American River Common Feature Natomas Basin Reach I

Sacramento’s Natomas Basin is one of the most flood-prone areas in the country. Improvements to the levees in Natomas were first authorized in 1996 as part of the American River Common Features project, aiming to strengthen the levee system throughout the Sacramento region. However, severe floods in 1997 and 2006 exposed major issues with water seeping under the levees, leading to an expanded scope of work. 
 
The U.S. Army Corps of Engineers, in collaboration with the State of California and the Sacramento Area Flood Control Agency, is now working to reinforce 42 miles of levees around the Natomas Basin. The Natomas Basin Reach I project focuses on the section of levee next to the American River on Garden Hwy between Gateway Oaks Blvd, Interstate 5 (I-5), and Northgate Blvd, requiring installation of approximately two miles of slag cement-cement-bentonite (SCCB) cutoff walls to control water seepage under that section of the levee. 
 
This project required extensive traffic control to maintain access to the park, to the highway, to residential areas, to businesses and restaurants, and to a community bike path in addition to keeping project personnel safe. Garden Highway is one of the busiest highways in the Sacramento area. 

We held numerous logistical discussions with various agencies (USACE, SAFCA, City of Sacramento, California Department of Water Board, Central Valley Flood Control Board, and CalTrans) to accommodate the construction of a SCCB slurry wall, relocation of a 24-inch waterline, relocation of a 4-in sewer line, and a replace and removal of the valve box and discharge pipes (two 24-in and one 12-in) at sump 58. To address access and traffic issues, we divided the project into two phases, allowing traffic and local businesses to remain unaffected by our work from Truxel Road to Northgate Blvd and from Truxel Road to Gateway Oaks Drive. 

We assisted businesses in setting up banners and signs along detour routes to ensure customers could reach the restaurants. Traffic detours did not delay commuters using Garden Highway for their daily routes, and we even provided bicycle routes during construction. 

We installed 494,000 square feet of a SCCB cutoff wall to a max depth of 61 feet along the center of the levee crown (Graden Hwy), with an average depth of 45 feet. 

Before starting the slurry wall construction, we sawcut and removed AC/AB, installed vibration monitors, implemented protective measures for endangered species, and installed SWPPP features, including a silt fence on each side of the levee crown. To mitigate the impacts of high temperatures on the SCCB slurry line, we covered the slurry line with  a white plastic over the length of the line (1-mi). 

Forgen operated its own long-reach excavator with an extended boom and stick, along with SCCB mixing plants for the slurry wall installation. We installed a 24-inch waterline to reroute utilities and removed existing 12-inch and 24-inch waterlines. We also raised existing pipes for Sump 58. 

The project required us to install a landside drainage blanket below the I-5 bridge, consisting of zoned earth embankments with precise transitions from one material to another. We used the spoils from the cutoff wall construction to create a berm at Reach 19-A, located a couple of miles west of I-5 along Garden Highway. We relocated, spread, shaped, and compacted the spoils, transporting them in watertight dump trucks to the Reach 19-A berm. 

For project finalization, we flattened the levee slope, hydroseeded, and installed a CLSM cap and AC paving to restore the road after the cutoff wall installation. 

The project schedule and total cost changed from the initial award due to multiple design changes introduced by the Corps after the project started. These changes arose from unknown conditions uncovered during the work and added unforeseen complexities. We collaborated with the Corps during redesign and resequencing to minimize delays. 

The installation of the SCCB cutoff wall significantly strengthened and improved the levee, which had previously been vulnerable to flooding due to water seepage. After 28 days, the hydraulic conductivity of the cutoff wall was measured to be lower than the specified threshold of 1 x 10⁻⁷ cm/s, demonstrating its effectiveness in preventing water seepage. 

Additionally, the Unconfined Compressive Strength (UCS) of the wall fell within the specified range of 50 to 300 psi, ensuring the structural integrity required for long-term flood control. Specifications required a minimum UCS of 30-psi at 3-days, which the developed mix comply without hesitation. As a result, the cutoff wall successfully mitigated water seepage under this section of the levee, enhancing the overall flood resilience of the area. 

The USACE rated our team as Exceptional across all categories in the Contractor Performance Assessment Report (CPARS): Quality, Schedule, Management, Regulatory Compliance, and Safety. The project was the winner of two awards with the ASCE: 2021 ASCE Sacramento Section Project Award Winner – Outstanding Flood Management Project and 2023 ASCE Region 9 Outstanding Flood Management Project. 

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