Threading the Needle: Design and Construction Over and Alongside Subway Tunnels-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes the design and construction of a five-building residential, retail, and office development on a two-acre site in dense downtown San Francisco. BART subway tunnels run directly beneath the property, while a Muni tunnel lies nearby. The project also involved demolishing existing buildings, retaining a historic hotel, and creating public open spaces.<br /><br />The principal challenge was constructing basements and foundations without causing adverse temporary or permanent effects on the tunnels, neighboring buildings, or utilities. BART and Muni requirements included minimum tunnel clearances, analysis of soil-stress changes, movement monitoring, and groundwater recharge if levels fell by more than two feet. The site also contained potentially liquefiable soil and compressible marsh deposits.<br /><br />The team designed five support-of-excavation systems: four deep soil mix (DSM) cutoff-wall systems and one soldier-pile-and-lagging system. Phased construction, tieback placement, bracing, and coordination with future excavations were incorporated into the designs. DSM ground improvement was selected to reduce liquefaction risk and transfer loads to competent soil. Groundwater-flow and soil-structure interaction models evaluated construction, long-term, and seismic conditions.<br /><br />Quality assurance included DSM strength testing, coring, and load tests. Monitoring showed acceptable track movement, and BART accepted the tunnel-protection analysis. The presentation concludes that finite-element modeling, groundwater analysis, ground improvement, and carefully phased shoring enabled the project to address its complex urban constraints. The development received the 2023 ASCE Region 9 Outstanding Urban/Land Development Project award.

Keywords

San Francisco development

BART tunnels

Muni tunnel

deep soil mix

support of excavation

groundwater recharge

liquefaction mitigation

soil-structure interaction

historical hotel retention

urban redevelopment