Date of Award

Spring 2012

Project Type

Dissertation

Program or Major

Civil Engineering

Degree Name

Doctor of Philosophy

First Advisor

Pedro de Alba

Abstract

Liquefaction flow slides triggered by earthquakes or heavy rainfall in saturated granular soils have produced great damage in landslide-prone areas worldwide. A major aspect that needs more study is how the 'residual strength' remaining in the liquefied material evolves at the high strain levels imposed as the slide progresses. Strength of liquefied granular soils is usually studied in the lab by means of the triaxial test, since the strains required to trigger liquefaction are low, compared with those observed after it has been produced. More sophisticated devices are necessary in order to apply the high strains and shear strain rates that could replicate those of typical flowslides: In particular, these are required to investigate the behavior of the resistance of the flow, which is termed residual undrained strength (Sur). Preliminary tests by de Alba and Ballestero (2004) with a modified version of the triaxial cell suggested that the residual strength was not a constant number, but depended on the velocity at which the liquefied soil was being sheared (i.e., the shear strain rate). However, in order to be able to control the strains and the shear strain rates, a more sophisticated machine is necessary: the ring shear device (RSD). The RSD is designed to apply a horizontal shearing stress (cyclic or monotonic) to a ring-shaped granular soil sample. This permits the application of very large total strains and controlled strain rates to the specimen. An RSD was designed and built at the University of New Hampshire with National Science Foundation support. A testing program using the current version of the RSD was carried out using a fine uniform sand, "Holliston sand". Results suggest that the residual strength is rate-dependent and that the data can be interpreted using the Herschel-Bulkley model. This model implies that shearing resistance increases with strain rate, but that the increase diminishes in an exponential fashion (i.e. flattens out) at high strain rates. Finally, data were compared with results from other RSD's and with data obtained from liquefaction case histories Seed and Harder (1990); the latter provided a reasonable match with residual strengths from this study.

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