Liquefaction mitigation of silty soils using dynamic compaction
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Liquefaction of saturated loose granular sand and non-plastic silty sand deposits during seismic loading has caused significant damage to structures and highway systems in almost all major earthquakes. Sand deposits densified by dynamic compaction (DC) are more resistant to liquefaction, and have performed well during earthquakes. Silty sand deposits appear to densify and perform well when improved by DC supplemented with wick drains. The current practice for evaluating feasibility and choosing the operational parameters of the DC technique at a site depends mainly on field trials, past experience at similar sites, and empirical equations based on reported records. Rational analytical methods are needed to improve the state of practice. This dissertation presents an analytical simulation model for the densification process of saturated sand deposits without wick drains, and silty deposits supplemented with wick drains during DC. Pore pressure generated during DC processes is simulated based on an energy based liquefaction model. The densification during dissipation is modeled using consolidation theory. Based on the model effects of silt content, hydraulic conductivity, initial soil density and techniques' operational parameters such as energy per impact, number of impacts per location, impact grid pattern, impact grid spacing, wick drains spacing, and time cycle between impacts on the densification of soils improved by DC have been studied. The model performance has also been verified through documented case histories and found to compare reasonably well. A rational design procedure has been developed for liquefaction mitigation of loose sand and non-plastic silty soils. The design model has been used to determine the densification achievable using DC in silty deposits supplemented with wick drains. A design procedure and design examples are presented. The computational methodology presented herein is a powerful tool for design analyses of DC taking into account the site conditions for different deposits and operational parameters. The model is expected to advance the use of DC in sands and silty soils, and reduce the reliance on expensive field trials as a design tool.