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Sliplining Under San Francisco’s Hectic Embarcadero Area
Due to the obstructions in place during the construction of the original force main, there are multiple double offsets throughout the line, ranging from 5 to 20 degrees. At a few locations, the bends are within five feet of each other, creating double offset joints which further complicated the sliplining process.
The site itself posed numerous restrictions. The Embarcadero is a major thoroughfare with vehicle traffic counts exceeding 40,000 per day, and with many shops and businesses along the bay, pedestrian traffic is non-stop throughout the day and only reduces during the early dark hours of the morning. The city of San Francisco required construction activities to be performed between the hours of 8 p.m. and 5 a.m. when lanes were permitted to be reduced from three to a single lane with all three traffic lanes reopened to traffic after 5 a.m. A 24-hour shutdown to slipline the pipe could only occur during the weekend period when the vehicular and pedestrian traffic were slightly more manageable.
During construction, there also were a number of street closures for special events, including San Francisco Giants major league baseball play-off games, followed by Word Series games and parades. Additionally, a number of local fun-runs and similar events impacted the progression of the work. Each of these special events required suspension of construction activities which delayed completion.
Subsurface soil conditions and historical and environmental issues also affected the project. The predominately soft marine soils contained fill material, much of it debris from the 1906 San Francisco earthquake.
The consistency of the soils required full solid shoring for any excavations. Due to the proximity to the San Francisco Bay, dewatering of the excavations was not feasible because of the influx of bay water and the influence of bay tides. The shoring had to include watertight interlocking capability and a seal slab. Longer sheets were required to create a seal in the bay mud, 50-feet below ground surface in an effort to minimize dewatering.
The existing fill material around the pipe is comprised of building and ship debris used to expand the Bay shoreline more than 100 years ago after the 1906 earthquake. As a result, Shimmick was required to retain the services of a historical archeological consultant in the event debris of historical significance was encountered.