Water Works Engineers
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24th Avenue Sanitary Sewer Relief Line,
City of San Mateo, CA

Water Works Engineers was selected by the City of San Mateo to design the 24th Avenue Sanitary Sewer Relief Line Project- upsizing of approximately 3,000-LF of 6” VCP with 10” and 12” HDPE & FPVC and an additional 1,500-LF of new 12” Fusible PVC or HDPE to accommodate capacity limitations in the collection system and future growth in the sewer basin.

Preliminary design services included detailed topographic survey; right-of-way assessment and procurement; environmental constraints analysis and CEQA mitigated negative declaration; encroachment permit needs analysis and application package development; CCTV condition assessment of the entire alignment; and current and ultimate-build-out peak wet weather flow capacity analysis. The work was in City, County and Caltrans Right-of-Way, which required significant encroachment permitting coordination. Criteria from each of these categories was used to identify major and minor constraints for 5 separate alignment options and multiple trenchless and traditional pipeline replacement methodologies. A net-present worth financial analysis was completed to assist with the selection of a preferred alignment given the City’s objectives.

Two of the key concerns on the project were unknown soil conditions due to close proximity of existing utilities, concerns about unconsolidated raveling soils and significant groundwater. To alleviate these issues, Water Works completed design phase potholing and utility coordination to verify exact location of utilities and confirm ground conditions. The selected alignment required replace in place to avoid existing utilities, and required crossings of an existing 10-ft x 12-ft concrete storm culvert using trenchless construction methodologies (bore and jack or pipe ramming). It also required replacement of approximately 60 sewer laterals; 1500-LF of sidewalk; and 3000-LF of roadway paving and slurry seal. Detailed dewatering and trench sub-grade stability requirements were included in the design to minimize affects of groundwater and poor soil conditions. Final bid documents included plans and technical specifications used in combination of City Standards and project specific design details.