All Together Now

Simultaneous Construction of Pipeline Wasn't Planned

Permits were approved nearly at the same time for Sacramento County Regional Sanitation District's $800-million capacity improvement program, causing construction to start concurrently on all three pipeline segments. When the project is completed next fall, 50 mi. of new pipes will carry wastewater under the Sacramento River to a regional treatment plant.

By J.T. Long

When the rush of tunneling ends next year, more than 50 mi. of new sewer pipeline will carry wastewater from the fast-growing Natomas and West Sacramento areas under the Sacramento River to the Sacramento Regional Wastewater Treatment Plant.

Neal Allen, principal engineer with the sanitation district, said the most challenging aspect of the entire project is the boring of two 2,000-ft. long, 15-ft.-diameter tunnels 40 to 60 ft. under the silt and gravel floor of the Sacramento River (photo courtesy of the Sacramento County Regional Sanitation District).

The Sacramento Regional County Sanitation District, which has identified 120 mi. of pipeline capacity improvements for the next 20 years, didn't plan to construct all $800-million worth of interceptor lines at once, but a delay in permitting meant that instead of staggering major projects, three big ones- the Bradshaw, Arden and Lower Northwest Interceptor-are all boring, pouring and pumping at once.

"Everything broke [the permitting process] at once," said Neal Allen, principal engineer with the sanitation district.

LNWI Is Divided Into 9 Contracts

More than half the work, $600 million, is contained in the Lower Northwest Interceptor, a 19-mi. pipeline being built with the aid of more than 22 contractors and subcontractors using state-of-the-art digging equipment.

To assure competition and contracting capacity, SRCSD divided the LNWI into nine contracts. They include two new pump stations, five pipeline projects and 14 tunneled crossings, two of which will be under the Sacramento River. During peak wet weather, the system will handle as much as 200 million gallons per day.

Despite one of the wettest winters and springs in recent memory, "the projects are proceeding on schedule," said John Buttz, LNWI program manager and vice president of MWH of Broomfield, Colo.

A Vital Link

The LNWI alone required the acquisition of right-of-way for more than 230 parcels, 122 permits and satisfying a stakeholder list of more than 5,000 people. It will link some 200,000 homes, many in the fast-growing Natomas and West Sacramento areas, to the Sacramento Regional Wastewater Treatment Plant in Elk Grove, which was last expanded in 1982.

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Major components of the LNWI include a pair of $115-million pump stations designed by Omaha, Neb.-based HDR Inc. The stations will ultimately include six 2,000-hp pumps with self-cleaning elements and odor control systems. Full standby power will be provided for both stations from six 2000-KW diesel generators.

Allen said the most challenging aspect of the entire project is the boring of two 2,000-ft. long, 15-ft.-diameter tunnels 40 to 60 ft. under the silt and gravel floor of the Sacramento River, a $44-million contract. The decision to go with one big tunnel using a manned Lovat Tunnel Boring Machine (TBM) was not an easy one. SRCSD considered horizontal direction drilling and paired microtunnels (methods used on other segments of the overall project), including the $14-million Arden Parallel Force Main, where 10,400 ft. of 60-in.-diameter parallel tunnels are being dug under the American River's soft clay soil using unmanned TBMs.

Dave Young, senior design manager for consultant Hatch Mott McDonald, a Millburn, N.J.-based consulting engineering firm that was awarded the $44-million design contract for the northern and southern Sacramento River crossings, said, "We had to convince the district that the large diameter pressurized tunnel using a tunnel boring machine was hands-down the best alternative for the Sacramento River crossings. It was equal in cost, but superior in schedule and operations and maintenance."

They Look Like Space Capsules

The method chosen called for two 30-ft. diameter Lovat TBM machines that look something like space capsules fitted with a cutting wheel at the front that turns the rock and gravel into a slurry in a pressurized chamber and then dumps the sludge out the back into spoil carts that are wheeled to the surface at a six-degree angle like a train. As the TBM moves forward, it lines the tunnel with a pre-cast concrete lining.

Upon completion of the tunnels, twin-welded steel pipes are installed and the void is filled with low-density cellular concrete, a lightweight foam and cement slurry that includes suspended air bubbles.

The decision to use this method was based largely on the type of soil. The Sacramento River contains gold mining tailings, a condition that Ray Costa, senior project manager for San Diego-based Engineering firm Kleinfelder Inc., which performed engineering work for parts of the project, described as "digging through marbles the size of basketballs." The larger shaft size and increased horsepower of the Lovat TBM can deal with these materials without getting stuck.

The area also has a high water table. In order to dig the launch and receiving pits for the TBM, excavation subcontractor Blue Iron Inc. of Woodbridge had to de-water by pumping groundwater out of wells. The discharge then had to be treated and sent through temporary pipelines to two sites predetermined by the Central Valley Regional Water Quality Control Board.

Another factor was space. The large TBM could not be used on the Arden project because space constraints from levees and urban development didn't leave room for the angled approach tunnels required by SRCSD for maintenance reasons.

In contrast, the Bradshaw Interceptor, which runs through the heart of the urbanized area, used a combination of microtunnels and open cut methods. The 36- to 120-in.-diameter reinforced concrete pipes are buried 30-ft. deep. The path crosses three high voltage easements, five creek channels and one railroad. Work was divided into five sections to space out impacts to residents.

"Schedule risk was the deciding factor," said Neil Mann, construction team lead for the project and an MWH employee. "We can't delay this project because this capacity is needed now for all the development going in."

Because of flood concerns near the levee, teams can only work during the spring and summer, a requirement that will stretch the entire project out over two years with completion expected in fall 2006, just in time to serve all the residential plumbing now under construction in the region.