Funding: NEESR-SG; Understanding and Improving the Seismic Behavior of Pile Foundations in Soft Clays PI; M. Muraleetharan; Award #0830328

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.

nees@UCLA's linear shaker on the pile cap

Media event held on-site