A significant Project challenge was locating and transporting the specified backfill materials. The closest specified material source was approximately 30 miles north in Nicholas, CA on a 130-acre site, where the groundwater table, at times, was within 3 to 4 feet of the ground surface. Occasionally, when the excavation took place below the groundwater table, the project team had to get creative to successfully excavate, transport, and place the overly saturated soft soils. Constructing channels to divert the daylighting groundwater from the excavation and pumping the water away from the work area is one example of the team’s creative approach.

Cutoff Wall Construction The project required construction of more than 1.3 million square feet of cutoff wall ranging in depth from approximately 45 to 135 feet. Cutoff walls were constructed using conventional open trench methods and include a 168,000 SF slag cement cement bentonite cutoff wall, a 650,000 SF soil bentonite cutoff wall, and a 550,000 SF deep mix method cutoff wall. There were numerous utility crossings through the proposed alignment of the cutoff wall, such as sewer, cable, and electric lines, that were researched, potholed, protectedin-place, removed, or replaced by MOJV. To complete the cutoff wall construction on schedule in one construction season, MOJV employed six separate headings that were completed simultaneously, which all required crew, equipment, supervision, and management on a large scale. Crew sizes varied between 50 and 75 personnel.

Borrow Site Management MOJV managed the 130-acre borrow site located in Nicholas, CA, which is operated in Reclamation District 1001 and licensed under CEQA as ID Number 91-51-0004. Overburden removal occurred prior to excavation across the active borrow area at a depth of 2 feet. Excavated overburden was stockpiled for future use as reclamation soil. Approximately 108,141 CY of lean clay was excavated from the borrow site for use as levee fill. MOJV used a Komatsu PC1250 excavator working on top of timber crane mats for stability on the soft soils as excavation proceeded. The in situ clay was saturated and required blending with soils from the full 5-foot depth of excavation using low-ground-pressure CAT D6 dozers for handling and blending soft and saturated clay with sandy material from levee degrade. Upon sufficient blending of levee fill, the blended fill was loaded into legal highway trucks that used a dedicated haul route within the borrow area footprint. Site access was challenging given the soft nature of the subgrade; dedicated haul routes were planned and used to ensure that trucks could access the borrow site. Following completion of excavation operations, the overburden soil was placed directly over the top of the soft subgrade where groundwater was now present at the surface. 

Additional Project Components MOJV was responsible for providing and maintaining all temporary facilities; implementing Storm Water Pollution Prevention Plan requirements; complying with permit requirements and environmental mitigation measures; developing staging areas; coordinating with Agencies and stakeholders; managing the borrow site; reusing excavated soils that met specifications as levee fill after blending with borrow site soils; managing and disposing of excess excavated soils; removing and replacing 2.5 miles of asphalt-paved bike paths; accommodating underground utilities and replacing six pipe crossings (HDPE and steel); removing temporary facilities; and restoring and seeding all disturbed areas.

Earthwork Earthwork on the soft soil borrow site was accomplished using low-ground-pressure equipment (including CAT D6 dozers) and a Komatsu PC1250 excavator that worked on top of timber crane mats for stability. The project team dug channels to divert daylighting groundwater from the excavation and pumped the water away from the work area. Sandy material generated by levee degrade was blended with fill material (lean clay) at the borrow site using similar means before being trucked to the Project Site for levee construction.

Fill Material The total volume of fill material excavated in shallow groundwater elevations was 108,141 CY. Ultimately, MOJV used 151,004 compacted CY of blended fill material comprising lean clay from the borrow site and sandy soil from levee degrade operations for levee construction. The fill material consisted of lean clay. It was excavated from a borrow site managed by MOJV and blended with suitable sandy soil generated during levee degrade operations.