During a Dec. 15 presentation to the Gridley City Council, City Engineer Dave Harden said new in‑river drilling conducted this fall revealed that the soil beneath the Feather River is far less stable than earlier borings suggested. Designed by Freepik, www.freepik.com

 

GRIDLEY, CA (MPG) - Once again, Gridley’s unusual subsurface conditions are disrupting a major public works project. The city, which was previously forced to alter infrastructure plans due to unexpected soil behavior, is now facing similar complications on its Feather River Sewer Crossing replacement effort.

During a Dec. 15 presentation to the Gridley City Council, City Engineer Dave Harden said new in‑river drilling conducted this fall revealed that the soil beneath the Feather River is far less stable than earlier borings suggested, eliminating micro-tunneling as a feasible option for replacing the city’s 1967 sewer line that carries all wastewater across the river. 

City officials said the discovery forces the city back into a full reevaluation of alternatives. 

“The results here are not what we were hoping for,” Harden told the council. 

Chief Administrative Officer Elisa Arteaga said the city is working under a $1.3 million Clean Water State Revolving Fund planning grant, with about $520,000 remaining. Bennett Engineering Services is under contract to complete environmental review, permitting and preliminary design. All required permits have been secured, and the city had adopted its Initial Study and Mitigated Negative Declaration when complications arose.

Harden said the new borings showed that the trench zone originally believed to provide 10 to 15 feet of suitable material pinched out to only a few feet, leaving the tunnel path dominated by gravels and cobbles that could collapse into the bore.

“To make micro-tunneling feasible, we would have to go roughly 35 feet deeper than planned, to nearly 100 feet below the river,” he said. “That adds significant cost and risk.”

Because the geotechnical results directly affect the selection of a feasible alternative, Bennett Engineering has paused the Draft Feasibility Study until the city determines a new path forward. Harden said the city will meet with tunneling contractors and industry specialists early next year to evaluate constructability and cost. With micro tunneling no longer viable at the planned depth, the city is revisiting alternatives previously set aside, including rerouting the sewer line north to cross the Feather River at the East Gridley Road bridge.

Harden said the option would add roughly two miles of pipeline and require evaluating pump station upgrades, levee‑adjacent construction and permitting implications. He also noted the State Water Resources Control Board may require the city to revisit regionalization concepts, including potential connection to Biggs, depending on long term feasibility and funding.

Arteaga recommended, and the council supported, a no cost amendment to the Bennett Engineering task order. The amendment reallocates funding from the design phase to feasibility and environmental tasks and limits design work to the 30 percent level until a viable alternative is selected.

The council authorized a budget transfer that shifts $60,000 into the feasibility task and $30,000 into environmental documents, reducing the plans and specifications budget by $90,000. No additional city funds are required, Arteaga said.

The city will also request a grant extension from the State Water Resources Control Board. The current final disbursement deadline is Aug. 27, but the city is seeking an extension to Jan. 30, 2029, to allow time for additional analysis, revised environmental documentation, and development of a new amended project report.

Harden emphasized that the existing river bottom pipe remains operational but cannot be replaced in its current location under modern permitting rules.

“Our job now is to find the most fiscally and technically feasible solution,” Harden said.