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 effec- tive 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 ap- proach 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 flexibil- ity 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.
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 Proj- ect– 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 remedia- tion technique 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 remedi- tion 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 us- ing 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
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June 2022 csengineermag.com
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