WSIP Tunnels Help Reshape San Francisco’s Future
By Jim Rush
The San Francisco Bay area is well known for its striking vistas, iconic architecture and cultural diversity. Since gaining momentum as a growth area in the mid-1800s following the Gold Rush, the area has continued to flourish; it is now perhaps best known for its tech sector that continues to drive growth in the region.
More than 7 million people live in the San Francisco Bay area, which includes the cities of San Francisco, Oakland and San Jose. To meet the converging demands of aging water infrastructure, population growth and the need to resilient infrastructure in a seismically active area, the San Francisco Public Utilities Commission (SFPUC) is in the midst of its $4.6 billion Water System Improvement Program (WSIP).
The ambitious program is the largest infrastructure program undertaken by SFPUC and one of the largest in the United States. The program involves 81 construction projects, including the construction of three major new tunnels. The primary purpose of the program is to provide seismic reliability to SFPUC’s 2.6 million customers in Alameda, Santa Clara, San Mateo and San Francisco counties.
The need for a safe and reliable water system for San Francisco came into sharp focus following the earthquake of 1906. In the aftermath of the quake, fires burned for three days without water to fight them, destroying much of the city and displacing 250,000 residents. Construction of the new water system began in 1914 and put into service in 1934, bringing water from the Hetch Hetchy Reservoir in the Sierra Mountains 160 miles to the San Francisco Peninsula.
As part of the WSIP, three major tunnels are being constructed to upgrade or replace portions of the systems that were built in the 1920s and 30s. They are the New Irvington Tunnel, Bay Tunnel and New Crystal Springs Bypass Tunnel.
New Irvington Tunnel
The New Irvington Tunnel (NIT) is being built parallel to the existing Irvington Tunnel, which was built between 1928 and 1932 and extends from the Sunol Valley to Fremont on the eastern side of the San Francisco Bay, connecting the Alameda Siphons to the Bay Division Pipelines and Bay Tunnel (when operational). When completed, the New Irvington Tunnel will allow the original tunnel to be taken out of service for inspection and repair. Because if serves as a critical link in the water delivery system, the Irvington Tunnel has not been out of service since 1966.
The NIT consists of 18,660 ft of tunnel constructed with conventional mining techniques including hand mining, drill-and-blast and roadheader excavation. The contract was awarded to a joint venture of Southland/Tutor Perini in 2010 for $226.6 million. NTP was issued in August 2010 with mining commencing in March 2011. Crews are on pace for completion in mid 2015.
As of Sept. 23rd, crews had 235 ft remaining to mine. Following excavation, a final liner of 8.5-ft diameter steel pipe, manufactured in 50-foot lengths by Northwest Pipe, will be installed.
The project includes the construction of two portal structures (the Alameda West Portal on the eastern end and the Irvington Portal on the western end) and one intermediate shaft (Vargas Shaft). Crews can mine a total of four headings (at one point they were mining four concurrent headings), one each from the portals and two from the 115-ft deep, 41-ft diameter Vargas Shaft. The section of tunnel between the Irvington Portal and the Vargas Shaft is already complete and steel liner installed.
Ground conditions consist of highly variable ground ranging from hard rock to flowing ground, with high groundwater inflows. Because of the variable ground, including the potential for squeezing ground in fault zones, the tunnel designers decided not to use a TBM for this project.
“The design team thought that the presence of seven secondary fault zones and the associated squeezing ground at or near these zones made the use of a tunnel boring machine likely to get stuck and prone to work stoppages. The resulting delays to dig around the machine to free it was deem too risky in terms of miner safety and schedule delays and costs,” said David Tsztoo, Assistant Sunol Regional Project Manager for SFPUC.
“The designers specified that this tunnel be excavated using conventional excavation methods —roadheaders and controlled detonation where hard rock cannot be excavated by the roadheaders. Roadheaders were deemed easier and less expensive to procure for the project than a TBM. The use of multiple roadheaders in multiple headings also offered opportunities to shorten the construction schedule and save cost.”
To build the tunnel in this manner required crew members to adapt to the quickly changing ground. Crews used steel sets with timbering as initial support in addition to, when necessary, steel invert struts. Additionally, a program of pre-excavation probe drilling and grouting for groundwater cutoff was used extensively. As of September, crews had pumped more than 7 million lbs of grout into the ground. Groundwater inflows of more than 300 gpm have been encountered in the 2-in. probe holes as well as multiple fault zones and shear zones.
“Controlling the groundwater was a big factor in this project,” said Dan McMaster Project Construction Manager for Hatch Mott MacDonald, which is providing construction management services for the owner. “During construction of the original Irvington Tunnel, inflows of up to 1,200 gpm were encountered, which could be disastrous. Finding that groundwater out ahead of us and cutting it off with pre-excavation grouting is extremely important.”
The use of conventional mining methods instead of a TBM created an additional challenge for the contractor. “There were only a handful of people on the job who had experience with this type of mining, which is completely different from most of the work going on today,” said Curtis Bahten, General Superintendent for Southland/Tutor Perini. “We had to train our guys to timber properly, set the steel, do the controlled detonations and watch for the different ground types. Given the nature and difficulty of the work, it takes a different breed to do the job.”
Despite the challenging work conditions, crews have maintained an average advance rate of 200 ft per week. “It took about six to nine months for the mining crews to gain the necessary experience to work at an average combined two or three heading production rate of 200 ft per week, and do so safely and consistently,” Tsztoo said. “After two years of mining, the New Irvington miners are very competent conventional tunnelers, with that ‘sixth sense’ ability to ‘read the ground’ and being able to avoid imminent rockfall, or the cool-headed ability to react with groundwater counter measures to keep the tunnel heading from flooding.”
An additional challenge presented itself to the team after tunneling commenced when the tunnel was re-classified from “Potentially Gassy” to “Full Gassy” by CalOSHA. This happened in June 2011 after detection of significant methane gas in one of the tunnel headings. CalOSHA reclassified the entire tunnel and the impacts affected four headings at the time. The ventilation system had to be upgraded, gas monitors trained and stationed at each tunnel heading for each shift, and hot work permits required to avoid sources of ignition during mining operations.
Environmental and social concerns also played a key role in the successful completion of the tunnel. One of the major areas of concern was the Sheridan Valley area, in which crews were permitted to dewater from the surface to mitigate impacts of groundwater inflows during construction. However, homes in the area in which the surface dewatering systems was set up relied on groundwater as their water supply. Making matters worse, when the original Irvington Tunnel was constructed more than 80 years ago, many of the groundwater wells were left dry – a sore memory that lingers among the many family descendants who still live in the area.
“The most challenge environmental issue we faced was dealing with the groundwater wells in the Sheridan Valley,” Tsztoo said. “To help mitigate potential impacts we needed to provide supplemental water to homeowners because we knew we would impact the groundwater wells in that area. Studies showed that we would impact up to 33 wells, and we ended up affect about half that number. For those people who were affected, we set up tanks and trucked in commercial potable water, and provided irrigation water from the dewatering wells. Those mitigation measures helped garner a lot of goodwill in the community and helped allay a lot of concerns.”
At the western end of the project, SFPUC coordinated with residents who would be impacted by the construction of the Irvington Portal. Some residents wanted a sound wall to mitigate construction noises, while others wanted unobstructed views from the hillside. The solution: sound walls constructed with transparent sound barrier panels, a concept imported from Europe.
With the many challenges associated with this project, all project participants agreed that teamwork was a vital component to achieving success. “The city staff, the construction management team, and the contractor partnered this project,” Tsztoo said. “A partnering charter was signed by all parties to work together as one project team. The city staff, construction manager and inspectors, and the contractor work on daily basis to collaborate on construction issues, and produce the best project that we can. Potential risks were also discussed early on and a risk register was developed and used as a tool to track these risks and identify measures to mitigate each risk if it occurred during construction.”
“As a team, we have met the challenges of the New Irvington Tunnel.”
The new Bay Tunnel – the first bored tunnel to be built spanning San Francisco Bay – will link the existing segments of Bay Division Pipeline (BDPL) Nos. 1 and 2 and the future BDPL No. 5 in the East Bay with those on the San Francisco Peninsula. The existing portions of BDPL Nos. 1 and 2, which were built in the 1920s and 30s, lay along the bay floor and on trestles that cross over environmentally sensitive marsh land. The pipe and the trestle are in a deteriorated condition. The Bay Tunnel will bypass these environmentally sensitive wetlands.
Tunnel mining, using a Hitachi Zosen TBM built in Osaka, Japan, has been completed – 6 months early – with lining operations under way. The contractor expected to finish liner placement by the end of the year. The tunnel extends 5 miles under San Francisco Bay at depths of up to 100 ft and was constructed using a specialized earth pressure balance TBM. The excavated diameter is 15 ft with a final tunnel lining of 9-ft diameter welded steel pipeline (like NIT, the pipe is being manufactured by Northwest Pipe in 50-ft lengths).
Vertical shafts were constructed at each end, and it will connect the BDPL Nos. 1, 2, and 5 piping manifold. The spoils resulting from excavation during this project were to be used as part of the South Bay Salt Pond Restoration Project, conversion of adjacent salt ponds to marshland. The portion of the existing BDPL Nos. 1 and 2 that are replaced by the tunnel will be capped on each end and will be abandoned in place.
The tunnel is currently under construction by the contracting team of Michels/Jay Dee/Coluccio, which was the low bidder at $215.3 million. Construction began in April 2012 and is expected to be completed in 2015. Jacobs Associates led the tunnel design team, which also included URS, Fugro West, and Telamon Engineering.
Ground conditions consist primarily of the San Antonio Formation, which contains interlayered clays, silts and sands. Included in the expected geology was a 700-ft long reach of Franciscan Complex bedrock, which can contain hard and abrasive materials different from the majority of the alignment, leading the selection of an EPB TBM.
The TBM launch shaft and primary work location is located in Menlo Park on the western side of the Bay. The work shaft, built using diaphragm slurry wall construction methods, is approximately 58 ft in diameter and 110 ft deep. The exit shaft is located in Newark on the eastern side of the Bay. It is approximately 28 ft in diameter and 74 ft deep. Due to the sensitive environment and wetlands in the area of the shaft, crews froze the ground prior to excavating the shaft. The contractor installed 50 freeze pipes, each 128-ft long, that formed the 28-ft diameter outer portion of the shaft with additional pipes installed through the center to freeze the bottom of the shaft. The freeze was used to prevent movement of groundwater and to avoid leakage during construction.
Challenges included difficult ground conditions, the proximity of California’s largest active seismic faults (the San Andreas and the Hayward faults), and the presence of environmentally sensitive habitats which required that no intermediate construction shafts be built.
New Crystal Springs Bypass Tunnel
The smallest of the three WSIP tunnels, and the first to be constructed, was the New Crystal Springs Bypass Tunnel (NCSBT). The tunnel will provide redundancy to the existing Crystal Springs Bypass Pipeline (CSBPL) and will improve the delivery reliability level of service. The existing CSBPL is a 96-in. pre-stressed concrete cylinder pipe that was installed in 1969 below the hillside along Polhemus Road in San Mateo County. This pipeline is a critical link in the transmission system, transmitting all of the water from the East Bay to the Peninsula and City of San Francisco. The pipeline and soils in this area are subject to failure during high precipitation or major seismic events.
Construction of a 4,200-ft long, 12-ft diameter tunnel, which was built to house 96-in. steel liner, was completed by Shank/Balfour Beatty, which was the low bidder at $55.7 million. Final construction completion is expected in October 2011.
Jim Rush is editor of TBM: Tunnel Business Magazine.
Photos are courtesy of the San Francisco Public Utilities Commission