The New Crystal Springs Bypass Pipeline is nearing its end with all excavation complete and its water-carrying steel pipe being grouted into position within the tunnel structure using cellular concrete. As if to emphasise the purpose of the project the area experienced a significant tremor (4.1 magnitude) just before a planned community site visit. The project’s groundbreaking ceremony took place two days before the 103rd anniversary of the 1906 Great San Francisco Earthquake and Fire, which cut off local water supplies. The New Crystal Springs Bypass Tunnel water supply route provides a critical link to help ensure water to San Francisco and San Mateo counties after a major earthquake.

In addition to its critical place in the WSIP with seismic upgrading, there was additional motivation for the move to replace the existing Crystal Springs Bypass Pipeline. A major landslide occurred during the winter of 1996-97 on the northeast side of the Polhemus road, threatening the structural integrity of the existing prestressed concrete, steel-lined pipeline lying 7-10ft (2-3m) below the surface of the hillside bordering the road. This raised concerns about future landslides, whether from seismic activity or extreme wet weather, affecting system reliability. As a result of subsequent studies into the replacement or repair alternatives, a deep tunnel was chosen as the preferred option. The scheme was designed by a joint venture of Arup and Brierley Associates, and Jacobs Associates were appointed as construction manager.

Dimensions
The new tunnel is 1280m (4200ft) long and 2.44m (96in.) finished diameter with a welded steel liner, and is due for completion in spring next year. At the time of T&TI’s visit tunnel excavation had been completed and the steel liner was being fixed in position within the tunnel with cellular concrete annular backfill.

The tunnel construction contract was awarded to the lowest qualified and responsible bidder, the M L Shank Co/Balfour Beatty joint venture, in October 2008 at an amount of USD55.674M.

The tunnel work is located along the Polhemus Road in San Mateo County, with most of the construction site occupying the location of decommissioned Polhemus Fluoridation Station. The site, which is surrounded by residential and equestrian plots, will be fully restored to a natural state at the completion of construction, including modification of surface drainage.

In addition to the tunnel, excavation work included:
• two shafts, of 9.765m (32ft) diameter at the south end for most working, and of (18- FT) diameter at the north end of the drive;
• the construction of valve vaults;
• control and monitoring systems and;
• a 20-kW emergency power system.

In addition the contractor installed inclinometers along the Slope bordering the Polhemus Road and on each shaft site to monitor stability.

On completion of the new pipeline it has to be connected to the existing Crystal Springs Bypass Pipeline and Tunnel, other supply pipelines in the vicinity and back to the Crystal Springs Reservoir. This is being accomplished in a shallower excavation on the construction site with sheet-piled walls to accommodate valves and connections.

The existing Crystal Springs Bypass Pipeline will remain in partial service to provide redundancy for inspection and maintenance of the new tunnel.

Geology
Jacobs Associates Principal responsible for the project, Victor Romero, explained that the contractor itself was allowed to select the depth of the tunnel (within limits). This allowed work to take place in the best ground horizon found, as long as pipeline water flow could be maintained with a gradient of 0.2-3.0 per cent. The ground is in the Franciscan Formation or Assemblage, which is of very mixed (heterogeneous) material affected by numerous seismic disturbances and bordering the San Andreas Fault in the San Francisco Peninsula. Typical materials include volcanic greenstones, deep-sea cherts, greywacke, limestones, serpentinites, shales and metamorphic rocks formed by high pressure through tectonic movements. Although the design documents include the Geotechnical Baseline Report, Romero commented, “We just couldn’t ‘join up the dots’ in the section from site investigation. So the contractor had to expect a range of ground.”

The contractor selection of the horizon could be based on the best horizon found during shaft construction, of relatively ‘good’ sandstone for launch of the TBM, resulting in a constructed grade of 1.2 per cent from a launch shaft depth of 50.3m (165ft) to meet the designed reception shaft depth of 18.3m (60ft).

M L Shank, led by veteran Mike Shank, is a small but well-known tunnelling specialist in North America with a wealth of experience. It is known particularly for concentrating on one project at a time, the previous one being the Narragansett CSO in Providence, Rhode Island, and for designing its own TBMs for custom build, usually by Hitachi Zosen of Osaka, Japan.

For Crystal Springs Mike Shank had the Hitachi Zosen single-shield tapered from the 3.71m (146in.) at the cutterhead down to 3.63m (143in.) at the rear of the shield in case of squeezing ground. Shank specified 17in. (432mm) hard-rock cutter discs from Herrenknecht and a Rotek main bearing.

The back-up system included an erector for expansion pre-cast lining segments, a railed gantry, and a belt conveyor.

Shafts
Before tunnelling could begin both shafts had to be sunk, using drilling and controlled blasting during daytime hours only throughout the first half of 2009. A special shaft-drill jumbo was employed to get through the hard ground. Blasting had to meet noise and vibration limits in view of the proximity of dwellings.

Initial support of the shafts in the superficial ground material was by soldier piles and steel rings with timber lagging. At bedrock one cast in situ concrete ring holds the upper structure, after which the remaining depth of shaft in more solid rock was supported by sprayed concrete over steel rings.

In July the TBM shield arrived on site from Japan to join the TBM components and trailing gear that Shank then fabricated and fully assembled on site. It was eventually launched in November last year. Some weekday night work was necessary during the TBM launch, but all noise was kept within the noise limits set out in the environmental impact report.

The project team has focussed on being a good neightbour and maintained open dialogue with the local community by providing timely and periodic progress notices, community meetings, and by maintaining a 24-hour ‘hotline’ to respond to questions and concerens. The team was successful in dispelling early community concerns.

Progress
Once the TBM was launched and joined by the back-up system from the south shaft, progress was better than had been feared, despite frequent breakdowns of the conveyor discharging spoil to rail haulage trucks, Romero reports. Another teething problem was that the shield taper, designed to cope with squeezing ground, made the TBM difficult to steer as shown by laser guidance. To improve steering Shank installed a fin-based anti-roll mechanism that worked, with all initial problems being overcome by the end of December 2009.

Shank’s general manger Jerry Stokes confirms that the TBM started off in good sandstone, and that reasonable drive progress was made despite the concerns over geological conditions. Groundwater make was also surprisingly low in the shafts and tunnel, at around 20g/min. Stokes also says that Shank usually bought their TBMs from Hitachi Zosen as they were prepared to build the shield machines to Shank’s design or modifications.

Shank also designed and built a water solids settlement and treatment system on site to ensure the removal of contaminants, especially heavy metals, before site water rejoins the general supply.

The TBM installed an expanded precast concrete temporary lining required in anticipation of squeezing ground, but there was little occurrence of this potential difficulty. Mucking out was by the TBM discharge belt conveyor into railhauled muck cars that were lifted up the south shaft via a crane-lifted guided platform. A depression at the back of the construction site, adjacent to residential neighbours, could be filled and raised slightly, allowing all 24 000yd3 (18 350m3) spoil to be retained on the 3.5-acre (1.4- hectare) site, thus reducing trucking needs.

From January this year there was a good start after Christmas with an average advance rate on 10h-working of 18.3m (60ft) per day and a record of 25.3m (83ft) per day. Breakthrough to the north shaft was achieved on 24 March, so tunnelling over 1,220m (40,00ft) took three months.

Pipeline installation
Once tunnelling had been completed, installation of the pipeline could begin. Ameron International fabicated the water pipe. The thin-section pipe used at Crystal Springs came in 32-ft (9.75-m) lengths. Shank welded these in the shaft bottom for carrying into the tunnel on a special carrier, also designed and fabricated by Shank within four weeks.

At the north shaft the TBM had been stripped out of all useful equipment to leave the sacrificial shield itself as a temporary support encasing the first length of water pipe. All the steel pipe was in position on 26 May, after which backfilling could begin. A relatively high-density cellular concrete mix was employed by sub-contractors to fill the annulus by placing through holes formed in the steel liner. The heat generated by concrete curing aggravated the warm humid conditions in the tunnel.

The mix was designed and placed to provide sufficient support for the pipe once cured, but to avoid too much flotation force tending to lift the pipe. Even so, the pipe had to be braced internally in its circular section to avoid ovalisation by the competing forces. The whole backfilling operation was scheduled to last two months.

Throughout the project keeping to schedule is of high importance since water supply shutdown periods have to be strictly limited. The contract also carries financial penalties for overruns. The best time for shutdowns is in the winter when water demand is less, so the scheduled completion for the new pipeline this autumn must be met to allow the main supply to be interrupted as the new pipe is connected to the old early in 2011.

Commenting on the project, SFPUC regional project manager Husam Masri says, “we are seismically upgrading and improving the regional water system to prepare for the future. This will provide additional relaibaility to out system ensuring our customers’ needs are met at all times. I am very proud of our project team, both our construction management staff and contractor. They are very committed to quality work and delivering this project on budet and on schedule.”


Victor Romero does a site inspection as sheet piling of the valve and manifold chamber continues in background Complicated welding required for a pipeline tee-joint at the base of the North Shaft. In the background is a covered butterfly valve [Photo: Jacobs Associates] View down the South Shaft showing different types of support [Photo: Jacobs Associates] Rear of the Shank Hitachi Zosen TBM in action with thrust rams and spoil discharge conveyor [Photo: Jacobs Associates] The Shank designed and fabricated TBM back-up gantry running on rails [Photo: Jacobs Associates] The 3.5-acre area on the South of the construction site accommodated all spoil for restoration