Big bore

Third phase of Memphis sewer project requires 72-in pipe placement

By Candy McCampbell

The third phase of Eutaw Construction Co.'s Wolf River Interceptor Relief Sewer project in Memphis seems fairly simple: lay 18,000 lin ft of pipe.

But the pipe is 72-in-diameter gravity sewer pipe and has to be laid 20 to 25 ft deep. And it has to go under a major thoroughfare and a major gas transmission line.

"It also includes two 200-ft-long tunnels - one under four high-pressure gas mains and one under a roadway (Germantown Parkway)," says Josh Layton, project manager with Eutaw Construction, which is based in Aberdeen, Miss.

The $12.4 million sewer project, for the City of Memphis, will serve the Germantown area and eastern Shelby County and runs roughly parallel to the Wolf River. Eutaw Construction's contract is for Phase 3C. Phases 3A and 3B, which crossed the Wolf River and ran almost 3.7 mi, were handled by Robinson Construction Co. of Perryville, Mo.

advertisement

"We're boring and jacking in the 72-in. pipe using a tunnel-boring machine," says Jimmy Dodson, an owner of Memphis Road Boring, the subcontractor handling the that part of the job. The electric- and hydraulic-powered machine moves in front of the pipe and jacks, or pulls, the pipe in behind it.

A muck car, located behind the machine operator's station, runs on a track in a continuous in-and-out cycle all day, removing soil and returning for another load. The soil in the first tunnel was sandy silt.

The pipe length is 20 ft, so two lengths can be laid in an average day, Dodson says. The hole for the 72-in. pipe is 76.4 in.

"This job is not your everyday job," he adds. "This is a large sewer job." Dodson says his workers typically work with smaller 20- to 48-in. pipe.

Work started on Feb. 7, and the tunnel under the Texas Gas Transmission LLC lines - with less than 1.5 ft of fall - was completed a week later, Dodson says.

As of late February, work had not begun on the second tunnel.

The job is scheduled for completion on Dec. 31. Eutaw Construction received the notice to proceed in October.

The rest of the pipe in the 3.4-mi job is being laid in a traditional open trench where workers are protected by more than $100,000 worth of trench safety shields, Layton says.

The trench is being dug with a 60-metric-ton excavator equipped with a 5.5-cu-yd digging bucket. Also on the equipment list are a pair of 40-metric-ton excavators, two wheel loaders, a bulldozer and a 35-ton end-dump truck.

The pipe is made of centrifugally-cast, fiberglass-reinforced polymer mortar pipe, or CCFRPM, which Layton calls "a better fit" than concrete for a project like this one.

"It was cheaper than concrete," he adds. "It doesn't require a liner and it's much lighter than concrete." A 20-ft "stick," or pipe section, weighs 350 lbs per ft., or 3.5 tons. A 20-ft length of prestressed concrete cylinder pipe weighs 1,600 lbs per ft, or 16 tons a stick.

The lighter pipe can be handled without a crane, Layton says.

The pipe that goes into the tunnels is "beefed up," he says. "It is also made of CCFRPM, but it's thicker and designed to have a much higher compressive strength so it can withstand the pushing (or jacking)."

Because of the pipe's strength, workers in the pipe are protected by the pipe itself and only have to wear hardhats and safety vests, he says.

But the depth of the pipe on this job "makes this a dangerous line of work" due to the threat of cave-ins, which presents a challenge not encountered on most jobs, Layton says. Eutaw counteracted by ramping up its safety program for its 20 workers at the site and making sure supervisors have training in trench safety.

As gravity sewer projects go, this is a big one," Layton adds. "There aren't many sewer systems that require such big pipe in this part of the country."

The next challenge will be dewatering the area near Germantown Parkway, where the second pipe bore will occur.

"The groundwater table in this area is at a depth of 10 ft and the pipe is to be installed at a depth of 25 ft," Layton says. Workers are using well points to pull the groundwater table down below the bottom of the trench, he adds.

The trench will be lined with limestone supplied by Fullen Dock & Warehouse Inc. of Memphis.

Roy Rothenberger, aggregate sales manager for Fullen Dock, says he has hauled about 9 tons of an estimated 50 tons of rock needed for the project.

The project started with the installation of about 41,000 lin ft of silt fence by Charles Hill, owner of Charles Hill Contractors of Memphis. Hill will also install about 9 tons of stone for riprap on the trench after the pipe is laid.

Layton says one area of the project goes through a natural area, where the working space will be narrowed from 100 to 80 ft. Eutaw will also go through a wetlands area that has to be returned to its original state.