Date of Award

5-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Ravichandran, Nadarajah

Committee Member

Andrus , Ronald D

Committee Member

Juang , C Hsein

Abstract

The purpose of this research was to further understand the behavior of pile foundations in unsaturated soils subjected to lateral loading. Recent case histories show the importance of incorporating unsaturated soil mechanics in geotechnical engineering practice for the design and construction of resilient and cost effective systems. Unsaturated soils are a three phase material- solid, liquid and gas, resulting in three interfaces. Among the three interfaces the liquid-gas interface, also known as the contractile skin, plays a critical role in the mechanical and flow behaviors of unsaturated soil. The effects of the contractile skin are measured in terms of matric suction. Though unsaturated soil mechanics can be beneficial in engineering design, the concern of practitioners is the selection of the appropriate value of matric suction for the site based on rainfall and infiltration data. In addition to studying the behavior of pile foundations in unsaturated soils a method is proposed for characterizing an unsaturated soil profile for a site based on reliability concepts using a sample shallow foundation design.
The research is divided into two types of lateral loading on piles in unsaturated soils: static cyclic loading and dynamic earthquake loading. A typical long and slender bridge pile in unsaturated soils is studied using geotechnical centrifuge modeling and finite element modeling. The development of a centrifuge model and test procedure for studying unsaturated soil-pile coupling behavior is developed. The soil container on the centrifuge is divided into three regions to allow for a cost effective way to collect a lot of data on a destructive model. The first region tests the dynamic response of a bridge pile with a representative superstructure at the top subjected to a given base motion, the second region tests the behavior of bridge pile subjected to slow cyclic loading, and the third region records the free field dynamic response for the applied motion at the base of the model. The steady state infiltration method was performed to create uniform degree of saturation profile while the centrifuge is spinning. Tests were conducted at two different degrees of saturation. Also, a dry test and a fully saturated test were also conducted for the purpose of comparing with the unsaturated responses. Data was collected on these four centrifuge tests for the three regions. Comparisons of responses are made between one dry sample and two different unsaturated soil profile samples. Overall the centrifuge tests provided useful data and the lessons learned from the test procedure will be applied to future physical models.
The dynamic behavior of a bridge pile in unsaturated soil is simulated using an improved simplified finite element model, which incorporates the Rayleigh damping model into the formulation. In the modeling, the stress-strain behavior of the soil is modeled using an elastoplastic constitutive model for unsaturated soil based on a bounding surface concept. The pile is modeled by Timoshenko beam elements using a linear elastic model. The response of the pile and the soil is investigated at three initial degrees of saturation. The results show that the coupled soil-pile interaction is not largely affected by the range of initial degrees of saturation in this study.
Since there is still a significant amount of work to create deterministic equations, p-y curves, and numerical models for laterally loaded piles in unsaturated soils. The method proposed for characterizing an unsaturated soil profile for a site using reliability methods was tested with a shallow foundation. The method uses Monte Carlo simulation to determine the bearing capacity of a footing using a semi empirical equation. The matric suction term in the equation is solved for using data from the U.S. National Climatic Data Center and U.S. Geological Survey National Water Information System. The results show increases in bearing capacity using the new method with factors as large as 2.7 times the capacity compared to deterministic approaches using saturated soil parameters. The paper also discusses the effect of the depth factor on the new dominating cohesion term in the bearing capacity equation. The results show that an increase in footing size results in smaller factors of increase in bearing capacity as suction increases the value of the cohesion term.

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