The Dam, which was constructed in the early 1960s, extends more than 8,300 feet from bank to bank, includes left and right earth embankments and a 10-unit powerhouse. It also includes a concrete spillway featuring radial gates that measure 50 feet wide and nearly 70 feet high.
In February of 2014, workers noticed a slight misalignment in the roadway deck curbs and handrails located at the top of the spillway during a routine maintenance inspection. Further investigation showed that the top of Pier 4 had moved more than two inches downstream. A follow-up dive inspection revealed a 65-foot long by two-inch wide crack that spanned the full length of Monolith 4.
The owner of the dam, Grant County Public Utility District, immediately mandated a drawdown of the dam’s reservoir to relieve pressure on the spillway monoliths.
After extensive investigation, it was determined that the crack was caused by mathematical errors in the initial design calculations and late changes to the final spillway design. It is also suspected that there may have been problems curing the concrete the weekend it was poured due to unusually hot temperatures.
The Public Utility Districts invested several million dollars ($3 million at Wanapum Dam alone) on fish ladders to allow the native spring Chinook salmon reach their upstream spawning ground destination while the waters of the Wanapum Reservoir were lowered.
Post-tension anchors were selected as the repair solution
for Monolith 4, along with drilling lift joint drains above the gallery to
reduce uplift pressure resulting from cracking within the body of the dam.
The restraining force necessary to stabilize the dam required
thirty-five 61-strand tendon anchors with lengths up to 260 feet installed
through the dam structure.
The most grueling challenge on the project was the drilling to advance
16-inch boreholes necessary to install the 61-strand tendons, which required
careful planning and precision through highly variable and highly fractured
basalt rock. In this case, the fractured basalt rock could take more than a
half dozen attempts of grouting and re-drilling to achieve water tightness.
An additional sixty-nine solid bar anchors, a majority of which were 3-inch diameter, were installed under water through the ogee spillway. The entire drilling, grouting and anchor tensioning processes were performed with the assistance of divers and using remote cameras to allow engineers at the surface to witness the underwater operations.
The cracks in Monoliths 2 and 4 were repaired to reduce seepage,
decrease water pressure and improve stability. The grouting program included
patching and sealing followed by an intricate sequence of cement grout
injections.
The District developed a two-part strategy to restore the reservoir
elevation as soon as possible. An
intermediate pool raise was proposed after repair of the cracked monolith and
some preventative measures were installed. The reservoir would be restored to the maximum pool level
after completion of all remedial measures.
The collaborative working relationship between the District, MWH, Nicholson and Kuney Goebel JV resulted in a very successful project at Wanapum Dam. The consistent team approach harbored creative solutions to overcome unique challenges in design, planning and construction phases of the project. It is also with this collaboration that the team was able to meet the schedule-demanding milestones to successfully rehabilitate Wanapum Dam with quality and safety at the forefront.
