Date of Award

Summer 2019

Project Type

Dissertation

Program or Major

Civil Engineering

Degree Name

Doctor of Philosophy

First Advisor

Majid Ghayoomi

Second Advisor

Majid Ghayoomi

Third Advisor

Jean Benoit

Abstract

While soils below the groundwater table are fully saturated with water, soils above the groundwater table are partially saturated, as water can rise above the groundwater table through different mechanisms such as capillary rise. In the literature, these soils are commonly referred to as unsaturated soils. The groundwater table fluctuates seasonally; thus, soil moisture profiles may continuously change during a year. Foundations of a considerable portion of structures are placed on the surface or shallowly embedded in soils, which are unsaturated. Properties of soils below the foundations can significantly alter the seismic response of the structures. Since the soil moisture may impact these soil properties, it is expected that the fluctuation of the groundwater table would influence the seismic response of the surface structures. This dissertation evaluates the effects of the depth of the groundwater table on the seismic response of soil-foundation systems.

Three sets of seismic centrifuge experiments were conducted to assess the effects of structural and foundation masses as well as inertial interaction on kinematic transfer functions when the specimens were excited with a suite of seismic motions. Three physical models, including a structure-foundation system, a single foundation, and a single light foundation, were tested. Dry sandy soil specimens were prepared in a laminar container; then the physical models were placed on the soil surface and tested atop in-flight shake table inside a geotechnical centrifuge. Lateral and rocking transfer functions, as well as incoherence parameters, were estimated. Results show that kinematic interaction can be captured better with the single foundation physical models when the effect of inertial interaction is reduced from the soil-structure seismic response.

One of the foundation physical models was tested in another set of dynamic centrifuge experiments, while it was placed on layers of sandy and silty sand soil layers with various groundwater tables. A set of experiments was also conducted on dry soil layers. The groundwater level was lowered during the centrifugation to stimulate the capillary rise process. The soil specimens were excited with a series of scaled earthquake motions. The results indicate that as the groundwater level was lowered, the soils became stiffer leading to lower free field and foundation settlement, lower maximum lateral soil deformation, lower mean period of the free field motion, higher strain-dependent natural frequency of the soil layers, and higher seismic soil amplification factors. In addition, incoherence parameter of lateral kinematic interaction was increased while the one associated with rocking kinematic interaction did not follow a clear trend.

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