Deep Foundation Challenges for a Combined Sewer Overflow Treatment Station in Seattle-PPT Presentation

PPT Presentation

Pdf Summary

The presentation describes geotechnical challenges and construction methods for a combined sewer overflow treatment station in Seattle. The site’s industrial history, contamination, obstructions, nearby structures, limited access, tight schedule, and compressible, liquefiable soils complicated design and construction. High seismicity and strict elevation tolerances for equipment, weirs, and large pipes added further constraints.<br /><br />The project team divided work into four contracts, addressing contamination early to reduce schedule and change-order risks. Site-specific seismic analyses, supported by field and laboratory testing, substantially reduced design estimates compared with simplified methods: design peak ground acceleration fell from 0.55g to 0.44g, estimated liquefiable thickness from 150 to 15 feet, and predicted building-foundation settlement from 45 to 10 inches. The process building used rigid inclusions, which generated no spoils and accommodated extensive underground utilities; water preloading and monitoring helped manage settlement and verify performance.<br /><br />Dewatering was restricted near the property line and prohibited at the influent pump station/equalization basin until its tremie seal reached full strength. Vibration and deformation monitoring protected nearby structures and utilities.<br /><br />The basin case history covered construction of a secant-pile enclosure, excavation without dewatering, placement of a thick tremie slab, and subsequent interior dewatering, leak sealing, lining, and installation of interior structures. The staged approach addressed difficult groundwater conditions while supporting safe construction.

Keywords

geotechnical engineering

combined sewer overflow

Seattle

liquefaction

seismic analysis

contamination remediation

rigid inclusions

dewatering

secant piles

construction monitoring