This award is an outcome of the NSF 08-519 program solicitation "George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research (NEESR)" competition and includes the University of Oklahoma (lead institution), Iowa State University, San Jose State University (a predominately undergraduate institution), Earth Mechanics, Inc., and Advanced GeoSolutions, Inc. This project will utilize the NEES equipment sites at the University of California, Davis and the University of California, Los Angeles.
Pile foundations are an integral part of many civil engineering structures. The seismic behavior of pile foundations is a very complex problem with interactions between soils (solid skeleton, pore water, and pore air), piles, and superstructure. This complexity is further exacerbated when weak soils such as soft clays and liquefiable loose sands surround the pile foundation. The behavior of pile foundations in liquefiable sands has been studied extensively; however, similar investigations for soft clays or seismic response of piles in improved soils have been rarely performed. The current seismic design practice calls for avoiding inelastic behavior of pile foundations by restricting their lateral displacements because it is difficult to detect damage to foundations following an earthquake. Limiting the lateral displacement of a pile foundation is relatively easy to achieve in competent soils. In the case of weak soils, the current practice is to use an increased number of more ductile, larger diameter piles that are difficult to design and expensive to construct. An innovative, and perhaps more cost-effective, solution to this problem is to improve the soil surrounding the pile foundation. For structures undergoing seismic retrofit with existing pile foundations in weak soils, in certain instances, improving the soils may be the only option to improve the seismic behavior of the foundation. This technique is not widely used in seismic regions due to lack of fundamental understanding of the behavior of improved and unimproved soils and the interactions between them as well as with the piles during earthquakes. As a first step in a long term objective of understanding and improving the seismic behavior of pile foundations in all weak soils, the proposed research will focus on soft clays. Soft clays are quite prevalent in earthquake prone areas of the U.S., but have received little attention from the research community.
Following are some of the unanswered research questions that have to be addressed before ground improvement can be used as a viable option to enhance the seismic response of pile foundations in soft clays in routine design practice: (1) What are the effective techniques for improving soft clays around pile foundations for both seismic design and retrofit? (2) How can we analyze, simulate, and design pile foundations in soft clays with ground improvement for earthquake loads? (3) How do individual piles and pile groups, with and without ground improvement, behave during seismic events and how can we validate our analysis and simulation tools and designs? And (4) how can we translate our understanding into a useful design methodology to benefit the broader earthquake engineering community?
The intellectual merit of this work is that the above mentioned research questions will be systematically addressed using a multidisciplinary team consisting of structural and geotechnical engineers and industrial partners who have extensive experience in ground improvement techniques and seismic design of pile foundations. Innovative centrifuge and full-scale field tests using NEES facilities and equipment, simplified analysis methods, and sophisticated fully coupled simulation techniques will be utilized to understand and improve the seismic behavior of pile foundations in soft clays. The research results will be translated into a useful design methodology and tools that will benefit the entire earthquake engineering community immediately as well as influence the long term practices. Simple analysis methods will serve the immediate needs of the industry while sophisticated simulation techniques are expected to show the limitations of the simple analysis methods and impact the long term industry practices.
In addition to benefiting the earthquake engineering community, the broader impacts of the proposed project include the integration of the proposed research into education at K-12 and undergraduate and graduate levels using the knowledge gained from innovative curriculum projects currently underway or already implemented. The proposed education plan includes a seismic design project that spans multiple courses for undergraduate and graduate students, a web-based simulation competition for high school students, and an adventure scenario based learning module for middle and elementary school students. Data from this project will be made available through the NEES data repository (www.nees.org).
Pile foundations are deep foundations used to support major structures such as bridges and tall buildings that cannot be directly supported on soft soils. The piles carry the heavy superstructure loads to a firm bearing layer such as the bedrock beneath the soft soil. Piles are typically steel pipes or long, slender, reinforced concrete cylinders driven or cast-in-place into the ground before the overlying structure is built. Designing pile foundations in soft clays for earthquake loads is a challenging problem. One solution is to use large diameter and/or large number of piles. An innovative, more cost-efficient, solution to this problem is to improve the soil surrounding the pile foundation. For structures undergoing seismic retrofit with existing pile foundations in soft soils, improving the soils may be the best option to improve the seismic behavior of the foundation. This project systematically addressed the following research questions using a team of structural and geotechnical engineers from academia and industry: (1) how can we analyze, simulate, and design pile foundations in soft clays with ground improvement for earthquake loads? (2) how do piles and pile groups, with and without ground improvement, behave during seismic events and how can we validate our analysis and simulation tools and designs? and (3) how can we translate our understanding into a useful design methodology to benefit the broader earthquake engineering community? More specifically, the project team investigated a ground improvement technique called Cement Deep Soil Mixing (CDSM). Small scale centrifuge and full-scale field tests, sophisticated computer simulation tools, and simplified analysis techniques were used to determine that CDSM is indeed effective in improving the seismic behavior of pile foundations in soft clays. For example, it was discovered that depending on the extent of ground improvement around a pile, the resistance of a pile to lateral loading can be increased 4 to 5 times that of a pile in the unimproved soil. Recommendations were developed for practicing engineers and disseminated through technical papers and a webinar. All the experimental data collected was curated and uploaded to the NEEShub for archiving. One post-doc, 7 graduate students, 7 undergraduate students, and a High School student were mentored and trained in various aspects of seismic behavior of pile foundations and collection and manipulation of large experimental data sets. A successful "hands on" educational module was developed using edible pile, soil, and construction materials to expose Middle School students to behavior of pile foundations in soft soils during earthquakes. Middle School students in Oklahoma and Iowa participated in the learning activities. A group of Middle School science and mathematics teachers in Oklahoma also participated in the learning activities to evaluate the potential for implementing the module in their classrooms.
