Opportunity
The Lower Northwest Interceptor pipeline is a 19-mile (30-kilometer) sewer line that carries wastewater from West Sacramento and the rapidly growing north of Sacramento County to the Sacramento Regional Wastewater Treatment Plant near Elk Grove.
The pipeline includes two tunnels that pass beneath the Sacramento River: the northern tunnel east of the I-80 Interstate Bridge and the southern tunnel north of Sacramento’s Freeport Bridge.
“By itself,” said the Sacramento Business Journal, “the interceptor is one of the biggest infrastructure projects now underway in California, and likely one of the biggest in the Western states.” The project, the article went on, “involves some of the most spectacular construction the Sacramento region has ever seen.”
Scheduling made a difficult project even more difficult. A delay in permitting meant that the Sacramento Regional County Sanitation District had to start construction on three major pipeline projects at once: the Bradshaw, Arden, and Lower Northwest Interceptor. The Lower Northwest project alone required about 122 government permits of various types.
Another challenge was that only three companies made the pipe needed for the Lower Northwest, and only one of these was in the region. Each 12-foot (3.6-meter) section of 10-foot-diameter (3-meter) reinforced concrete pipe weighed 30 tons. “They don’t sell it at the Home Depot,” said Mary Snyder of the sanitation district.
Solution
Noting the risks of tunneling under the river, The sanitation district awarded the contract for the design of the two big river tunnels to Mott MacDonald.
To minimize the risk, Mott MacDonald recommended the use of a state-of-the-art pressurized-face soft ground shield Tunnel Boring Machine (TBM). “The large, manned machine, which looks like a spaceship, uses a cutting wheel at the front to turn rock, gravel and dirt into slurry in a pressurized chamber.”
Excavated material was dumped out the back of the TBM onto a long conveyor, which hauled it to the surface. Workers used precast concrete to line the tunnel walls as the TBM moved forward.
Mott MacDonald's Dave Young 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.”
This tunneling method, ordinarily used for larger transit tunnels, was a first in North America. Despite difficult ground conditions, the tunnels were excavated at a 6% downgrade to extend below projected scour channels in the river. Horizontal conveyers were used to remove muck, since the V-shaped alignment made the use of muck cars impractical.
California Construction described what happened next:
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 … described as "digging through marbles the size of basketballs."
The larger shaft size and increased horsepower of the TBM was able to deal with these materials without getting stuck.
Outcome
The project was completed in 2006, $1 million under original contract price and 34 days ahead of schedule. It was expected to serve approximately 200,000 households and to carry peak wet-weather flows of as much as 200 million gallons (757 million liters) per day.
Mott MacDonald's recommended construction methods enabled tunneling on the two crossings to run in parallel rather than in series, which saved the sanitation district the cost of a second tunnel boring machine.
Project cost estimates prepared by Mott MacDonald held constant through the design phase and the Engineer's Estimate was within 2% of the low bid. No time-lost accidents were experience during construction.
Sustainability was a critical issue on this project:
- Critical impacts to the local environment were avoided due to our construction recommendations and the contractor's ability to control tunneling-induced surface settlement during the first tunnel crossing.
- Early performance results prompted the Reclamation Board to waive the six-month construction blackout period usually enforced to protect the river and natural habitat.
- Potential flooding was mitigated by controlling settlement of the levees.
- Old oak trees, wetlands, and trout and salmon runs were undisturbed by deep tunneling.
- Habitat for the endangered Valley Elderberry Longhorn Beetle was spared by special fencing.