3D Seepage and Stability Analysis on Oroville Dam, Tallest Earthen Embankment Dam in United States: Finalist of Bentley's YII Geotechnical Engineering Awards Featured

Oroville Dam is the tallest earth dam in the United States, rising up to 770 feet, and is the key element in California’s State Water Project. The dam is almost 40 years old, however, and the last two five-year inspections resulted in recommendations for an improved seepage monitoring program. HDR Engineering was tasked with developing that program and, due to the dam’s height and steep embankment, more traditional design methods would not be sufficient.

To monitor the dam's seepage level and comply with regulations, HDR Engineering used Bentley's applications SVDESIGNER, SVFLUX and SVSLOPE to conduct -for the first time ever- a 3D seepage and stability analysis with supplemental 2D analysis of the Oroville Dam model.

The biggest challenge HDR had to face while setting up the model was inserting the multiple surfaces from the different dam zones and then meshing the model. With the help of Bentley's SoilVision team, they converted the model and develop surface meshes by using a “layered cake” approach. The developed mesh contained 407,561 nodes and 222,930 elements, an extremely large mesh for a complicated internal zoned dam.

Foundation topography and valley effects were incorporated into the seepage analysis for a more accurate simulation. Cross Section I, being the largest section of the dam, includes the seepage collection system, downstream of the dam. Sections II and III were used to evaluate the total seepage from the dam by analyzing 2D cross sections and compare them with the flux line summing of the entire 3D model seepage. 

The downstream shell of the dam is unsaturated. Bentley's fine element solver was able to handle the large hydraulic conductivity contrast between the clay core and the rocky downstream shell. 

Features such as elevating the slope surface above the dam gave HDR Engineering a good idea of the volume of the slip surface. The factor of safety shading also provides a good visualization for regulators to get a better idea about the type of slip surface: deep seated vs shallow or a highly nonlinear. 

Using Bentley's applications allowed HDR Engineering to increase their proficiency with the tools quickly and ensure that they were on the right path for completing their analysis. The knowledge gained by performing the seepage analysis using Bentley's tools, allowed HDR Engineering to confirm the in-situ measurements made at the dam and provided insights on how the seepage flows were crossing the dam cross section and flowing below the dam as well.