Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Doctor of Philosophy

First Advisor

Majid Ghayoomi

Second Advisor

Jean Benoit

Third Advisor

Ricardo Medina

Abstract

Accurate prediction of seismic ground response plays a vital role in earthquake-resistant, sustainable, and cost-effective design of infrastructure. Seismic response of unsaturated soil layers may differ from that of saturated or dry soil deposits. Importantly, surficial soil layers are often partially water-saturated, however, site-specific design procedures only consider ground in either dry or fully saturated conditions. Further, seasonal water table fluctuation introduces additional uncertainty in site-specific response analysis. This research investigates the effect of soil-water interaction on seismic site response of soil layers both experimentally and numerically. To that end, initially, preliminary numerical analyses were performed using the DEEPSOIL software considering uniform suction profiles where partial saturation was considered by changing the unit weight and dynamic soil properties as a function of degree of saturation. Further, a set of centrifuge experiments were conducted to study the influence of partial saturation on seismic response of sand layers under scaled Northridge and Kobe earthquake motions. Steady state infiltration was implemented to control and provide uniform degrees of saturation profiles in depth. Partial saturation led to higher surface-to-base motion intensity amplification than dry conditions; especially in low period ranges. On average, the amplification of peak ground acceleration was inversely proportional to degree of saturation at surface, demonstrating higher amplifications at lower degrees of saturation. It also varied with depth, showing higher values for unsaturated conditions near the ground level. The lateral deformation and surface settlement of partially saturated sand layers with higher stiffness were generally lower than that in dry soil. Although neglecting the effect of partial saturation in sand layers might be conservative with respect to seismic deformations, it may result in underestimating the surface design acceleration spectra. To assess the uncertainty in site response analysis due to water table fluctuation, further parametric analyses were performed using DEEPSOIL program. Water table fluctuations considerably affected the seismic response of sand and silt layers, with a more significant divergence in sandy soils. Variations in small-strain shear modulus and unit weight, due to water table fluctuation, led to considerable change in site response, while the change in damping and modulus reduction factor caused insignificant difference in acceleration amplification, mainly due to the incompetency of their formulations to account for partial saturation.

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