Among the many achievements from the SFPUCs water infrastructure project is a 21-mile long pipeline from the City of Fremont to near Redwood City, called the “Bay Division Pipeline Reliability Upgrade Project.” The pipeline includes a 5-mile tunnel underneath the San Francisco Bay called the “Bay Tunnel.”
The Bay Tunnel is significant in that it is the first bored tunnel under the San Francisco Bay. It replaces pipelines built in the 1920s and 1930s. These were either lain along the bay floor or across trestles that cross sensitive marsh lands.
The presence of environmentally sensitive habitats on the Bay margins precluded using cut-and-cover pipelines, which require digging a trench, placing piping, and filling back the remaining open earthwork. The result of adhering to good environmental custodianship meant that a 5-mile long tunnel would replace the trenching. The surface displacement for this method required only two surface openings, one for launching and one receiving shaft.
These two shafts are located on properties owned by SFPUC in the city of Menlo Park (launching shaft @ ~58 foot diameter and 110 ft deep to insert with additional foundation depth) and city of Newark (receiving shaft @ ~28 foot diameter and 74 ft deep). The tunnel is the first of its kind built under the Bay, utilizing an articulated earth pressure balance tunnel boring machine to effectively counteract the hydrostatic pressure and sandy/silty soils of the tunnel environment.
[According to Wikipedia: “Earth pressure balance tunneling is a method during which excavated material is used to support the tunnel face while being plasticized using foams/slurry & other additives to make it transportable and impermeable.”]
One additional factor of construction methodology was used to build the receiving shaft. Interestingly, Crews froze the ground prior to shaft excavation. Fifty freeze pipes of 127 foot depth were installed forming the 28 foot diameter of the shaft. Additional pipes were installed through the center to freeze the bottom of the shaft. This method was applied to prevent movement of groundwater and to avoid leakage.
The following shot shows the entrance shaft bottom with fully assembled TBM [tunnel boring machine]. Existing BDPLs No. 1 and No. 2 are south of the shaft with the existing Ravenswood Valve House. Extra care was taken in working in close proximity to the operating 1920’s era BDPL pipelines.
PG&E transmission towers are west of the shaft. A substation was installed on-site converting 115kV from the PGE transmission towers to 12kV for site use.
The above tunnel was used as a people mover and concrete segment transport. The below tunnel highlights the rail, overhead muck conveyor, and other auxiliary ultilities. The 10-mile, 4-part, tunnel/shaft/overland belt/magazine muck conveyor system greatly increased production efficiency, and allowed the Contractor to maintain high excavation rates of up to 200 feet per day.
The Bay Tunnel was completed on May 20, 2015, within the baseline schedule and well below the baseline project budget. In recognition of its success, this project was bestowed with multiple awards, in 2013, 2015 and again in 2016. The most prestigious awards were awarded 2013 and 2015. These were: the “Highly Commended Tunnel Award for the Tunneling Project of the Year (up to $500 million)” awarded by The International Tunneling and Underground Space association in 2013; and the “American Public Works Association Project of the Year” in 2015 (a national award).