Hashash to Study Massive Urban Excavation
The five-level Transit Center will serve both bus and rail and will include a 5.4-acre rooftop park. (Photo: Business Wire)
Underground construction projects in heavily built areas are necessary for continued and sustainable urban life, but they present distinct challenges. CEE Professor Youssef Hashash has long been at the forefront of the development of advanced computational and monitoring tools that help construction professionals avoid damage to surrounding structures during urban digs. Now an extensive project in downtown San Francisco has presented a unique opportunity for a team of researchers led by Hashash and funded by the National Science Foundation (NSF) to study an urban excavation of unprecedented scale.
The NSF this month announced a grant of nearly $780,000 to fund Hashash’s project, “GOALI: Performance of deep and wide excavations in congested urban areas.” The NSF’s GOALI program, Grant Opportunities for Academic Liaison with Industry, promotes university-industry partnerships. Hashash, along with Professor Michael Riemer of the University of California at Berkeley and industry partner Dr. Nick O’Riordan of ARUP North America Ltd., will study the excavations for the Transbay Transit Center (TTC), a massive dig in the soft soils of San Francisco Bay area. Over a three-year period, they will instrument the site, gather data, conduct field and laboratory soil tests, and develop new computational models for large-scale urban digs. This has been made possible through close collaboration with the Transbay Joint Powers Authority, the authority responsible for the project.
During excavations in urban areas, care must be taken to keep disruption of local activity to a minimum and avoid damage to surrounding structures. This can be complicated by adverse soil conditions. Construction professionals use advanced computational tools to predict ground response and sophisticated sensor systems to monitor excavations, but the reliability of predictive models is limited by the available performance data from construction projects. The massive TTC excavation will offer researchers an opportunity to employ extensive instrumentation to collect data that can be used to create models of exceptional accuracy for large-scale projects in urban areas.
“A suitable analogy for this project is the equivalent of conducting open-heart surgery on a patient running a marathon,” Hashash says.
The TTC is a nearly $2 billion public works project designed to replace the existing Transbay terminal in downtown San Francisco with a modern regional transit hub that will connect 11 transit systems and will be the terminus point for the high speed rail line from Los Angeles. The construction of the TTC includes an open cut excavation that is about 200 feet wide, 60 feet deep, and more than 1,500 feet long in relatively soft soils. The excavation is adjacent to several high-rise and low-rise buildings including one of the tallest buildings in San Francisco. The TTC will have to be constructed with minimal disruption to infrastructure and economic activity in the area.
Field instrumentation will include bracing strain gages, extensometers, and the first development and use in the U.S. of flushable piezometers to measure potential negative pore pressures in the unloaded clays. Data from the laboratory testing and field monitoring programs will be used in inverse analyses to generate new soil constitutive models and to update predictions of excavation performance. Analyses will also be conducted to better understand the impact of long-term unloading on the seismic response of the underground structures.
The project leverages the more than $10 million in public investments in the geotechnical aspects of the project to acquire unique data on excavation and soil response beyond what is currently available in current empirical databases and numerical models, Hashash says.
“It will significantly advance state-of-the-art excavation modeling techniques,” he says.
The knowledge gained during the TTC dig is expected to inform future underground projects being planned in Los Angeles, Seattle, New York and Miami.
“Our ability to achieve urban sustainable living is critically dependent on development of underground space in ever-challenging environments at an unprecedented scale,” Hashash says. “Through knowledge gained on this project, we will develop the expertise and technical tools to confidently construct these types of structures with minimal disruption to the urban environment.”
Photos: Business Wire