Priest Rapids Spillway Performance and Stability Assessment

Abstract Only - The Priest Rapids Dam, constructed in the late 1950’s, is located on the Columbia River in Central Washington and is owned and operated by Grant PUD. The spillway is constructed of twenty-three concrete monoliths consisting of a 12-foot wide reinforced concrete pier with concrete ogee sections on either side for a total width of fifty-two feet. There are twenty-two radial tainter gates that can discharge up to 1.34M cfs. Construction of the spillway was uniquely sequenced to facilitate the flowing Columbia River. This sequencing, along with other contributing factors, played an important role in the structure’s performance.
Following the 2014 Wanapum spillway incident, Grant PUD analyzed the internal stability of the Priest Rapids spillway monoliths. The stability analyses indicated that critical failure planes would likely be located within the monolith, not at the base. This paper will discuss the recent and on-going proceedings following the initial stability analysis.
An initial field investigation, performed in 2017, included a small set of core holes that were drilled in the Priest Rapids spillway monoliths for testing and analysis, plus monitoring and drainage. The 2017 drilling program also included installation of instrumentation at the spillway foundation. Pressure readings from both the new instrumentation and drilled core holes indicated a possible connection between the core holes and the reservoir, leading to a second field investigation in early 2018. The phase two drilling investigation included over a couple hundred core holes through three concrete horizontal lift joints in the spillway structure. It soon became apparent that one of the internal lift joints had experienced separation and had varying hydraulic connectivity with the reservoir.
A non-failure emergency condition of the EAP was activated in March 2018 and the reservoir was lowered 3.5 ft. to ensure structure stability while investigation, monitoring, and analysis continued. Through recently-installed automated instrumentation, lift joint drain monitoring, and stability analyses, the District was able to terminate the EAP condition and lift the operational reservoir level restriction. Currently the District is completing additional analyses to address global structure stability for all load cases and planning the path forward for stability improvement work.