Design Approach for a Spillway Replacement in Weak California Rock

Inspections at Santiago Creek Dam in Orange County, California following the 2017 Oroville Dam event showed the spillway has reached the end of its design life and is in poor condition, requiring replacement to improve safety, pass larger flows, and improve access to site facilities during spills. The proposed replacement spillway features a complex design and geometry due to limited space between the embankment dam and adjacent site features. To accommodate the proposed spillway, cut slopes about 200 feet high will be required at the left abutment. The geology at the left abutment consists of a thick sequence of marine sandstone and shale, which are intensely weathered in outcrops. The shale unit in particular presents significant engineering challenges due to its relatively low strength, which is exacerbated by the deep weathering profile sometimes more than 40 feet deep in borings. This paper describes the geologic characterization of the left abutment and along the spillway chute, and the development of material and strength parameters for these units. The engineering solutions to overcome the difficult geologic conditions are described, including the stability analysis approach and challenging design of the cut slopes, the design of the spillway control structure and retaining walls, and the foundation criteria and treatment. Constructability challenges and risk are discussed.