District Starts Sewer Rehabilitation Project

March  28, 2012

 Napa Sanitation District is breaking ground on a project to reduce wet weather sewer flows in an area of southwest Napa. This area south of downtown Napa, bounded by Madrona Street to the north, Hemlock Street to the south, South Jefferson Street to the east and South Seymour Street to the west, is a portion of a larger area that the District refers to as “Basin L”. Basin L was identified in the District’s Collection System Master Plan as an area with significant inflow and infiltration.

Inflow and infiltration (I&I) is water that flows into sewer pipes from groundwater and stormwater. Groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint failures, and faulty connections. Stormwater (inflow) rapidly flows into sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in manhole covers. Most I&I is caused by aging infrastructure that needs maintenance or replacement.

The Basin L Pipeline Rehabilitation Project comprises sewer main pipe in roadways and lower lateral lines that run from the sewer main to the property line, which will be rehabilitated utilizing a method called pipe bursting. Pipe bursting is a trenchless method of pipe replacement, where a breaking device called an expander head is pulled through the defective pipeline from a launching pit. As it travels through the pipeline toward the receiving pit, the expander head breaks the pipe into many small pieces, pushing the pieces into the surrounding soil. New pipe is attached to the back of the expander head, replacing the line immediately.

Pipe bursting is effective when old clay pipes are being replaced. In other areas, particularly lower lateral lines, the pipes may be lined instead to plug cracks and holes. These methods of sewer repair and replacement are preferred, because they protect road surfaces by reducing the need to dig trenches in the roads. As with all major sewer projects, this work has been coordinated with the City of Napa’s Public Works Department and their schedules for road resurfacing.

The Basin L project also includes the installation of new manholes and rebuilding of existing manholes, as well as sewer main sag repairs. “The I&I flow reduction achieved by this project will reduce the chance of sewer overflows in the area, and decrease impacts to the treatment plant,” explains Tim Healy, District General Manager.

The District received 10 bids for this project; Mountain Cascade’s bid came in the lowest at $1,086,962. Construction is expected to take place from early March through July 2012. Post-construction flow monitoring will be performed to evaluate the success of the project. 

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