Date of Award

Spring 2023

Project Type

Dissertation

Program or Major

Civil Engineering

Degree Name

Doctor of Philosophy

First Advisor

Majid Ghayoomi

Second Advisor

Jean Benoît

Third Advisor

Yashar Eftekhar Azam

Abstract

Strong earthquake motions often cause severe damage to buildings and foundation systems, during which the interaction between the soil, foundation, and structure may dominate the seismic response. Most shallow foundations are located on, or embedded in, unsaturated and partially saturated soil deposits. Unsaturated soil layers are particularly common in zones above the water table where water can rise through different mechanisms like capillary action. Additionally, the degree of saturation throughout a soil deposit can vary both seasonally and yearly due to groundwater table fluctuation related to infiltration and evaporation. Properties of soil layers below foundations impact the seismic response of structural systems. Since soil moisture impacts soil properties, it is expected that changes in groundwater table depth would impact the seismic response of foundations and structures. However, the understanding of the mechanisms by which the degree of saturation and water table depth influences the foundation and structural response needs improvement. This dissertation aimed to evaluate the effect of the depth of the groundwater table on the seismic response of soil-foundation-structure systems and to extend current seismic design guidelines leading to the implementation of rocking foundations in practice.Three sets of dynamic centrifuge experiments were conducted on four physical models representing three prototype structures. The prototype structures included elastic and inelastic single-degree-of-freedom structures as well as single- and two-span bridge systems. The elastic single-degree-of-freedom structure and bridge systems were designed to incorporate rocking foundations, while the inelastic single-degree-of-freedom structure incorporated structural fuses designed to guide plastic deformations to above-ground structural locations. Physical models were slightly embedded in sandy silt layers with various groundwater table depths and subjected to a series of seismic motions. The experimental findings highlight the influence of the groundwater table depth on changes to the foundation and structural deformations and rotations, foundation-level overturning moments, period lengthening, and damping ratios. Furthermore, design procedures to predict several seismic response properties of a structure resting on unsaturated soil layers are derived in this research based on the fundamentals of unsaturated soil mechanics. These properties include the overturning moment capacity of the foundation, the initial rotational stiffness of the foundation, and the period lengthening and foundation damping ratio. Properties derived from these design guidelines are compared to the experimental results to judge the viability of implementation in practice or signify the need for further improvement.

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