Author(s): Tony L. Wahl; Bryan J. Heiner

Linked Author(s): Tony Wahl

Keywords: Spillways; hydraulic jacking; uplift; boundary layer; risk

Abstract: The 2017 failure of the spillway chute at Oroville Dam highlighted the need to better understand and prevent hydraulic jacking. Such failures originate from offsets into the flow that develop at cracks or open joints that experience differential movement. Modern spillways are usually constructed with multiple defenses, including keyed and reinforced joints, waterstops, and subsurface drainage, but older structures often lack these features or are in deteriorated condition. Flow striking an offset stagnates at the boundary, which produces high pressure and can drive large flows into the foundation beneath the chute lining. Design and evaluation of anchors, drains, and joint remediation concepts requires estimation of uplift pressures and flow rates into the joints. This article reviews recent laboratory tests in a supercritical open-channel flume that produced equations that relate uplift pressure and joint discharge to the joint properties (offset height, gap width, and orientation), bulk flow properties (velocity head and flow depth), and the boundary layer velocity profile in the chute. The new equations provide a three-fold improvement in the accuracy of uplift pressure prediction compared to previous methods and give the first accurate predictions of flow rates through joints. This paper then applies the new equations to the Oroville case to demonstrate the variation of hydraulic jacking risk factors along the length of a prototype spillway chute for a range of discharge conditions. Important insights are noted for designers and spillway risk evaluators, and sensitivities to offset height, gap width, and chute roughness are illustrated.

DOI: https://doi.org/10.3929/ethz-b-000675921

Year: 2024