Wanapum Dam Repaired Using Post-Tensioning Anchors-Paper

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Wanapum Dam, part of Washington’s Priest Rapids Hydroelectric Project, developed a major crack in spillway Monolith 4 after about 50 years of service. Discovered during a routine inspection in February 2014, the crack was approximately 65 feet long and two inches wide. The reservoir was immediately drawn down, and the lake and downstream areas were closed while the damage was investigated.<br /><br />The investigation attributed the fracture to original design calculation errors and later design changes, which created tensile stresses at the upstream face of the concrete. The crack followed a horizontal construction joint roughly 85 feet below normal pool level. Although cracking was visible only at Monolith 4, repairs were planned for the other spillway monoliths as a precaution.<br /><br />Nicholson Construction joined the emergency repair effort and carried out coring, crack grouting, and installation of 35 long, multistrand tendon anchors through the dam. The project also required 69 solid-bar anchors installed through the spillway, mostly underwater. These were selected over multistrand anchors because underwater grouting and post-tensioning would be more difficult.<br /><br />Installing the underwater anchors presented significant challenges. Crews worked from platforms suspended above the spillway, using specialized equipment to drill and install anchors in submerged sockets. Full-scale dry rehearsals helped refine the procedures, while specialized packers limited grout loss into the water. Close coordination among divers, drillers, engineers, and project managers enabled the team to complete the complex emergency repairs successfully.

Keywords

Wanapum Dam

Priest Rapids Hydroelectric Project

spillway crack

Monolith 4

concrete fracture

routine inspection

reservoir drawdown

design calculation errors

tendon anchors

underwater repairs