Sacramento Sewer Expansion Deals with High Water Table
A Village of Contractors
The $61-million West Sacramento Force Main, a pair of 3.9-mile, 54-inch diameter pipes stretching from the Northern Sacramento River Crossing across the northern portion of the West Sacramento to the proposed West Sacramento Transition Structure, was designed by Englewood, Colo.-based CH2M Hill. San Diego-based Berryman & Henigar teamed up with Walnut Creek, Calif.-based Covello Group as construction managers and Livermore, Calif.-based Mountain Cascade is the contractor. The firm will tunnel-bore the first 8,000 ft and open-cut the final 32,000 ft.
Brown and Caldwell, Walnut Creek, Calif., designed the $37-million, 120-in.-dia Southport Gravity Sewer. Concord, Ca.-based Harris & Associates is the construction manager for the 3.3-mile stretch of PVC-lined, reinforced concrete pipe that runs through southern West Sacramento and ends at the South River Pump Station. Steve P. Rados, Inc., of Santa Ana, Calif. is the contractor.
The team of Cambridge, Mass.-based CDM and Davis, Calif.-based West Yost & Associates designed the $53-million, 5-mile Yolo Force Main. Harris & Associates is the construction manager once again and Las Vegas Paving is the contractor.
The design consultant for the $35-million Natomas Force Main was Overland Park, Kansas-based Black & Veatch. The construction manager for this 3.2-mile stretch of 60-in diameter pipe was Alberta, Canada-based EPC and the contractor was Mountain Cascade.
The design consultant for the $43-million, 2.4-mile Sacramento Force Main was URS Corp., San Francisco, Calif. The construction manager on the two 66-inch-dia pipes is EPC and the contractor is Rados.
A Really Big Drill
The most challenging aspect of the project are the two 2,000-ft long, 15-ft-dia tunnels under the silt and gravel floor of the Sacramento River, a $44-million contract. Dave Young, senior design manager for consultant Hatch Mott McDonald, Millburn, N.J., says, "We had to convince the district that the large diameter pressurized tunnel using a tunnel boring machine was hands-down the best alternative. It was equal in cost, but superior in schedule and operations and maintenance."
Two other methods were considered: Horizontal Direct Drill and pairs of 10-ft micro-tunnels using remote-controlled machines. Other project segments employ both methods, but for the river crossings, "schedule risk was the deciding factor. We can’t delay this project because this capacity is needed now for all the development going in," says MWH’s Mann.
That decision paved the way for the ordering of two 150-ton, 30-ft earth pressure balanced Lovat drilling machine that bore into the earth and spit dirt out onto a discharge conveyor belt.
To make maintenance easier, the tunnels will be drilled at 6û angles–a requirement that means the carts carrying away the debris will have to fight gravity to get to the surface with their loads.
The two repurposed machines will deal with the water pressure in the surrounding earth by creating a pressurized bentonite slurry. A screw conveyor at the bottom of the cutter head will line the tunnel with 13.5-ft dia gasketed precast concrete segmental linings as it goes.
Upon completion of the tunnels, twin steel pipes will be installed inside the tunnels and the void will be filled with low density cellular concrete, a lightweight foam and cement slurry that uses air bubbles as aggregate.
A number of factors make the process more difficult for tunnel contractor, Chesterfield, Miss.-based Affholder. "The water table in Sacramento is a challenge," says Matthew Crow Sr., resident engineer Los Angeles-based office of Parsons Brinkerhoff Construction Services, Inc., the tunnel construction manager. He cites a combination of natural and imposed conditions that make the project even more challenging than the 35-km East Central Interceptor Sewer and Northeast Interceptor Sewer project that required his team to use similar technology to handle growth and deteriorating infrastructure in the southern city.
Just digging the 7.6-m by 15.2-m, 14.8-m deep launch and receiving shafts kept subcontractor Blue Iron scrambling. The Woodbridge, Calif.-based firm faced tight dewatering restriction, under permits set by the Central Valley Regional Water Quality Control Board. To protect the environment, the agency allowed only two discharge points, where water from the pit could be cleaned and returned to the river. The contractor brought in divers for the final 10% of excavation in the receiving pit. Starting May 9, they were scheduled to spend a week hand-guiding a tremie tube to plug the floor of the south shaft tunnel.
Sheet piles are commonly employed in high water table regions. Cranes place members, and separation can be problematic. To eliminate splitting, the contractor brought in a hydraulic vibratory ABI Mobilram with 120 tons of driving force to pound divided sheet metal into predrilled holes. This avoided the need for cranes.
The crew also had tight site restraints as they tried to disturb a minimum of endangered beetle habitat and were under the watchful eye of neighbors who literally trained video cameras on the job site to ensure no work started before 7 a.m.
Also as a precaution, 20-foot berms were added to the launching pits to give what Woodbridge, Calif.-based Blue Iron, Inc., Drilling Manager Chris DellAringa calls 100-year flood protection, "even though we won’t be working in the rainy season."
That seasonal limitation means the teams will be working for two years with completion expected in Fall of 2006, just in time for all the new plumbing now under construction in the region.