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Sliplining Under San Francisco’s Hectic Embarcadero Area
Two of the primary benefits of trenchless construction and rehabilitation technologies are their capabilities of replacing or rehabilitating failing pipelines in locations where it is impractical or impossible to perform extensive excavations.
Also, trenchless methods are valuable when it is essential to limit surface damage and disruption of services and normal surface activities.
Due to repeated failures, the city of San Francisco found it necessary to make emergency repairs to portions of a 36-inch steel sanitary sewer force main buried under the Embarcadero, a busy shoreline drive adjacent to the bay, and a major visitor and tourist destination throughout the day, evenings and weekends. An additional complication was the force main crossed underneath two transit rail lines owned by separate agencies. Because the force main serves as an integral part of the city’s wastewater system, it could not be shut down for more than 24 hours at a time.
The San Francisco Public Utility Commission (SFPUC) made the decision to install new HDPE pipe inside the failing pipe by the trenchless method of sliplining. The project was initiated under an emergency-procured design-build contract.
Lockwood, Andrews & Newnam was the design engineer for the project; Shimmick Construction was the prime contractor and also did the sliplining. Ultimately, 1,100 feet of 28-inch diameter HDPE pipe was sliplined.
Planning is important on any project, but this complex San Francisco sewer rehabilitation could not have been successfully completed without a comprehensive design plan taking into account the many challenges and solutions to the conditions and situations that would be encountered.
The existing 36-inch outside diameter steel pipe was installed in the mid-1970s with some sections buried as deep at 15 feet. The pipe was lined with tar and had been repaired multiple times throughout the years using various methods, including internal joint seals.
Complicating the entire project were site challenges, including marine soft soils (locally referred to as bay mud), fill material, and contaminated material, to name a few. Furthermore, due to the congested corridor, several utilities, including a duct bank containing eight 2-inch conduits was subsequently installed directly above the majority of the force main. The 36-inch force main was installed through an existing BART (Bay Area Rapid Transit) platform that was backfilled in the 1970s.