Rock Island Dam monitoring boost tracks concrete growth, guides preventive work

ROCK ISLAND — Chelan County PUD commissioners approved a $1.413 million budget increase Oct. 19 for a Rock Island Dam monitoring project intended to help engineers track concrete expansion and plan preventive work before substantial damage develops.

The increase brings the project budget to $7.118 million. The work includes installing instruments at 27 locations to monitor dam movement and concrete stress, along with testing concrete and underlying bedrock to improve predictions of how the structures will change over time.

“This is kind of a once in a generation take at improving our instrumentation and monitoring of Rock Island Dam,” PUD dam safety engineer Brandon Little told commissioners.

Little said the project will collect samples representing the different vintages of concrete at the dam, along with bedrock samples, to check design assumptions and support detailed three-dimensional computer models.

The testing and monitoring will help the PUD assess alkali-silica reactivity, or ASR, and predict future concrete growth. Engineers will use those findings to develop measures to manage the expansion.

Little said the modeling will begin with the dam’s drop gate structure, with Powerhouse 1 and the spillway likely to follow. The PUD could develop a series of smaller models or eventually combine the structures into one larger model.

The goal is to identify where and when intervention would be useful, allowing crews to act before large-scale concrete damage occurs. Little said saw cutting is a likely approach, though other methods will also be considered.

Measurements from the new instruments will allow engineers to compare actual changes at the dam with the models’ predictions, refine those predictions over time and evaluate whether interventions are working.

The project’s higher cost reflects increased estimates for concrete testing, conduit routing, drilling and access, along with additional instrument cleaning requested by the Federal Energy Regulatory Commission.

The presentation identified approximately $500,000 in increased concrete testing costs, $325,000 for revised conduit routing estimates, $200,000 to apply prevailing wages to remaining drilling activities and $100,000 for difficult drilling access. Additional instrument cleaning accounted for about $50,000.

Commissioners also asked how the equipment would measure changes and how long it would remain useful.

Little described one instrument, an inclinometer, as a tube installed in the dam into which a sensor can be lowered. By measuring tilt at different depths, engineers can track deformation in multiple directions over time.

The sensor can be replaced if damaged, while the installed tube can remain useful for decades as long as it remains intact and accessible, he said.

Other monitoring equipment measures pressure and movement at joints. Little said many of the instrument types are also used at Rocky Reach Dam, although Rock Island’s age, geometry and ASR issues call for a more extensive monitoring system.

Following the improvements, Rock Island will have the PUD’s most robust monitoring system at its generating facilities, he said.

“The instruments that we're proposing are not novel or new for our facilities,” Little said. “They're very common throughout the industry and well-supported with long service histories.”

The Oct. 19 action increased the individual project’s budget without increasing the overall Rock Island system budget. Staff expect to bring a broader system budget increase to commissioners Nov. 16, reflecting accelerated progress on multiple projects.

Andrew Simpson: 509-433-7626 or andrew@ward.media