Foundation Design For Cut And Cover Tunnels Using Cement Deep Soil Mixing

Published: Proceedings-DFI/EFFC 11th International Conference on Piling and Deep Foundations, 21-23 May 2014, Stockholm, Sweden. Copyright Deep Foundations Institute (DFI)

21 May 2014
Wei-Yu Chen, Fugro Consultants, Inc., Andrew Bro, Fugro Consultants, Inc., Shahriar Vahdani, Fugro Consultants, Inc. Andy Herlache, Fugro Consultants, Inc.

The Presidio Parkway Project will reconstruct approximately 1.6 miles of highway, including seven new bridges, three new cut-and-cover tunnels, and a new undercrossing connecting the Golden Gate Bridge to the City of San Francisco, California. The project site is approximately 5.6 miles from the San Andreas Fault, capable of producing magnitude 8 earthquakes. Seismic hazards include strong ground shaking, soil liquefaction, ground settlement, and lateral spreading. The Main Post Tunnels (MPT) are two parallel cut-and-cover concrete tunnels approximately 1100 feet long. The MPT site underlain by loose sandy soils susceptible to liquefaction and lateral spreading. Excessive racking and permanent displacement due to lateral spreading are identified as the main seismic vulnerabilities. Analyses showed that without ground improvement, tunnel racking of over 5 inches and permanent lateral tunnel displacement of up to 15 feet could be anticipated. Cement Deep Soil Mixing (CDSM) is proposed to mitigate slope displacements and increase lateral stiffness of the foundation materials to reduce tunnel racking. A series of 2-D fully dynamic time domain analyses were performed to evaluate the performance of CDSM, confirm that performance criteria are met, and optimize the CDSM geometry and replacement ratio. The use of CDSM reduced the permanent tunnel displacement from over 15 feet to less than 1 inch. Tunnel racking was also reduced significantly. The adopted analytical approach presented herein can be considered for future tunnel design projects in seismically active areas.


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