San Mateo leaders and residents dreaded heavy rain forecasts, because big storms meant sanitary sewer overflows onto city streets. Overloads came from the conveyance of sewage from three areas: San Mateo, the Town of Hillsborough and Crystal Springs County Sanitation District. A mandate from the Regional Water Quality Control Board required a rapid solution.
After much negotiation, the four entities agreed to construct a dedicated sewage relief line to transport Hillsborough and District wet weather sewage flows directly through San Mateo to the regional treatment plant. Harris & Associates was chosen to provide design services for the relief line, which involves placing a large-diameter pipeline under congested commercial and residential streets.
Key challenges stemmed from the sewer line path, which traverses a major artery at El Camino Real Boulevard, crosses environmentally sensitive San Mateo Creek and passes under the busy Caltrain corridor to reach the treatment plant.
- Calculated risks and rewards of four alignments through downtown that had enough room for the pipeline, which ranged from 27 to 30 inches in diameter for about two miles
- Conducted hydraulic analysis to confirm pipe sizes
- Provided various technical reports for the partners to assist with the decision making process
- Coordinated with the affected utility agencies
- Coordinated with Caltrans and Caltrain to secure the necessary permits
Harris also performed the following tasks during construction:
- Reviewed all contractor submittals and Requests for Information, and making recommendations to the city
- Visited the site and clarifying contractor questions in the field as necessary
- Provided the city with the necessary documentation for change order processing
One of the project’s major corridors was congested with underground utilities and already contained three existing sewer lines. Based on the results of the Decision Matrix, Harris recommended upsizing one of the existing lines to carry average dry weather and peak wet weather flows, instead of adding an additional line in that segment.
This strategy not only achieved the project’s hydraulic goals, it also replaced a badly deteriorated sewer line that would have required eventual replacement.
Beyond the Blueprints
Open cutting through San Mateo Creek would have created many environmental challenges and potentially delayed the project. Harris opted for an inverted siphon to tunnel beneath the creek.
After careful evaluation, the team chose the less expensive pilot tube guided auger boring method—which significantly minimized the risk of frac-out (the escape of drilling mud into the environment) and produced a shorter siphoned pipe.