Leroy Anderson Dam, located about ten miles south of San Jose, California, is a 240-foot-high embankment dam retaining a 90,000-acre-foot reservoir, that was built in 1950. The dam is located approximately 1.7 kilometers west of the active Calaveras Fault, and the dam footprint is crossed by the conditionally active Coyote Creek Fault Zone. The embankment is vulnerable to seismic deformations due to potentially liquefiable materials underlying the embankment shells and within the upstream shell. The dam lacks an engineered filter and drainage chimney between the core and downstream shell and is vulnerable to cracking caused by foundation fault offset of up to 4 feet. Failure of the dam during a seismic event could result in catastrophic flooding through San Jose and adjacent communities. To address the seismic vulnerabilities, the Santa Clara Valley Water District (Valley Water) initiated the Anderson Dam Seismic Retrofit Project (ADSRP), which will include replacing the existing dam with a new dam that meets modern seismic safety standards.< br>Due to site constraints, the dam will be replaced at the same location as the existing dam leading to significant scheduling challenges during construction. Removal and replacement of the dam is planned to occur over a five-year period, with a series of four interim dam heights. These will include two years with lowered sections of the existing dam, two years with partially completed sections of the replacement dam, and a fifth year to complete the final replacement dam section. The geometries and elevations of the interim dam sections were determined based on balancing the volume of earthwork that can be completed each construction season with risk of overtopping of the partial-height dam sections during extreme winter storm events. The project design includes a new diversion tunnel system to bypass reservoir inflows during the winter seasons. Each interim height dam section will be protected with temporary articulated concrete block lined spillways on the downstream slopes, installed following the end of earthwork before each winter season to address the residual risk of more extreme inflow events.< br>This paper will provide an overview of the project design including key design criteria, construction sequencing considerations for removal and placement of the dam, geotechnical analyses of the interim dam sections, and evaluation of construction schedule risks and considerations. This paper will also briefly discuss risk mitigation strategies, surveillance monitoring, Emergency Action Planning, and coordination with dam safety regulatory agencies.
