Located 35 miles southeast of Seattle, Washington, the Howard Hanson Dam protects a population of 75,000 people and the fourth-largest continuous warehousing district in the United States, as well as two major seaports handling 4,000,000 containers at $75 billion per year.
In January 2009, the dam’s water level reached a record high. Sediment-laden seepage through the embankment reached alarming levels, and investigators discovered two subsidences in the dam. The U.S. Army Corps of Engineers (USACE) reclassified the dam as high-risk, putting Howard Hanson in the top 10 dams at risk of failure in the U.S.—with an estimated failure probability of one in three. An emergency grout curtain was installed, but the dam quickly needed a permanent fix.
The project’s joint venture team contracted Kleinfelder to provide construction quality control, project administration, geotechnical support, and onsite laboratory services to mitigate seepage in the right abutment of the dam, along with the two subsidence areas. With two USACE-validated laboratories and three USACE-trained project managers, the team maintained an around-the-clock schedule to perform drilling operations, drain installations, soil and material testing, installation of temporary shoring, and soil excavation—all within 110 days of project notice.
In February 2012—one of the top ten rainfall events in the area’s history—the dam repairs performed as designed and constructed. One of the first projects completed in the new USACE DSAC Risk Management System, the dam validated the ability of the new program to prioritize repairs and minimize public risk.