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The Effect of Fines Content and its Type on Shear Wave Velocity and Liquefaction Resistance of Sandy Soils

Akbari Paydar, Nima | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45600 (09)
  4. University: Sharif University
  5. Department: Civil Engineering
  6. Advisor(s): Ahmadi, Mohammad Mehdi
  7. Abstract:
  8. Liquefaction is one of the most destructive geotechnical phenomena, and so the determination of soil liquefaction is very important. Shear wave velocity measurement is one of the methods of the soil liquefaction potential evaluation. The use of this method has increased substantially in recent years, due to its advantages compared to other methods, especially for microzonation of liquefaction potential. However, proposed curves for liquefaction evaluation which are based on the shear wave velocity measurement are more conservative than curves based on SPT and CPT, especially for sands containing fines. Local information from sites that have been liquefied previously and the shear wave velocities have been measured on them before the occurrence of liquefaction is very limited. Hence, liquefaction can only be judged from surface observations such as sand boils, or large settlements. Factors such as the existence of a superficial non-liquefiable thick layer would prevent the appearance of these surface indications. The lack of information about the type and percentage of fine materials, and the lack of awareness about whether the soil layer is fully saturated or not, can severely affect this local information. Most of the in-situ shear wave measurements used in this method are post earthquake properties, and do not exactly reflect the initial state of the soil before earthquake.
    Several studies have been conducted to establish a correlation between shear wave velocity and liquefaction potential; however, the accuracy of provided curves for correlate the liquefaction resistance with shear wave velocity decreases with the increase in the fines content. Also, researchers have reported different results on the effect of fines on the liquefaction resistance (regardless of the shear wave velocity). On the other hand, the results concerning the effect of fines on the shear wave velocity are also limited. One of the weaknesses of the existing correlations for evaluating liquefaction resistance from shear wave velocity is the lack of considering the effect of fines type (fine plasticity) on the provided curves. It is worth noting that, no research has been done on the effect the fines type on the correlation between liquefaction resistance and shear wave velocity; so that, on current proposed curves, only the amount of fines is taken into account and so the influence of fines type has been inadequately addressed.
    Given these issues, it is necessary to conduct an independent study to examine the effects of fines of plastic or non-plastic type on the liquefaction resistance and shear wave velocity of sand.
    The main objective of this study was to investigate the effect of fines content and fines type on the shear wave velocity (and also small-strain shear modulus), the liquefaction resistance of sands and the correlation of these parameters. There are different methods to establish a correlation between shear wave velocity and liquefaction resistance. The most controllable method is the correlation establishment based on laboratory tests, which is used in this thesis. Liquefaction resistance of materials having different densities has been obtained using cyclic triaxial tests and shear wave velocity has been measured using bender elements tests. In order to measure the shear wave velocity and liquefaction resistance on a single sample, the bender elements were assembled on the cyclic triaxial apparatus and shear wave velocity has been measured several times before conducting the cyclic triaxial test. A Semi-empirical, semi-analytical method was proposed to correlate the liquefaction resistance and the shear wave velocity and using the results of conducted tests and the proposed method, the correlation between shear wave velocity and liquefaction resistance is obtained for all the tested soils. Clean sand, silty sand, sandy silt and clayey sand materials with different percentages of fines have been used as test materials. For this purpose, five different types of materials consisted of two types of clean sand, silt and two types of clay have been used. in addition to the tests conducted in this study, similar results for other soils have been collected from different papers. Using the method presented in this study, the correlations between shear wave velocity and liquefaction resistance of these materials are also obtained. Based on the research findings, the liquefaction resistance, the small-strain shear modulus and the correlation between shear wave velocity and liquefaction resistance of sand containing fines depend both on the amount of fines and the fines type. Also, based on the results obtained in this study, it is found that the correlation between shear wave velocity and liquefaction resistance of the soil is soil specific. Finally, using these results and the results of similar previous studies, an alternative method is proposed for liquefaction evaluation based on shear wave velocity.
  9. Keywords:
  10. Liquefaction Resistance ; Sands ; Shear Waves Velocity Estimation ; Correlation Between Shear Wave Velocity and Liquefaction Resistance ; Fines Amount ; Fines Type

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