Design and Construction of Underground Basin Using Composite Slurry Wall 500 Feet Away from the Pacific Ocean-Paper

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The Pacifica wet-weather equalization basin was built to reduce storm-related sanitary sewer overflows. Located about 500 feet from the Pacific Ocean and near homes, the underground tank needed to fit a restricted site while storing 2.1 million gallons. It is approximately 70 by 145 feet and 45 feet deep. Shallow groundwater, variable soils, high earth and seismic pressures, and tight budget and schedule posed major challenges.<br /><br />The design uses a composite perimeter wall: a 3-foot slurry wall serves first as excavation support and groundwater cutoff, then combines with a 2-foot cast-in-place liner wall to form the permanent structure. This approach preserved usable tank space and avoided a separate support-of-excavation system. Temporary steel-pipe bracing supported staged excavation; the 5-foot composite wall, base slab, and hold-down micropiles resist permanent loads and uplift.<br /><br />Engineers used PLAXIS to assess staged excavation behavior and SAP2000 to analyze permanent structural loads, including seismic effects and differing water levels in the tank’s two chambers. Construction required careful site logistics, controlled slurry excavation and tremie concrete placement, watertight panel joints, and surface preparation and dowels to ensure composite action.<br /><br />Unexpected sand layers on the west side prompted an additional bracing level and deeper wall embedment. Despite these conditions and the shallow water table, the slurry walls were installed with limited seepage. The project demonstrated that an integrated slurry-wall system can meet demanding space, performance, schedule, and budget constraints for an underground water tank.

Keywords

Pacifica

wet-weather equalization basin

sanitary sewer overflow

underground tank

slurry wall

composite wall

groundwater cutoff

seismic loads

staged excavation

micropiles