CS4A - Concurrent Session 04A: Replacing 5000-lb Trunnion Pins on a Giant Radial Spillway Gate

Abstract Description: Tainter Gate 12 at Wanapum Dam had been out of service for 4 years, limiting the crucial ability to pass flows on one of the nation’s largest rivers. Like many on the Columbia River, this is a truly massive spillway gate, standing 65-ft tall and 50-ft wide. In early 2020, workers observed improper rotation of the pin indicating high trunnion friction. The gate was immediately placed in emergency-use-only status until repairs could be made. Like many things in 2020, this work was postponed for the global pandemic. Add in questions about the capacity of the dam’s resident cranes, competing priorities, and a host of other issues, and years slipped by without progress. With that time, our institutional knowledge of how to perform repairs began to degrade.

The project was jump-started in 2023 with a new commitment to the FERC to complete repairs by the end of 2024. Our first task was research. We combed through old records, interviewed contractors and manufacturers, and employed the only in-house inspector remaining who’d worked on the project to upgrade our trunnions. Our goal was a fool-proof plan involving personnel access, gate stabilization, pin swapping, and thorough testing. We endeavored to leave very little to chance once work began.

Grant PUD, dam owner, ultimately elected to hire a contractor to perform the work using nearly all District materials, equipment, and tools. The contractor provided the crane, the crew, and years of experience on similar projects. The District spent nearly 18 months planning, building tools, milling precise replacement pins, and negotiating a contract. All these efforts culminated in a successful 4-week project for our contractor.
Intent on leaving a better record of how we accomplished this project, a post-project effort was undertaken to document the procedure. The contractor submitted lessons-learned and provided a fantastic library of project photos. District personnel compiled daily reports and our own photos and focused especially on parts of the project where information had been missing before. We were able to close this project knowing that we’ve left the District in a better position to handle this particular problem in the future.