Pacific States Environmental Contractors, Inc.
Dumbarton Quarry is located in the City of Fremont, adjacent to the San Francisco Bay. For more than 40 years Dumbarton Quarry produced crushed rock and asphaltic concrete products meeting California Department of Transportation specifications for roadway construction. In 1997, operation of the quarry was extended to 2007 and a final second use plan based on agreements was reached with the East Bay Regional Park District (EBRPD). The visionary plan described site reclamation topography that would prepare the site for subsequent conversion to a regional public park that included filling the excavation with water for public recreation uses. However, the planned lake had no identified water source or drainage outlet. Procuring a water source and permitting an “internally draining basin” created obstacles in implementation because of changes in environmental and regulatory conditions.
Dumbarton Quarry Associates (the Applicant) and EBRPD developed an alternate plan that involved backfilling of the entire excavation, requiring over 6 million cubic yards of fill. The fill would come from regional construction projects, where excess cut from grading requires offsite disposal. In 2017, the plan was again revised to accommodate an additional 1.5 million yards of fill.
The Dumbarton Quarry Regional Recreation Area will be a 91-acre regional park with more than 100 campsites, an amphitheater, event center, restrooms, and an expansive meadow with views of San Francisco Bay, the East Bay hills, and the Peninsula. The park will provide trail connections to the 30,000-acre Don Edwards National Wildlife refuge with the 980-acre Coyote Hills Regional Park, linking to the San Francisco Bay Trail and hiking in the wetlands and grassy hillsides of the southern East Bay shoreline.
Key issues of the project surrounded the geotechnical stability of over 300 feet of fill in a seismically active region. Fill would need to be stable to support future park facilities and public use. Lack of predictability of the quantities and timing of fill availability was a challenge for reclamation timing and the financial assurance cost estimate. Visual compatibility of fill as viewed from surrounding residential, commercial, and regional park viewpoints was a community concern.
Benchmark Resources developed a reclamation plan for this unique project that fulfilled current requirements of the California Surface Mining and Reclamation Act, demonstrated the geotechnical stability required, and satisfied the visual compatibility concerns of the community. Geotechnical requirements were met through an approach of adaptive management using dynamic compaction, testing and phasing that accommodated differential settlement. Visual compatibility was resolved by a contour grading plan that re-created a portion of the pre-mining ridgeline, as demonstrated by topographic simulations of topography from 50 years earlier.