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A Numerical Study of Piezocone Penetration test (CPTu)in Saturated Soils under Partially Drained Conditions

Mashinchian, Mohammad Javad | 2024

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 57565 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ahmadi, Mohammad Mehdi
  7. Abstract:
  8. The Piezocone penetration test is a common geotechnical field test used to assess soil properties. In interpreting the CPTu field measurements, soil drainage conditions are mostly considered completely drained or undrained; however, partial drainage conditions govern such soils as silts or clayey sand mixtures. The studies show that neglecting partial drainage conditions causes soil consolidation characteristics to be underestimated. Most studies have been conducted using calibration chambers and centrifuge tests on clayey soils. Due to the complications in modeling the piezocone test, few numerical studies have been performed in partially drained conditions, especially on coarse-grained soils. Among the challenges of numerical modeling of CPTu, one can mention the difficulty of modeling soil structure under large strain scenarios and complicated soil-water interaction behavior. In the first part of this study, using the advanced hypoplastic constitutive model and finite element method, piezocone penetration tests were modeled. The behavior of Firoozkooh sandy soil at different drainage conditions and relative densities was analyzed. Then, the effect of cone penetration on the surrounding soils was discussed. It was shown that drainage conditions and the soil relative density significantly affected the trend of changes in excess pore water pressure generated around the piezocone. The dissipation test is a widely used field test for studying the consolidation behavior of soils under different drainage conditions. However, there are challenges in determining the soil consolidation coefficient by this test. One complication is the presence of partial drainage conditions. Additionally, non-standard dissipation curves add complexity to the interpretation of soil consolidation behavior. These two challenges complicate the interpretation of soil consolidation characteristics. To address these issues, in the second part of this study, an advanced hypoplastic constitutive model was employed to numerically investigate the piezocone penetration process and dissipation test in silty sands. Different drainage ranges were determined using the obtained backbone characteristic curve. Then, different types of non-standard dissipation curves were studied, and the factors contributing to their formation under partially drained conditions, different non-plastic silt content, and different dissipation depths were discussed. In the third part of this study, finite element numerical analysis was used to determine the soil consolidation coefficient under different drainage conditions in clayey soils, employing the Cam Clay constitutive model. The results of the piezocone test were verified against the existing laboratory measurements. The drainage characteristic curve of the clayey soil under study was determined. The variations of excess pore water pressure in the radial and vertical directions of the piezocone were numerically examined at various depths and drainage conditions. Lastly, a practical approach for estimating the soil consolidation coefficient was proposed, considering the aforementioned limitations. By considering different drainage conditions and utilizing advanced numerical techniques, the proposed method offers a more reliable estimation that can be applied in geotechnical engineering practice
  9. Keywords:
  10. Excess Pore Water Pressure ; In-Situ Piezocone Penetration (CPTu) ; Hypoplastic Constitutive Model ; Partial Drainage ; Non-Standard Dissipation Curve ; Saturated Soil ; Finite Element Method ; Granular Soil

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