Water Resources Engineering, Inc.
When built, the proposed $160 million operable gates and appurtenant structures will be the largest facility of its kind in the United States.
The US Bureau of Reclamation (Reclamation) and the California Department of Water Resources (DWR) propose to implement the Franks Tract Project to improve water quality and fisheries conditions in the Sacramento–San Joaquin Delta (Delta). Reclamation and DWR are planning to install operable gates on Threemile Slough to reduce seawater intrusion and positively influence movement of fish species of concern to areas that provide favorable habitat conditions. The project will include a navigation lock to allow boat passage when the gates are up, as well as levee improvements and removable floating booms to protect the gates.
Water Resources Engineering (WRE) was the prime contractor responsible for the feasibility study of the operable gates. TERRA provided geotechnical engineering design services as a subcontractor to WRE. After a number of options were evaluated, Reclamation and DWR decided to implement a 550-foot wide, 30-foot tall removable barrier composed of ten bottom-hinged gates lifted and lowered by hydraulic cylinders. TERRA evaluated the geotechnical conditions within the project area and provided geotechnical engineering recommendations for the gate foundations and the stabilization of the levees along Threemile Slough in the vicinity of the gates. Liquefaction of the soils beneath the levees and the gate foundations was a significant issue.
Levee stabilization measures considered and carried to the feasibility-level design by TERRA included cellular retaining wall structures combined with densification of the native soils by vibro-replacement, and reinforced concrete walls constructed to a depth of 100 feet using the slurry trench method combined with treatment of the native soils with Cement Deep Soil Mixing. Pile support was recommended for the gate foundations which are to be constructed within a cofferdam.