with r_u=1.0 at liquefaction or failure
  9. Keywords:
  10. Liquefaction ; Initial Static Shear Stress ; Cyclic Triaxial Test ; Cyclic Shear Test ; Membrane Compliance ; Particles Separation ; Well-Graded Gravelly Soil

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