Case Histories - Fox Wins: Sewer Sinks to New Depths

The City of Bakersfield, CA, wanted to open the rugged foothills northeast of the city for residential development. The area had been annexed into the city in the 1970s but growth was slow. Municipal officials turned to Porter-Robertson Engineers for recommendations on providing sewer service to the area. Three options were considered by the Bakersfield consulting firm: Develop a community leach field system, construct a package treatment plant, or run a new sewer line to the proposed development from an existing treatment facility. The up-front cost of running the line was more than the other options. However, officials determined that in the long run it was the more cost- efficient option. The projected fees for the community leach system and the package treatment facility would have been far in excess of the projected $700 per dwelling fee to connect to a new sewer line.

A number of thermoplastic pipe materials were considered along with vitrified clay. Bids ranged from $4.7 to $7.2 million. The project was won by Speiss Construction of Santa Maria, CA. The material of choice for the large diameter requirements of the line was Vylon PVC pipe. Over 20, 000 feet of 21 to 30 inch pipe was installed by the contractor at some unexpected depths.

Habitat Conservation Plan

As the consulting firm worked on the planned route of the new line, Jim Movias, principal planner with the Bakersfield Development Services Department, was putting the finishing touches on a Habitat Conservation Plan. The goal of the plan is to protect the area's natural habitat and meet the requirements of state and federal endangered species acts. Consistent with the plan, a biostudy was conducted to identify wildlife within the proposed corridor of he pipe. The study revealed that within pipe's path were a number of dens belonging to the endangered San Joaquin Kit Fox.

The Kit Fox dens were located in a fairly deep ravine directly along the pipe route. To avoid disturbing the dens, the alignment of the pipe was moved 300 feet p the hill. The move increased the burial depth of the pipe to approximately 37 feet, necessitating stacked trench boxes.

DeWayne Starnes, P.E. with the City's public Works Department observed, "We ere comfortable with placing the product at the extreme depths necessitated by these conditions."

In addition to the challenges of working at this depth, construction crews had to adapt to a variety of soil conditions including cobble boulders and blue-clay. Silty-and runs through the foothills making trench walls unpredictable.

Under the supervision of the Bakersfield Construction Supervisor, Gerry Claassen, a contract inspector from Porter Robertson Engineers monitored the quality of construction. Soil samples were sent to an independent lab for testing. Bedding consisted of screened native material. Ninety percent compaction was achieved in the pipe zone; 85 percent was achieved one foot above the pipe to the surface. The pipe passed a low-pressure air test the first time. A go-no-go steel mandrel was pulled through the line. Results were within the 5 percent deflection specification.

According to the City of Bakersfield, the pipe, which has been in service for over one year, is performing to standard without leakage. The performance of the pipe is no surprise to Fred Porter of Porter Robertson Engineers, who frequently visited the site during construction. "The pipe has a unique four-finned gasket that really locks those joints together."

Since completion of the line, a new magnet school has been constructed in the area and residential development is moving at a rapid pace.