Strengthening the Sewer System

Oxnard Upgrading to Keep Up With Population Boom

Two general contractors are adding about 45,000 ft. of new sewer interceptor pipes. Another contract calls for a new headworks facility to come on line late next year.

By Kathy Lee Scott

Oxnard's wastewater department is in the midst of a $100 million project to upgrade the headworks portion of its treatment plant, replace or add roughly 10 mi. of new sewer pipes and eliminate five lift stations in its sewer collection system.

The two contracts of the Redwood Trunk Sewer Project will replace about 47,000 ft. of sewer interceptor pipes and be completed in November (photo courtesy of city of Oxnard).

To service its burgeoning population-the Ventura County city has added 65,000 residents in the last 20 years, Oxnard's city council approved the project with funding from developer fees ($30.1 million) and $71.4 million worth of 30-year revenue bonds. System improvements are expected to handle Oxnard's wastewater needs for several decades.

The major upgrade to the 102-year-old city's infrastructure includes three main contracts:

• The two contracts of the Redwood Trunk Sewer Project will replace about 47,000 ft. of sewer interceptor pipes. The $33.3 million southern half of the sewer project is slated to be completed in September, while the $21 million northern part should be done in November.
  
  
• The pipes will carry wastewater to a $40.2 million headworks (the third contract) being built just north of the existing sewage treatment plant on Perkins Road. The new headworks will come on line late next year.

"We got a little behind schedule," said Bob Stier, area manager for Chesterfield, Mo.-based Affholder Inc., one of the two general contractors awarded the sewer project.

The other GC is Sahuarita, Ariz.-based Don Kelly Construction Inc.

"We ran into different material than we expected," Stier added.

The crews encountered wood and steel debris, which was mined out by a tunneling machine, Stier said.

The headworks facility will remove much of the larger solids from the influent via four mechanical bar screens and a grit chamber before it is processed in the existing treatment plant. A state-of-the-art odor control facility will absorb odorous gases from the air vented from the various operating areas before releasing it to the atmosphere. The odor control facility is part of the headworks contract.

The existing headworks will remain in service until the new one is finished, said Mark Moise, operations manager of the wastewater division. Then part of it will be converted to a plant drainage system, he added.

Affholder crews are microtunneling the southern portion of the sewer system, which entails remotely guiding a Soltau boring machine to carve out a tunnel and install new piping. This section of the project encompasses almost 5 mi. of sewer.

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"They dig access pits for the tunnel-boring equipment, pour concrete for a base and line them with steel sheeting," said George Pendergrass, project manager of Concord-based Harris and Associates, the program manager overseeing the project. "After tunneling, they use a hydraulic ram to push the pipe."

Every 900 ft., the machine bores another pit, Stier said.

Pendergrass said microtunneling is the best way to install long lengths of pipe accurately.

"It's also the best way to lay pipe in a high-water table location," said Oxnard planner Gary Sugano.

The 42- and 60-in. Hobas pipes being installed lay about 30 ft. below ground and resist degradation from sewer gases, Stier said. The plastic composites are immune to sulfuric acid, Pendergrass added.

Don Kelly Construction has been using traditional open-cut methods to lay 3.8 mi. of sewer lines along the northern section of the system.

"We'll finish on time," said Scott Gossard, Kelly's project manager.

The new pipelines will connect to the city's new 20,000-sq.-ft. headworks, designed by Malcolm Pirnie of Carlsbad and built by Lake Forest-based ARB Inc. Construction of the headworks facility began in September and is scheduled to be finished in late 2006, Moise said.

"We initially wanted to construct it with the sewer system, but had to rebid it," Moise said. "The city engineer's initial estimate for the job was too low because concrete and rebar costs came in higher than expected."

The new facility will handle peak wastewater amounts of up to 80 million gallons per day. The existing headworks processes between 22 mgd and 30 mgd.

ARB industrial division manager Bob Zaiser said crews are using a shoring system to counter the high water table of about 7 ft. at the site.

The incoming pipes will join the headworks 17-ft. lower than the existing headworks because the lift stations were removed.
Crews are digging to 40 ft. below grade to construct the initial receiving area.

"We excavated around 60,000 yds. of soil," said Mike Hoban, ARB's project manager.