As part of the fix, crews will install lift-joint drains to allow officials to identify future issues. Installed equipment will include piezometer pressure gauges. A more complex component set for placement consists of 61 strand-steel anchors—pier tendons—each approximately 200 ft long and 10 in. in diameter. Every one of the dam's 12 monoliths will receive three tendons embedded as much as 100 ft into bedrock below the dam.
The anchors, as designed for this project, says Kevin Shanley, Max J. Kuney Co. project manager, get installed in a 16-in.-dia borehole and have a bond length of 35 ft. The anchors are grouted with a "neat cement grout designed to achieve a compressive strength of 4,000 psi in seven days," he says.
Due to the trajectory of the borehole through the concrete monolith, the 16-in.-dia boreholes require tight vertical and horizontal tolerances. To ensure workers achieve these specified drill tolerances, 3.5-in.-dia pilot holes now are being drilled using specialized tooling. Drill rigs will pull double shifts—20 hours a day, seven days a week—until the fourth quarter of 2014.
"The pilot hole is used as a guide for the subsequent drilling to enlarge the borehole to its final diameter," Shanley says. This repair method—more economical than adding concrete mass—is exacting, but not unusual. "Permanent post-tension rock anchors have been used for almost 30 years in America to stabilize existing concrete dams and their appurtenant structures," writes Donald A. Bruce of Geosystems.
"Designs of dams constructed in the first half of this century are often found to be deficient, and owners are obliged by law to take appropriate remedial action," Bruce observes.
Engineers are designing a grouting program for the original fracture, which closed up when pressure was relieved, says Shanley.
The Wanapum reservoir shoreline will remain closed during the current low-reservoir levels, and repairs to the spillway will likely keep reservoir levels low—below the normal minimum of 562 ft above sea level—until completion of the fix. Under the current elevation, Wanapum generates about 50% to 60% of its capacity and is passing spring runoff flows in coordination with other dam operators on the Columbia River.