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

Video Transcription

Video Summary

The presentation describes the geotechnical design of a two-acre, five-building development on Market Street in San Francisco, directly above BART tunnels and near MUNI tunnels. The dense urban site also required phased construction, preservation of a historic façade, and protection of an existing hotel and utilities.<br /><br />Key challenges included tunnel protection, groundwater control, potentially liquefiable and compressible soils, and restricted excavation support. The team designed deep soil-mix (DSM) cutoff walls, bracing and tiebacks, and used groundwater-flow modeling and recharge wells to limit off-site drawdown. DSM ground improvement mitigated liquefaction and transferred building loads to competent underlying soils, avoiding deep foundations near the tunnels. Load tests and strength testing verified the ground improvement, while finite-element analyses assessed tunnel impacts. Survey monitoring during construction confirmed tunnel movements stayed within expectations.<br /><br />The project was completed successfully and received an ASCE Outstanding Urban Land Development Project award.

Keywords

geotechnical design

BART tunnels

groundwater control

deep soil-mix walls

liquefaction mitigation