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Large-scale test validates new seismic criteria for California ports

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A hydraulic piston bolted to two sections of the pier slowly expands, subjecting the pier to lateral loads that might be experienced during an earthquake.
August 14, 2003 -- Structural engineers from the University of California, San Diego (UCSD) recently traveled to the Port of Long Beach to conduct large-scale tests on a 1,500 foot-long pier in order to test and validate new seismic design criteria for improved safety. These new regulations will apply to new and existing piers and wharfs, and are accompanied by new seismic criteria for marine oil terminals in California – a multi-billion dollar retrofit project that will span the next 10 years. Because of its involvement in the $80 million phase of the $500 million Port of Long Beach reconstruction project, UCSD stands to establish itself as one of the premiere institutions for port research.

“Not only did we test new seismic design criteria in relation to the 50-year old pier, we also had the unique opportunity to test the soil beneath it – a clayey silt sediment that is typically difficult to access,” said Scott Ashford, co-principal investigator and professor of geotechnical engineering at the university’s Jacobs School of Engineering. “This is the first test of its kind and would not have been possible without the collaborative efforts of those involved. We could not have done this study without help from the Port of Long Beach [$100,000], the State Lands Commission [$25,000], the Port of Los Angeles [$50,000], Sea Grant [$40,000], and industry,” said Ashford.

Pier Test Video
UCSD structural engineers recently traveled to the Port of Long Beach to conduct large-scale tests on a 1,500 foot-long pier in order to test and validate new seismic design criteria for improved safety.
Typically, it is rare that engineers have an opportunity to conduct full-blown, destructive tests on real marine structures. Because it was scheduled for demolition, the Port of Long Beach was able to cut up portions of the pier to provide access to Ashford’s team. It was divided into a 27 pile group, a nine pile group, a three pile group, and single pile groups. A pile is a 20-inch square, 80-feet long, pillar-like structure that supports the pier deck. The nine pile group, for example, measured about 20 feet square, while the 27 pile group was 60 feet long. The water beneath the pier has a depth of 40 feet.

Heavy machinery was used to core down the center of the piles to not only obtain data from the structures but also to gather critical information on the soil and its response to the seismic loads. Highly-skilled divers, provided by industry partners, removed corrosion and barnacles from the piles and applied the sensors with a water-proof epoxy.

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Instruments called accelerometers record how the pier moves in response to an applied force. By placing three accelerometers at right angles to each other, all accelerations in the horizontal and vertical directions are recorded.
After instrumenting the pier with the specialized, saltwater-proof sensors, the UCSD researchers conducted ambient and forced vibration tests on the pile groups. The ambient vibration test analyzed the low strain natural frequency of the system under normal conditions. This data allowed them to back-calculate the properties of the structure and the soil. The forced vibration tests consisted of rotating shakers to gyrate the piles at a high frequency (up to 5,000 pounds), and actuators to push and pull for lateral load testing (up to 500,000 pounds). “We used the adjacent portion of the pier as a reaction wall for the lateral load testing. We bolted the actuators in place and were able to move the piles in the cut-out groups by four feet from their original positions,” explained Ashford.

Ashford’s team also looked at the shadowing effect of multiple piles on one another. “There is a zone of influence in which the movement of a pile can impact trailing piles by destabilizing the soil. Inefficiencies develop when they are placed close together and when too many piles exist. Trailing piles may only have 20 to 40 percent of the lateral load capacity as a single pile,” said Ashford. Engineers are starting to move away from multiple pile groups to large, single diameter shafts of eight to 10 feet in diameter.

Ashford and Walsh’s experiments were designed to gather information on how to retrofit these piers, with a broader goal being to revitalize the nation’s port infrastructure. As they continue to analyze data gathered during the tests, what is learned will be shared with port authorities, oil companies and other interested parties in a series of educational workshops to be held at all major ports in the state. The results also have environmental implications. The State Lands Commission is in the process of implementing new design criteria for marine oil terminals ­ huge piers where supertankers pipe oil into holding tanks on land. The goal is to reduce the chances that moderate earthquakes will cause disastrous oil spills. There are about 60 marine oil terminals in ports such as Long Beach, Oakland and Los Angeles that may need retrofitting to meet new standards.

The initial results of the study are very promising and will be incorporated into California’s new seismic design criteria. The aging structure fared much better than expected, even though all of the piles were taken to failure. An ultimate goal is to develop better computer models that engineers and port designers can use to simulate how a structure might respond during an earthquake. The data gathered from this project is making it possible for Ashford and Walsh to truth-check these still primitive models. “These tests will give us some confidence that when we use our computer models they are predicting what will happen in reality,” Ashford said. “After it can be excavated, we will bring one of the piles back to the lab at UCSD for more in-depth analysis — a major endeavor due to the fact that it goes 40 feet below the ocean floor.”

     Troy Anderson, 858-822-3075,
     Christina Johnson, California Sea Grant, 858- 822-5334,

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