GOALI: Performance of Deeep and Wide Excavations in Congested Urban Areas

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The increase in urban population density in the US and many parts of the world requires major advances in our ability to extend civil infrastructure into the subsurface environment. Development of underground space is key to continued and sustainable urban living. A continuous challenge to underground space utilization in many urban areas is in constructing/developing that space in a heavily built environment in often challenging soil conditions. The Transbay Transit Center (TTC, http://transbaycenter.org/) under construction in downtown San Francisco is a $2 billion public works project designed to replace the existing Transbay terminal with a modern regional transit hub that will connect 11 transit systems and will be the terminus point for the planned California High Speed Rail from Los Angeles. The construction of the TTC includes an open cut excavation that is about 200 ft wide, 60 ft deep and over 1500 ft 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. The excavation is unprecedented in scale on the US West Coast, and will include an extensive monitoring program. A suitable analogy for this project is the equivalent of conducting open heart surgery on a patient running a marathon. This research project is an industry-university partnership between the University of Illinois at Urbana-Champaign, the University of California, Berkeley, and Arup to leverage this unique excavation project to enhance our understanding of the response of very wide and long excavations and to develop suitable computational models. The TTC excavations extend beyond our available empirical databases and validated models. The research project includes development of numerical models, performance of a supplementary laboratory testing program and installation of supplementary field instrumentation designed to answer specific research questions, leveraging current investments in the geotechnical aspects of the project. Field instrumentation will include strain gages on cross-lot bracing, extensometers and first development and use of flushable piezometers in the US 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. This research project will significantly advance state-of-the-art in excavation modeling techniques. The successful completion of this excavation and the acquired knowledge provides valuable knowledge that can be applied to large upcoming underground infrastructure investments in the US including San Francisco, 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. 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. We will be able to provide decision makers with the sound technical data that will alleviate societal apprehension and potential resistance that has traditionally impeded such projects. Among a number of educational activities, we have partnered with the Alliance for Minority Participation at UC Berkeley to engage underrepresented undergraduate students to assist with this research project. Students working on this project will gain invaluable real-world experience on a major construction project, and will interact with both researchers and consultants.