Uplift Pressures below Spillway Chute Slabs at Unvented Open Offset Joints

The catastrophic failure of the spillway chute at Oroville Dam in February 2017 raised concerns throughout the water resources industry regarding design, construction, and maintenance practices for concrete spillway chutes, especially joints and cracks that could allow penetration of high-pressure water into a chute foundation. The independent forensic team investigation found that hydraulic jacking was the most likely cause of the initial chute slab failure, highlighting a need for better analysis of the hydraulic jacking potential of existing spillways and more resilient designs for spillways that operate under high-velocity flow conditions. This paper reviews the Oroville Dam event and findings and previous laboratory testing performed to evaluate uplift pressures and flow transmitted through spillway joints. A reanalysis of previous studies was used to develop relations between chute velocity, joint geometry, and uplift pressure transmitted into a joint. Uplift pressure head in these relations is expressed in a dimensionless manner, either as a percentage of the velocity head in the boundary layer at midheight of the offset into the flow, or as a percentage of the channel-average velocity head. The first approach is potentially more useful for prototype applications, but the second method provides the best fit to the available experimental data. Additional research is still needed to quantify rates of flow through open joints, confirm relations between uplift pressure and boundary layer velocities, and evaluate the effects of aerated flow.