The Town of Hillsborough (Town), California is an affluent community located south of San Francisco. Hillsborough's trunk sewer system consists of approximately 15,000 lineal feet (lf) of vitrified clay (VC) pipe ranging from 8-inches to 18-inches in diameter. After flow monitoring
of the Town's sewer system was completed, it was determined that the trunk sewer was undersized and a rehabilitation program was developed. The design called for upsizing the pipeline by up to three diameters. One portion of the design which proved to be the biggest challenge was upsizing
of a 4000 lf section of sewer from 15-inches to 28-inches (outside diameter). This portion of the alignment runs through a wooded area adjacent to San Mateo Creek, and geotechnical reports indicated that rock was present. Because of limited boring locations, the quantity, quality, and location
of the rock were unknown. In addition, the land through which the alignment runs is owned by the San Francisco Water Department (SFWD) and is a throughway for 79-inch, 60-inch, and 44-inch diameter water mains which cross the Town's sewer several times. Environmental permitting requirements
coupled with SFWD construction constraints limited the possibility of using open cut installation for the upsize of this portion of the alignment. Therefore, multiple diameter pipe bursting was researched and selected as the option of choice. Because of the uncertainty regarding the subsurface
conditions and risks associated with bursting to upsize an existing sewer by three diameters, Carollo Engineers conducted a test burst on 660 feet of the alignment before finalizing the design. This paper will discuss the results of that test, lessons learned, design and construction considerations
for multiple diameter pipe burst upsizing, and recommendations for other municipalities considering this uncommon application of an existing technology.
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