South Tacoma Sanitary Sewer

(Tacoma, WA)  Landau Associates provided geotechnical engineering services to the City of Tacoma for design and construction of the South Tacoma Sanitary Sewer Trunk Replacement project. The existing 24-inch-diameter sewer pipe was replaced with 3,200 linear feet (LF) of new 36-inch-diameter HDPE pipe. Approximately 340 ft of the pipeline crosses beneath Tacoma Mall Boulevard and Interstate 5 inside a 54-inch diameter steel casing installed by pipe jacking methods. As part of the project, approximately 2,500 LF of existing storm sewer pipeline was deepened and upsized from existing 30-inch-diameter pipes to a 42-inch-diameter HDPE and 48-inch-diameter concrete pipe.

Geotechnical engineering services included compiling available geologic and geotechnical information in the project vicinity, completing borings along the pipeline route to characterize near-surface soil and groundwater conditions, completing geotechnical engineering analyses, and developing geotechnical engineering recommendations for design and construction of the pipeline. Geotechnical engineering recommendations were developed for construction dewatering, trenching and excavation support, pipe foundation support, pipe bedding, trench backfill and compaction criteria, loads on pipes, and resistance to lateral loads. Landau Associates assisted the City in preparing portions of the tunneling specifications. A Geotechnical Baseline Report was prepared to establish baseline soil and groundwater conditions for bidding purposes. We also provided geotechnical observations to the City during construction.