The San Francisco region of California, located on the San Andreas Fault, is renowned for being a hotspot of seismic activity. In 1906 a massive earthquake devastated the city, and more recently, the 1989 Loma Prieta earthquake reached 6.9 on the Richter scale and caused the death of 63 people.

Not only do such quakes cause fatalities and damage to property, but they can also imperil the local water supply. City engineers have long feared that a critical stretch of the Hetchy Hetchy Regional Water transmission system, installed in 1969, could fail during high precipitation and major seismic events.

The concrete cylinder pipe, 96 inches in diameter, transported water from the East Bay to supply the population of San Francisco. Located only 1.5–3 m below the surface, and passing next to a slope that had failed twice during the mid-1990s, there was a high risk that the conduit could be damaged during future earthquakes, with potentially devastating consequences.

Given this possibility, the San Francisco Public Utilities Commission (SFPUC) decided to replace the 1,280 m water pipeline, installing a new welded steel pipe within a tunnel bored deep into the bedrock.

Beginning in 2008, the New Crystal Springs Bypass Tunnel (NCSBT) is one of 85 schemes being conducted within a $US4.3 billion San Francisco Water System Improvement Program (WSIP).

Jacobs Associates, a San Francisco-based international engineering firm, provided construction management services for the NCSBT, and are also working on several other WISP projects, including the new Irvington and Bay tunnels. Meanwhile, Shank/Balfour Beatty JV were awarded the $US55.67 million contract for construction of the tunnel.

The first stage of the project involved the construction of a South (launch) shaft and North (receiving) shaft, which were excavated during July and August 2009 using a combination of mechanical excavation and drill and blast.

The shafts were then connected by a 2.4 m diameter tunnel, constructed by a specially designed single shield tunnel boring machine (TBM). The 47.2 m TBM was designed by ML Shank Co, while the shield was manufactured by Hitachi Zosen, and the cutters were provided by Herrenknecht. The machine also features five gantry cars and two muck conveyers, and has a torque of 593,920 foot-lbs and an operating thrust of 1,150,000 lbs.

Although many contractors name their TBMs to ward off bad luck, it was the preference of Mike Shank to leave his machine nameless. Instead, it was decorated with logos of the entities involved.

The TBM was designed to work within the sometimes unpredictable ground conditions of the San Francisco area, which include sandstone and Franciscan melange. The technical baseline report anticipated that the Franciscan melange may cause squeezing, so the machine had a tapered design, with a front diameter of 370.76 cm and a back diameter of 363.22 cm.

Fortunately, squeezing ground proved not to be a major issue. NCSBT Project Assistant Construction Manager Sarah Wilson said “We never saw any squeeze come on during excavation or over a weekend, or anything that interfered with erecting the initial support.”

In addition, the TBM featured 23 disc cutters 17 inches in diameter, which were capable of cutting through the relatively hard sandstone.

The major parts of the TBM arrived from Japan in July 2010, and the machine was assembled on-site, which took approximately two months. On 30 September 2009, the TBM sections were lowered by crane into the South Shaft, before being reassembled at the base. Tunnelling began 10 November, and rings of concrete segments were installed inside the tunnel to provide ground support.

Tunnelling occurred at a downhill grade of 1.2 per cent. This grade was determined by the contractor, who was able to select a grade between 0.2–3 per cent by determining the depth of the drive shaft. If there had been significant amounts of water on site, the downhill grade could have presented some challenges, but fortunately this did not become a matter of concern.

After travelling approximately 18 m per day, the TBM arrived at the North Shaft ahead of schedule on 24 March 2010, and was then disassembled. The shield remained in place in the receiving shaft, while the other components were backed out of the tunnel and removed from the South Shaft.

It is possible that these components will be recycled on future projects. “Based on the way the contractor removed them, I suspect he’s going to reuse them. They weren’t completely destroyed when he was using them [and] he removed them in sensible sections,” said Ms Wilson.

Once the tunnelling was completed in early May, construction began on the installation of a 2.4 m diameter welded steel pipe.

After this process is finished, the new pipe must be tied-in to the existing system, which involves shutting down the pipeline for approximately one month. This process is conducted in winter, when water demand is at its lowest, and the SFPUC re-routes the entire system to avoid affecting supply. The first shut down, to tie-in the receiving end, occurred in February and March 2010, while the second shut down, for the drive shaft end, will take place in January 2011.

According to Ms Wilson, the tie-in process is “not simple or trivial but it went perfectly during the first shut down, so we have high hopes that the second shut down will also go perfectly”.

One of the key challenges of the NCSBT project was to complete all necessary works without adversely affecting local residents. The project site is completely surrounded by family homes, and some occupants expressed reservations about having a construction project in their backyard. To mitigate these concerns, maintaining good public relations became a priority for the construction managers.

Jacobs Associates telephoned individual home owners each time a blast was due to occur, and a blog was used to inform the public about project activities. In addition, outreach events were held at the local Starbucks, where community members could drop in to have their questions answered.

Also, the contract required the installation of noise and vibration monitors, which would operate 24 hours per day. If anything approached the project limits, an alert was sent to the personal mobile phones of the project managers. Although the alert was sent on several occasions, these were false alarms and the limits were never actually exceeded.

Environmental protection was also an important feature of project management. “Especially being in California, there are a lot of environmental requirements that we’ve had to pay attention to, and really work with the permitting agencies to keep them apprised of little developments in the project,” said Ms Wilson.

In addition, the tunnel was classified as potentially gassy, and air quality was constantly monitored to ensure the safety of the 40 workers on site. On several occasions gas was detected and the tunnel was ventilated, but this never impacted the progress of work.

Although not scheduled for completion until September 2011, the NCSBT has already made excellent progress, with both the tunnelling and first tie-in completed ahead of schedule. Ms Wilson attributes this to a combination of favourable ground conditions, the TBM design and the positive attitude of the contractors. Given this track record, it seems likely that the project will come to a successful conclusion, securing safe water supply for San Francisco for many years to come.