Earlier this year, Nicholson Construction Co. finished the construction of a double-line grout curtain at the Howard A. Hanson Dam in western Washington State. This high-profile project was completed under a very strict schedule and heavy public scrutiny.
Howard A. Hanson Dam, built in 1962, is an earth embankment dam. The dam embankment is 235 feet high and 675 feet long. The composition of the right abutment foundation is partially bedrock and unconsolidated alluvial and landslide material. The right abutment has leaked since the dam became operational. A leak in January 2009 filled the reservoir higher than ever, prompting the U.S. Army Corps of Engineers (USACE) to reclassify Hanson Dam as “unsafe” with an “urgent and compelling” need for immediate action.
Given the unsafe nature of the dam and the limited available time prior to the beginning of the local high rain season, the USACE opted to reduce seepage on an interim basis by installing a grout curtain until a more permanent solution could be designed, financed and constructed. Pittsburgh-based Nicholson was awarded the interim grout curtain contract on July 9, 2009, after the completion of a fast-track, best-value request for proposal (RFP) process.
Nicholson’s primary work involved the installation of a 450-foot-long double-line grout curtain into the dam’s right abutment, which is comprised of fluvial, glacial, lacustrine and rockslide deposits of andesite, with dimensions up to 20 feet or more. The grout curtain depth varies from 90 feet to 170 feet, and the curtain extends 15 feet into bedrock for the first 170 feet, and into a silt horizon for the remainder of the curtain length. The curtain is made up of a high-mobility, balanced and stable grout. The grouting was monitored in real-time with Nicholson’s Grout I.T. system.
Nicholson’s substantial completion of the base contract before the specified deadline, along with acceptable refusal rates (6 lugeons in overburden and a 3 lugeons in bedrock) of the majority of stages, allowed the USACE to award an additional modification to the contract that extended the grout curtain in a rock septum between the dam and the right abutment.
It was determined that the grout curtain reduced the chance of flooding from a one-in-three chance to a one-in-25 chance, which was better than expected as an interim measure for the USACE. Seattle District Commander Colonel Anthony Wright thanked Nicholson with this statement: “I want to thank the Nicholson Team for the completion of emergency repairs to the Howard Hanson Dam. With literally billions of dollars of damage and potential loss of lives at stake, the repairs to this dam became my number one priority after the flooding in January 2009. Your team, with its depth of experience and talented field staff, took on the construction of a grout curtain barrier that was unique in the industry, complex and had a very challenging timeline. You met the tight construction schedule with a high-quality installation, and provided substantial relief to the citizens and businesses downstream. Please pass on my thanks to all of the Nicholson employees who helped make this project a success.”