South Fork Tolt River Project Post-Tensioned Anchoring System Seattle, Washington Back to Project List Post-Tensioned Rock & Soil Anchors Drilling Drilling and installation of 58-strand post-tensioned tendon anchors. Project Overview: The South Fork Tolt River Reservoir is located in a seismically active area, and a five-year dam safety review concluded the intake tower structure was susceptible to damage from a seismic occurrence exceeding the latest earthquake design criteria. The seismic retrofit design required installing six post-tensioned tendon anchors and stressing each anchor to a lock-off load of 2.5 million pounds. Each anchor comprises fifty-eight 0.6-inch diameter epoxy-coated tendons. Layne GeoConstruction successfully installed the post-tensioned tendon anchors in a single work shift using a sky-crane helicopter. Overall length of the tendons was 291 feet with a total weight of 14,000 pounds per anchor. at the time, the anchors were the largest ever used for seismic retrofit of a structure of this type. Installing the tendon holes for the anchors required drilling holes through the intake tower structure and into bedrock. The tendons holes were drilled through 3 feet thick steel reinforced structural concrete. Allowable deviation of the drill holes was minimal. Tthe tendon drill holes had to be guided through a maze of concrete with embedded pipes, valves, and hydraulic equipment crucial to the on-going daily operation of the intake tower. To accomplish drilling the tendon holes to the unusually tight alignment and curvature specifications required implementing a variety of close-tolerance drilling equipment and techniques.     Value of Work Performed: $2,300,000 Project Duration: 9 months Project Completion Date: August 1998 Project Reference: Project Owner City of Seattle Water Department Engineering Consultants Woodward-Clyde Consultants