OCSD Wastewater Storage and Reclamation Project

The project comprised a feasibility study and two design-level geotechnical investigations for two potential 12.5 million gallon capacity wastewater reservoir sites, referred to as Site 3A and Site 3C-2, located just outside the town of Occidental in West Sonoma County, California.

The report's purpose was to (1) identify geological and geotechnical constraints and advantages to development and (2) establish criteria for the design and Construction of proposed earthen construction storage reservoirs at the sites. Work performed included a detailed background literature review, geological field mapping, subsurface geotechnical explorations (test pits, soil and rock borings, CPTs and a seismic refraction survey), laboratory analysis and stability evaluations. Detailed investigation initially focused on Site 3A, however, Site 3A was judged inadequate for cost-effective Construction due to factors including high groundwater, weak expansive soils, liquefaction potential, inadequate slope and earthquake stability and asbestiform materials in serpentinite bedrock and overlying soils within the excavation footprint. As a result, the investigation was broadened, which led to the detailed investigation of Site 3C-2.

The results of the investigation for Site 3C-2 indicated favorable conditions for Construction and long-term usage. Site 3C-2 is underlain by rippable, highly weathered, weak sandstone conglomerate from the existing ground surface to 24 feet to 41 feet. This is underlain by more competent but still weak, weathered and variable graywacke sandstone and shale sedimentary sequences. Both units may be used for embankment fill construction. Excess material may be exported for use as fill on other sites. Stability analyses of the approximately 45 foot deep excavation below existing grade necessary to establish the reservoir show that the interior and exterior slopes will be stable in Construction, in long term static loading and during rapid drawdown conditions (sudden drainage of the reservoir). Seismic analysis based on the maximum credible earthquake (MCE) of magnitude 7.9 from the San Andreas Fault (which is approximately 12.16 km (7.6 miles) away and is the controlling design seismic source for the site) indicate an adequate factor of safety. No known active faults cross the site, so fault rupture is not anticipated. Serpentinite bedrock containing asbestiform minerals is located immediately to the southwest of the proposed 3C-2 reservoir footprint and extends to the depths explored. No evidence of the occurrence of serpentinite was observed in the excavation area. If the area of serpentinite remains undisturbed, it does not affect the construction feasibility of the 3C-2 reservoir.