In partnership with Brown & Caldwell (B&C), SAGE provided services for two improvement projects for the Contra Costa Canal in Oakley, California. The Contra Costa Canal (CCC) conveys water from Rock Slough to Pumping Plant 1 (PP1). Both were constructed in the 1930s by the Bureau of Reclamation, which owns the 300-foot-wide right-of-way where the canal is located, although a water district currently operates them.
The first phase of work, described as the Canal Replacement Project, consisted of replacing 2,100 feet of the existing canal, from PPI to an existing siphon at Marsh Creek, with a 10-foot-diameter reinforced concrete pipe. The purpose of the pipeline was to separate the canal from surrounding shallow groundwater, improving the quality of water that entered the pumping plant. The new pipe invert was constructed below the previous canal invert, the canal levees were leveled, and the excavated levee fill was used to backfill over the new pipe. Because the canal is tidally influenced, it was necessary to construct cofferdams to isolate the work zone and facilitate dewatering.
The second phase of work consisted of the construction of a new fish screen at the upstream entrance to the canal. The new fish screen replaced an outdated fish screen that did not meet current fish screen standards. However, the new fish screen is about twice as long as the original screen, necessitating that the canal levees be moved back to reduce entrance velocities. It was also necessary to dewater the confluence of Rock Slough and the CCC to allow construction activities.
SAGE provided geostructural consultation during design of the Canal Replacement Project, including:
- Feasibility analysis of potential cofferdam structures and locations
- Structural design of the selected cofferdam structure
- Analysis of seepage through the existing Marsh Creek levees and beneath the existing box culvert siphon
- Evaluation of the potential for internal erosion and/or piping at the location of the siphon penetration through the Marsh Creek levees
- Design of seepage control measures to mitigate through-seepage and piping concerns at both Marsh Creek and PP1
- Embankment and levee design criteria, including design of sand filters at pipe penetrations
- Keyway requirements for temporary levees at inlet/outlet structures
- Design Services During Construction (DSDC), including bid period assistance, submittal and RFI review, and as-needed construction observation
During the project, the environmental permitting constraints were revised, necessitating a total revision of the project design concept. SAGE worked with the project team to develop a phased cofferdam installation, dewatering, and deconstruction program to prevent leakage from Marsh Creek, which was an environmental goal for the project. We also assisted the team with constructability analysis and value engineering during the redesign phase.
For the fish screen project, SAGE performed the geotechnical analysis and design of the two new setback levees used widen the confluence of CCC and Rock Slough, which now serve as the permanent project levees. SAGE reviewed existing geotechnical data for the site, and performed supplemental geotechnical exploration to confirm existing soil stratigraphy. Because it was assumed the levees may need to be certified flood protection features in the future, they were designed following United States Army Corps of Engineers and State of California Department of Water Resources standards. The levees feature minimum 20-foot-wide crests, 3H:1V side slopes, and three feet of freeboard over the 200-year Design Water Surface Elevation (DWSE). Analyses included through and under seepage evaluations and static and seismic slope stability analyses. Coordination with the adjacent Reclamation District and their consultant during the levee design and permitting process was required. We also designed the three temporary cofferdams constructed across each arm of the confluence for construction dewatering. The cofferdams were in place for more than one winter season. The cofferdams were designed so that during winter the area could be unwatered to facilitate fish rescue, but then the pumps could also be shut off and the area allowed to refill.
SAGE performed feasibility analysis of potential cofferdam structures and locations and structural design of the selected cofferdam structures. The cofferdams were constructed using steel sheet piles buttressed with rockfill; fill was not allowed outside the cofferdam system. The cofferdams retained up to 20 feet of water/soft sediment. The amount of rockfill used for the cofferdams had to be balanced to accommodate tidal influences, including potentially both Lower Low Water and Higher High Water conditions, as well as variable water levels within the cofferdam system depending on the status of pumping.
SAGE’s services for the fish screen project also included preparation of construction plans and specifications for the cofferdam construction, as well as field review of cofferdam and levee construction.