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

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The 1621–1629 Market Street project transformed a two-acre site in San Francisco into five mixed-use buildings, each with a single-level basement. Construction took place above two BART tunnels and near Muni tunnels, in a dense urban area with poor, potentially liquefiable soils and groundwater close to excavation depth.<br /><br />To meet BART requirements and limit impacts to the tunnels and neighboring properties, the team modeled soil–structure interaction and groundwater flow. Analyses predicted tunnel movements of 0.1 inch or less and groundwater drawdown outside excavations below the two-foot limit. Recharge wells were installed as a contingency and restored groundwater levels when drawdown briefly exceeded that limit. Track surveys monitored tunnel movement during construction.<br /><br />Four excavations used deep soil mix (DSM) cutoff walls, which supported excavation and restricted groundwater flow; the shallower fifth used soldier-pile-and-lagging shoring. Staged construction allowed Building B to reuse about 40% of shoring installed during the first phase. Tiebacks and internal bracing restrained the walls, with anchor layouts adjusted to avoid tunnels, utilities, and other underground structures.<br /><br />Because deep foundations and soil-displacement methods could affect nearby tunnels, the buildings were supported on mat foundations over DSM-improved ground. An interconnected DSM grid transferred loads to competent soil and mitigated liquefaction by increasing the improved soil’s composite shear resistance, rather than densifying it. Load tests and strength and integrity testing verified the DSM elements.<br /><br />The combined design and construction measures provided foundation support, enabled phased excavation, controlled groundwater, mitigated liquefaction potential, and kept impacts to subway tunnels and nearby improvements small.

Keywords

Market Street project

San Francisco

mixed-use buildings

BART tunnels

Muni tunnels

deep soil mix

groundwater control

liquefaction mitigation

mat foundations

excavation support