Date of Award

1-1-2012

Document Type

Campus Access Thesis

Department

Civil and Environmental Engineering

First Advisor

Chunyang Liu

Abstract

It has been tacitly assumed that liquefaction does not occur in unsaturated soils during seismic events, because pore air behaves as a cushion and excess pore water pressure is difficult to accumulate. During recent earthquakes, some slopes composed of unsaturated soils experienced large deformation similar to fluid flow. One explanation for this phenomenon is that the unsaturated slopes completely lost their effective stress and reached a state of liquefaction. The field observation shows controversial phenomenon against current understanding on soil liquefaction. This work was motivated to solve this controversy by experimentally studying the following two questions: 1) are unsaturated soils liquefiable? and 2) how do the initial conditions, including relative density, effective confining pressure, and degree of saturation affect the liquefaction potential of unsaturated soils?

To answer the above questions, a series of strain-controlled undrained cyclic loading triaxial tests on saturated and unsaturated Nevada sand were conducted. The index properties studied included particle size distribution, maximum and minimum void ratios, and specific gravity. To provide data for future numerical modeling on unsaturated Nevada sand, hysteretic soil water characteristics curves under different relative densities were also measured. For triaxial tests on saturated Nevada sand, the effects of initial relative density (i.e. Dr=30%, 50%, and 70%) and effective confining pressure (i.e. ó_c0^'=50 kPa, 100 kPa, and 200 kPa) on soil liquefaction were studied. For unsaturated soil tests, besides initial relative density (Dr=50%) and effective confining pressure (ó_c0^'=100 kPa), the effects of initial degree of saturation (S_r0= 90%, 95%) on liquefaction were also investigated.

For saturated Nevada sand, the liquefaction potential decreased with an increase of relative density and effective confining pressure. When the other initial conditions were the same, the cycles needed to make the specimen liquefy increased with the relative density. For the same other initial conditions, the number of cycles required to make the saturated specimen liquefy increased with an increase in the effective confining pressure.

For unsaturated Nevada sand, the liquefaction potential generally decreased with an increase in effective confining pressure and an increase in relative density. When the initial degree of saturation was 95%, about 180% and 70% more cycles were needed to reach liquefaction for the loose Nevada sand (Dr=30%) and the dense Nevada sand (Dr=70%), respectively, if the effective confining pressure increased from 50kPa to 200kPa. When the initial degree of saturation was 95% and the effective confining pressure was 50kPa, about 45% more cycles were needed to make the specimen liquefy if the relative density changed from 30% to 50%,. When the confining pressure was 200kPa and the degree of saturation was 95%, the relative density did not play a significant role in effecting liquefaction of Nevada sand.

For the loose Nevada sand, the liquefaction potential decreased with a decrease in degree of saturation. When the confining pressure was 50kPa and the degree of saturation was 90%, the number of cycles required to liquefy doubled compared to the saturated case. When the effective confining pressure was 200kPa, twice of the number of cycles were needed for the sand to liquefy when the degree of saturation was decreased by 5%. For the dense Nevada sand, the degree of saturation did not play an important role on the number of cycles required to reach liquefaction under lower effective confining pressure (50kPa). However, when the effective confining pressure was increased to 200kPa, the number of cycles for liquefaction was significantly increased with the decreasing degree of saturation. When the degree of saturation was 90%, the 70% relative density specimen experienced 2.0% axial strain without liquefying.

Rights

© 2012, Jiting Xu

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