By Luke Carothers

The original island of the pelicans in the San Francisco Bay, Yerba Buena Island sits between San Francisco and Oakland, California. The first lighthouse was built on the island in 1875. During this time, locals used the island to raise goats–for which the island was renamed for a time–and the military soon found use for the space, establishing a naval training station in 1900. A few decades later in 1936, President Franklin Delano Roosevelt ordered the US Army Corps of Engineers to begin dredging the bay to build a new, 400-acre island on the shoals of Yerba Buena, which had previously represented a shipping danger. 

Named for the gold that potentially lies in the mud dredged up from the bay’s floor, Treasure island was originally constructed for two purposes: to host the 1939-40 Golden Gate International Exposition and serve as a future second airport for San Francisco’s growing population.  Several permanent structures were erected on the new island including “Building 1”–a 1938 Art Moderne building–and the terminal building meant to house and showcase new Pan American Airways Clippers. 

After the Golden Gate International Exposition ended, plans to use the development as a new airport fell out of favor and the Navy began using it as a major training station for World War II all the way through the Cold War. In 1997, the naval station on Treasure Island was closed, and the hangar buildings were converted for other uses such as sound stages for television and movies. Since this time, Treasure Island and its neighbor Yerba Buena have maintained a population around 2,000 residents. In addition, Treasure Island is home to several restaurants, schools, arts and athletics organizations, and a Job Corps campus.

Now, both Yerba Buena and Treasure Islands are poised to return to a similar level of prestige that came with the Golden Gate International Exposition.  The Treasure Island/Yerba Buena Island Development Project has plans to create a new San Francisco neighborhood with homes offered at below-market rates, extensive parks and open space, public art, hotels, restaurants, and other elements that would make the development an ideal destination for both San Francisco residents and tourists alike. To manage this development, the Treasure Island Development Authority (TIDA) began the planning stages of the project around 2011.

From a geotechnical perspective, building a new development on top of an island that was dredged from the bay’s floor over half a century ago was a tricky endeavor. To lead the geotechnical work on the development project, ENGEO started studying the island in 2003. ENGEO, one of the most comprehensive geoscience firms in the world, is serving as the Geotechnical Engineer of Record. As such, they have been working in a number of areas throughout the different phases of the project, doing everything from conducting the design-level geotechnical study to designing and overseeing a massive ground-improvement program, to designing new building foundations to providing construction-phase quality control, to setting up a Caltrans-certified materials testing lab on the island.

The plan for the development on Treasure Island/Yerba Buena Island was to develop approximately 200 acres on Treasure Island and over 85 acres on Yerba Buena Island. Development will include a new ferry terminal, 8,000 residential units, and over 100 acres of parks and open space. However, to begin any sort of large-scale development on the island, engineers had to quantify how the land would perform from loading related to new buildings and earthquakes. In 2014, 2015, and 2016 ENGEO conducted full-scale field tests and design-level geotechnical studies to support the first phase of development. Uri Eliahu, ENGEO’s President and CEO, notes that these first geotechnical studies were critical to understanding how the island would perform over time. Eliahu points out that, while the entire island has experienced some degree of settling, it is by no means uniform in distribution. The average settlement across the island since it was constructed is between 6- and 6.5-feet, but, on the north end of the island, the settlement is closer to 10-feet. This is contrasted by the south end of the island, closer to Yerba Buena, that experienced only about 3-feet of settlement.  

This contrast in settlement is due to the varying materials at different locations. Closer to Yerba Buena, Treasure Island rests on not only the sand pumped up from the bay, but also natural shoals that provide an additional source of stability. At other locations on the island, the soil rests on highly compressible natural bay mud, which, according to Eliahu, makes building on the island a much more involved technical process. Under earthquake conditions, the sandy soil could liquefy, and, when significant loads are placed on top of the mud, it has a tendency to compress significantly. The real challenge of this process comes when remediation is. According to Eliahu, the remediation techniques for these two processes are the complete opposite of one another. 

Eliahu is not shy about pointing out the enormous challenge that forms the crux of the Treasure Island/Yerba Buena Island Development Project– being that it is located on a manmade body of land that sits upon up to 100 feet of compressible soil and 50 feet liquefiable material, surrounded by water, nestled between the two largest faults on the continent. To both compensate for the compressible nature of the bay mud and the vulnerability of the sand to seismic activity, engineers had to take advantage of both static loading and dynamic energy. Static loading was essential to consolidating for the bay mud. This remediation tech- nique involved pre-compressing a location with a large static load to “squeeze all the water out from the mud,” says Eliahu. The key to this process is ensuring the temporary pre-fill is heavier than the foundation of the new building. In doing so, the building will impose less weight than the soil was previously under, preventing additional settling. For the areas of the island built on sandy soil, the key remediation technique was to densify the material by using dynamic energy. 

Treasure Island’s location meant that the compaction of this liquefiable material was paramount to safely building on the site. In 2015, ENGEO selected a site on the island to conduct a full-scale testing program for using dynamic energy to densify the material. Part of this testing involved using a 51-ton motor unit to densify material through vibro-compaction and replacement. Although this technology has a long record of success in Japan, it had never been used in the United States. This massive piece of machinery is outfitted with four probes shaped like H-piles. The arms are powered by two motors turning in the opposite direction, rapidly vibrating the arms up and down as they are lowered into the ground. As the machinery moves up and down, it displaces material, which the process was completed, the team then followed up by using a different, smaller tamping machine to densify the top layer of material. 

The result of this testing phase was clear: this technique was very effective at densifying the first 23-feet of material, but had almost no effect on materials below that, particularly the older, natural materials. Eliahu and his team conducted another round of testing to determine if another technique would need to be used to densify this deeper material or if it had some form of natural resistance. To get a better understanding of this deeper material, ENGEO carefully sampled the island soils and used many techniques, including scanning electron microscopy, cyclic shear testing, and geologic mapping. This refined analysis showed that the natural shoal material had a very complex composition that formed a “natural fabric” which made the underlying material resistant to movement. 

The setting of this new development means that it also has to con- tend with rising sea levels from climate change. Varying levels of settlement across the island mean that ENGEO’s team had to take an adaptive management strategy approach. Rather than building a wall of levees around the island, Eliahu and his team are taking a more innovative approach to combating sea level rise–monitoring the rising of the sea and applying that information to their settlement data. This allows them to come up with adaptable solutions that can be applied to specific areas of need on the island. The initial construction places the development area of the project about 3.5-feet above the 100-year sea level, which Eliahu believes, based on current sea level rise projections, makes the new development safe until at least the end of the century.  

As the Treasure Island/Yerba Buena Island Development Project continues moving forward, each step is being supported by advanced geotechnical engineering work from ENGEO. As a part of its adaptive management strategy for the project, Eliahu points out their ability to not only plan and perform complex geotechnical work and testing, but also to monitor the data it generates in real time. This provides the team at ENGEO both a better base of knowledge to work from and flexibility to adapt to changes in near real time. With recent milestones such as the completion of the ferry terminal and its opening for daily service, ENGEO’s attention to detail is coming to fruition and will continue to benefit the project beyond its ultimate completion. 


Luke Carothers is the Editor for Civil + Structural Engineer Media. If you want us to cover your project or want to feature your own article, he can be reached at lcarothers@zweiggroup.com.  

Comments