CASE STUDY: FEATHER RIVER WEST LEVEE PROGRAM CUTOFF WALLS-Paper

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The Feather River West Levee (FRWL) Program was launched in 2010 to improve flood protection for communities north of Sacramento, California. Its goal was to bring 35 miles of levee into compliance with state and federal standards and reduce flood risk for more than 34,000 properties.<br /><br />Slurry cutoff walls were the primary measure for addressing seepage beneath and through the levees. Construction used conventional excavation for walls up to 85 feet deep and deep soil mixing (DSM) for greater depths. By the end of 2015, approximately 6.65 million square feet had been installed; the program’s final reported quantities were about 6.97 million square feet of conventional wall and 770,000 square feet of DSM. Soil-bentonite and soil-cement-bentonite mixes were developed for site conditions. Quality controls included slurry and backfill testing, settlement monitoring, and permeability testing. All reported backfill permeability samples met the specified limit.<br /><br />The project faced archaeological and cultural-resource discoveries that disrupted construction and required coordination among the agency, contractors, regulators, and local tribes. Unexpected changes in the depth of competent foundation layers also prompted major design and contract adjustments, including a change from DSM to conventional excavation in some areas. Tight alignment constraints and the unusually large workload further challenged delivery in a seasonally limited, resource-constrained industry.<br /><br />The authors recommend more thorough cultural-site investigations during design and conducting verification drilling earlier, as part of the design process, to improve depth estimates and reduce construction-stage changes. They conclude that close collaboration among the owner, engineers, contractors, and subcontractors was essential to meeting demanding quality requirements and schedule constraints.

Keywords

Feather River West Levee

FRWL Program

flood protection

levee construction

slurry cutoff walls

deep soil mixing

seepage control

soil-bentonite

cultural resources

quality control