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Project: North Trunk Sewer Replacement

Client: City of Santa Rosa

For over seven years the City of Santa Rosa considered replacing a section of the North Trunk Sewer approximately 1,700 feet upstream from Chanate Road along Paulin Creek, ever since debris in the creek, swollen with winter rains, broke a section of exposed sewer pipe that was originally constructed in the 1950’s.

Once the City began to move forward with the project, varied interests of multiple owner-developers, public agencies, and special interest groups called for the development and study of numerous alternatives for sewer and access road alignment. A primary issue to consider was the property’s recent purchase by the Sonoma County Agricultural Preservation and Open Space District, designating the parcel as an Open Space Preserve. As such, it was preferable to find an alignment for the sewer to traverse the parcel as far from Paulin Creek as possible in order to preserve the riparian habitat.

The sewer alignment selected for construction required open cut trenching over 22 feet deep for nearly 600 feet, a bore and jack under Chanate Road and a narrow (25-foot) work corridor whose limits were closely monitored to prevent damage to surrounding native trees and plants. Additionally, portions of the alignment traverse the Rodgers Creek fault zone, which posed the threat of unstable trench walls and necessitated special consideration of pipe and joint materials. A drilled pier retaining wall was installed to stabilize a narrow section of the access road over the sewer to ensure reliable access for 40-ton vactor trucks. The access road was designed to blend with the surroundings and permit growth of native grasses and ground cover yet still provide all-weather access for service vehicles.