Pile Foundations on Soft Soils

Oceanic windmills largely rely on pile foundations. Photo Credit: Vattenfall Energy Company; Stockholm, Sweden.

Dr. Dominic Assimaki works with numerical methods in earthquake engineering and geophysics, including forward simulations of dynamic nonlinear soil response, soil-structure interaction and scattering phenomena in heterogeneous media, as well as inverse problems. She serves as an assistant professor of geosystems engineering in the School.

Defining the Problem
Pile foundations are primarily used for structures, piers and platforms constructed on loose or soft soils prone to liquefaction and lateral spreading during strong earthquakes. In the approach most widely employed in practice for the design of pile foundations in liquefiable soils, the pile stiffness estimated for stiff, non-liquefiable sites is uniformly scaled via empirical factors that account for the reduction of soil resistance during earthquakes due to liquefaction. As a result, the predicted response of piles in loose, saturated soils is a scaled replica of the response of piles in stiff soils. However, field and laboratory experimental data clearly show that there are significant differences between the two, and that the existing models lack fundamental aspects of pile behavior in liquefiable soils.

A Unique Approach
To bridge the gap between widely employed empirical models and computationally expensive numerical simulations, Dr. Assimaki’s research group developed a macroelement for dynamic analyses of piles in liquefiable soils. This macroelement captures the fundamental physics of saturated granular soil response to dynamic loading, such as dilation and seepage, while retaining the efficiency of simplified approaches for the analysis of dynamic foundation problems. The macroelement components were developed using three-dimensional fully coupled finite element (FE) analyses and validated via centrifuge and field experimental data. The FE simulations used for the parametric investigation and calibration of the macroelement were first validated using field data of blast-induced liquefaction experiments. In addition, the novel mechanical element has been integrated in the open source finite element platform, OPENSEES, used extensively by the international earthquake engineering communities. It has also been used in an NSF-funded project of seismic hazard assessment and mitigation of liquefaction in port waterfront structures.

Huge Impact Potential
Dr. Assimaki’s pile macroelement is advancing the state-of-the-art by efficiently providing realistic predictions of pile displacement in liquefiable soils. It will enable credible and cost-effective design solutions for critical infrastructure projects such as bridge foundations, waterfront structures, and highrise buildings. Currently, Dr. Assimaki’s team is using the macroelement as a building block for novel soil-structure interaction mechanical models of fixed offshore wind turbine foundations. Offshore wind-generated electricity is foreseen as a major contributor to the U.S. energy supply. However, commercialization is extremely cost-prohibitive due to the expense of wind turbine foundations. The foundations alone can account for up to 25% of the total cost of a wind farm. To date, the U.S. has no standards for the design of foundations in the offshore wind industry, but Dr. Assimaki is working to change that. She and her team of researchers envision that the macroelements for offshore wind turbine foundations will lead to cost-effective design solutions in a renewable energy market. Her pending European partnerships will provide the performance and operational data for calibration and validation of the models, giving her research enormous potential to make that vision a reality.

    Additional information:

From left to right, Dr. Assimaki identifies research experiments and findings; pile supported wharf damage due to liquefaction and lateral spreading during the M8.8  2010 Maule Earthquake in Chile.
Photo credit: Geer Association.

Detailed progressive deformation around a pile in cohesionless fully saturated soil. The bottom right deformation reveals the settlement in the vicinity of the foundation due to pile-induced liquefaction.