Wanapum Dam Repaired Using Post-Tensioning Anchors-Article

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In February 2014, workers inspecting Washington’s Wanapum Dam noticed misaligned curbs and handrails above the spillway. Further investigation revealed a 65-foot-long, 2-inch-wide crack running across Monolith 4, with the pier displaced more than 2 inches downstream. The operator immediately lowered the reservoir by 27 feet and restricted access to the reservoir and downstream areas.<br /><br />An investigation attributed the cracking to design-calculation errors and later design changes that increased tensile stress. High temperatures during concrete placement at a horizontal lift joint may also have weakened the joint. Analysis showed that cold-weather contraction and uplift pressure could worsen the crack and threaten the monolith’s stability.<br /><br />Repairs combined post-tensioning, drainage, and grouting. Crews installed 35 anchors, each containing 61 steel strands, through the spillway piers, with lock-off loads of 2,500 kips. They also installed 69 solid-bar anchors underwater through the ogee spillway. More than 100 drains were drilled from the gallery to relieve uplift pressure, and grout was injected into the Monolith 4 crack to reduce seepage and water pressure. Investigations also identified lesser cracks elsewhere, which were treated with grout.<br /><br />The work presented significant challenges, including drilling large, accurately aligned holes through fractured basalt and installing and stressing anchors underwater. The team developed specialized drilling, lifting, sealing, and stressing methods, supported by testing and rehearsals.<br /><br />Working with the dam owner, engineers, contractors, and regulators, the project team completed the repairs and returned the spillway to operation little more than a year after work began.

Keywords

Wanapum Dam

Monolith 4

spillway crack

post-tensioning

drainage system

grouting repair

uplift pressure

design-calculation errors

reservoir lowering

underwater anchors