Engineering Monograph No. 45: The fluid mechanics of hydraulic jacking

Authors: WAHL, TONY - Bureau Of Reclamation; Heiner, Bryan

Submitted to: Government Publication/Report
Publication Type: Government Publication
Publication Acceptance Date: 2/13/2025
Publication Date: 3/24/2025
Citation: Wahl, T., Heiner, B.J. 2025. Engineering Monograph No. 45: The fluid mechanics of hydraulic jacking. Government Publication/Report. Federal Agency engineering monograph. V-Government Publication.https://www.usbr.gov/tsc/techreferences/hydraulics_lab/pubs/EM/EM45

Interpretive Summary: In 2017, the Oroville Dam spillway in California suffered a catastrophic failure that brought to light the dangers of hydraulic jacking—a hidden but significant threat to concrete spillways. Hydraulic jacking occurs when water flows through cracks or joints in a spillway and creates high pressure beneath the surface. This pressure can lift the concrete slabs, leading to erosion, collapse, and potentially devastating consequences. Despite earlier research, the mechanics of this process remained poorly understood, leaving spillway designers and risk analysts without reliable tools to prevent future failures. To address this critical issue, researchers conducted new experiments between 2021 and 2023, simulating hydraulic jacking in controlled laboratory conditions. They tested how flow velocity, joint shapes, and different foundation conditions influence uplift pressure. Using a supercritical water flume and adjustable joint models, the team developed improved equations for predicting uplift pressures and water flow through cracks. These equations reduce uncertainty in predicting hydraulic jacking pressures by a factor of three compared to earlier methods. Importantly, the research also explored ways to modify joints—such as chamfering or rounding edges—to minimize the risks. The findings were incorporated into analysis tools used by dam safety engineers and applied to real-world cases, including the Oroville spillway, to better assess risks and identify vulnerable areas. This work equips engineers with more accurate tools to design safer spillways and retrofit existing structures, reducing the likelihood of catastrophic failures like Oroville. By improving our understanding of hydraulic jacking, this research contributes significantly to protecting critical infrastructure and public safety. USDA is an equal opportunity provider and employer.

Technical Abstract: Hydraulic jacking is a serious threat to concrete spillway chutes, illustrated by the catastrophic chute failure that occurred in 2017 at Oroville Dam, a California Department of Water Resources facility. Hydraulic jacking occurs when joints or cracks in a spillway chute experience differential settlement or surface damage that creates offsets into the flow. Stagnation of flow against such offsets converts the kinetic energy of the flow into high pressures that can be injected through open joints into the foundation, leading to uplift failure of slabs or foundation erosion followed by slab collapse. Laboratory experiments in the 1970s and early 2000s associated uplift pressures with chute velocity and joint geometry but details were still poorly understood at the time of the Oroville failure. To support the efforts of chute designers and risk analysts to mitigate against hydraulic jacking failures, new experimental tests were conducted by Reclamation from 2021 to 2023. New equations were developed for estimating uplift pressure, flow rate through joints and cracks, and the effects of various methods of remediating existing offsets. The new laboratory tests were performed in a supercritical flume furnished with a model joint where the gap width to offset height ratio was varied over a 725:1 range. The tests included measurement of boundary layer velocity profiles approaching the joint and uplift pressures associated with unvented (sealed) and vented foundation conditions. Uplift pressures were normalized to the velocity head near the boundary, which is related to depth-wise velocity profile exponents determined in the experiments and can be estimated for field applications from the chute friction factor. The normalized uplift varies with the joint aspect ratio and the flow depth to offset height ratio. The new relations reduce the uncertainty of modeled uplift pressures by a factor of about 3 compared to previous methods. The experimental work also led to new relations for calculating discharge through open joints and the results show that extremely large flow rates are possible. Testing of joints and cracks with irregular geometries led to useful relations for estimating uplift at joints that are chamfered, rounded, skewed, beveled, or otherwise relieved to reduce uplift. Finally, the new relationships developed in this research have been incorporated into spillway analysis software tools used at Reclamation. Example application to the Oroville spillway case is used to demonstrate trends in hydraulic jacking risks along the full length of a spillway and over wide ranges of discharge. USDA is an equal opportunity provider and employer.