Graduate Thesis Or Dissertation
 

Shear reinforcement effects of discrete columns in liquefiable soils

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  • Discrete columns, such as stone and soil-cement columns, are often used to improve the liquefaction resistance of loose sandy ground. In particular, stone columns are considered to increase resistance potential by densification, drainage, and reinforcement mechanisms. For silty soil, the shear stress reduction resulting from the reinforcing effect of stiffer discrete columns is considered as a main contributing mechanism to increase liquefaction resistance. Although limited studies have been found in literature on shear reinforcement mechanism, the actual deformation patterns of discrete columns and the level of shear stress reduction in soil has not been clarified yet. This study aims to investigate the reinforcing mechanism of discrete columns through numerical simulations and physical model tests focusing on (1) obtaining a better understanding on shear stress and strain distributions between discrete columns and surrounding soil, (2) quantifying the level of shear stress reduction in improved ground, and (3) investigating the effects of discrete columns for liquefaction-induced lateral displacement and settlement In this study, a series of linear-elastic and nonlinear three dimensional (3-D) Finite Element (FE) simulations for unimproved and improved soil profiles were performed covering a wide range of geometries, and ratios of shear moduli for the discrete columns and unimproved soil, and dynamic loadings. It is found that discrete columns and surrounding soil do not behave compatibly in shear (i.e., shear strain incompatibility). The results showed that shear stress reductions are far less than predicted by the assumption of shear strain compatibility. Based on numerical analyses, a new simplified design method was proposed, which better quantifies the level of shear stress reduction improved soil. It is found that for the range of parameters used in this study, shear stress reduction using stone columns in improved soil may not sufficient to prevent liquefaction triggering by shear reinforcement mechanism. Thus, it is recommended that shear reinforcement mechanism of stone columns should be considered as secondary rather than primary mechanism for ground improvement in silty soil. Though stone columns may not help to prevent liquefaction triggering in improved soil, they are found to be effective in reducing post-liquefaction lateral displacement. Dynamic centrifuge tests of two unimproved and two improved models were performed at NEES@UC-Davis, California. In first improved soil model case, the soil-cement columns were resting on top of a dense sand layer simulating free end boundary conditions for soil-cement (floating-type), while in second improved soil model the soil-cement columns were socketed into a cemented sand layer (fixed-type) simulating fixed-base conditions. The results of these centrifuge tests and associated analyses showed that, the soil-cement columns were relatively ineffective in stiffening the soil profile, reducing the cyclic stress ratios induced on the surrounding soils, or reducing the potential for liquefaction triggering. The soil-cement columns with fixed-base conditions were slightly more effective than the columns with a floating-type base condition, but they also did not significantly delay the onset of liquefaction during strong shaking. Analyses based on the assumption of shear strain compatibility, greatly over-estimated the shear reinforcing effects of the soil-cement columns. The soil-cement columns did, however, provide a means for supporting overlying structures even after liquefaction triggering occurred in the improved soil.
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