Challenges of Building the Deepest Basement in Downtown San Jose-Paper

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The 200 Park Avenue project in downtown San Jose required a 54- to 58-foot excavation for four basement levels. Subsurface conditions included shallow groundwater and deeper sand and gravel layers under artesian pressure. Dewatering could have caused settlement of nearby streets, utilities, and buildings, while excessive pressure beneath the excavation risked bottom blowout. The shoring system also had to limit wall movement under strict city requirements.<br /><br />The design team selected a tied-back cutter soil mix (CSM) cutoff wall instead of conventional soldier piles and lagging. Approximately 39 inches thick, the wall was reinforced with closely spaced steel beams and extended about 135 to 140 feet to cutoff the deeper water-bearing layers. Four levels of tiebacks provided lateral support. Dewatering and depressurization wells maintained groundwater at least three feet below the excavation bottom.<br /><br />Monitoring during construction found that wall deflections and adjacent settlement remained within acceptable limits. Outside the excavation, groundwater drawdown was generally one to two feet in the upper sand layer and three to four feet in the lower layer. Although the lower-layer drawdown exceeded predictions, observed settlement remained below one inch. The wells effectively controlled groundwater and artesian pressure, and the wall’s performance was consistent with design expectations. Overall, the CSM cutoff wall limited ground movement, protected neighboring improvements, and mitigated the risk of bottom blowout.

Keywords

downtown San Jose

200 Park Avenue

deep excavation

four basement levels

cutter soil mix cutoff wall

CSM wall

tiebacks

groundwater control

artesian pressure

dewatering wells