Nashville’s New Water Ways

Nashville, Tenn., contractor W.L. Hailey, now part of Layne Heavy Civil, has been a key player in the city’s upgrades for decades on everything from tunnels for high voltage transmission to open cut water and wastewater pipelines.

The contractor’s latest project saw them using various methods including small diameter tunneling to excavate a new water main through difficult ground conditions in the southeastern area of the city. To complete the project, crews conquered water inflows of 60 to 70 gpm in some sections.

The contractor used cut-and-cover, hand mining and a shielded Rockhead, manufactured by The Robbins Co., to excavate widely varying ground conditions in an urban setting. “This project is unique. Its challenges included rock excavation via trenching to conform to the project’s restrictions on controlled blasting.
Tunneling was done under 15 railroad tracks and open-cut pipe work was completed inside of a rail yard with up to 1,000 tractor-trailers entering and exiting per day,” said Brian Rieschick, project manager for Layne Heavy Civil.

Planning for the Population Boom

At 6,900 ft long, the Powell Avenue 36-in. water main will send finished water for drinking and residential use through the existing Powell Avenue Pumping Station.

“This project is part of the Metropolitan Government of Nashville and Davidson County’s Master Water Growth Plan. It’s intended to grow the water distribution system capacity for future demands imposed by population growth,” said Rieschick.

Nashville and its surrounding areas have experienced 21 percent population growth since 2000, and are currently home to nearly 1.6 million people. The new 36-in. ductile iron water line, along with the city’s planned modifications to an existing pump station, will result in a capacity increase of 10 million gals per day for the Metro Water Services of Nashville. “This project has been in the works for five years. With the water main and pumping station upgrades, we will be able to handle 24 million gallons of water per day on average,” said Sonia Harvat, public information officer for Metro Water Services of Nashville.

 In 2011, the utility named Layne Heavy Civil as general contractor for the $7.02 million project. The water main is expected to become operational in April of this year.

Full Spectrum of Construction Methods

Approximately 5,300 ft of 36-in. and 2,700 ft of 12-in. ductile iron pipeline were constructed using traditional open-cut methods. These areas were in softer geology and not below critical structures. A short 210-ft long, 6-ft diameter section under a roadway was also hand-mined using hydraulic splitters.

The excavation method for the section below the CSX Radnor Rail Yard, one of the largest and busiest yards in the country, was clear cut. “Shielded TBMs were the only method ever considered for this section, the specs required a shielded machine plus liner plate to reduce any chance of settlement below the rail yard,” said Rick Ryon, tunnel superintendent for Layne Heavy Civil. Ryon. The contractor selected a 60-in. diameter Double Shield Rockhead for 1,400 total ft of tunnels in competent limestone ranging from 8,000 to 12,000 psi UCS.

Long-term Tunneling Experience

Layne Heavy Civil has traditionally operated larger diameter TBMs, and is now gaining more experience at smaller tunnel diameters. “My first TBM project in Nashville used an 8.5-ft diameter Jarva/Robbins machine, about 27 years ago. We still own that machine, which was recently rebuilt by Robbins for another project in Nashville,” said Ryon. New hydraulic and electrical systems, as well as a new cutterhead, were provided for the 6th Avenue South Transmission Line. That 3,088-ft long excavation is currently ongoing for a high voltage cable tunnel required to relocate current overhead lines.

Ryon said the newer machines he has worked on offer several improvements: “The new machines are better electronically, they have more protective shields, and overall better capabilities.” The self-propelled Robbins Rockhead on the Powell Avenue project was designed for soft rock conditions, with 6.5-in. disc cutters mounted in the cutterhead. The machine operates much like larger diameter TBMs — as the cutterhead rotates, disc cutters ranging from 6.5 to 15 in. in diameter penetrate the rock face and create a “crush zone” through which fractures propagate. Material between adjacent crush zones is then chipped from the rock face. Muck scrapers scoop the muck into openings on the cutterhead called muck buckets, which transfer the material to a machine belt conveyor. Muck removal from the site is by either a belt conveyor or muck cars.

The machine can be launched from a shaft or pit, and can be either self-propelled using a gripper system or pushed forward with a separate pipe jacking system. During excavation, the tunnel is lined with a primary liner of the contractor’s choosing, including steel casing, ring beam and board, reinforced concrete pipe and four-flange liner plate.

The Robbins Double Shield Rockhead is typically used on longer utility installations (usually over 600 ft). It is also used for line- and grade-critical installations such as gravity sewers, as the machine can be continuously steered from an in-shield operator’s console. Ideal ground conditions are those above the water table–mixed ground such as alluvium, and hard rock ranging from 4,000 to 25,000 psi UCS.

Machine Launch and Excavation Limited Site Space

Tunneling at the Powell Avenue project was designated for two sections of 500 and 900 ft long, at nearly 90 degree angles and joined by an intermediate shaft. The first, 53-ft deep launch shaft was constructed in a tight 60-ft wide by 100-ft long footprint using drill and blast. The small space, combined with high-voltage power lines overhead, required careful crane operation with a limited range of motion to lower the components down the shaft.

A 26-ft long cradle was set at the bottom of the shaft for machine installation, but clearance did not initially allow for installation of the last gripper shield. Instead, the contractor utilized a launch pad, advancing the machine forward by pushing off of beams until there was enough space for the gripper shield to be installed.

Water in the Tunnel

The machine was launched on June 7, 2011, and relatively quickly, crews encountered some difficult challenges. An unexpectedly high volume of water flowed into the tunnel about 500 ft in at a rate of 60 to 70 gallons a minute. These flows were intermittent, but tended to spike after heavy rains. “Our biggest problem was keeping muck on the belt because of the water—we slowed down our excavation rate in this section to help with that. Also, our locomotives were electric, so we had to try and keep the drive motors from getting wet,” said Ryon.

Because the tunnel had no grade, the inflow of water did not naturally drain back to the launch shaft. At one point, the inflow was so great it required the use of two 4-in., 110 volt submersible pumps — one in the launch shaft and one in the intermediate shaft — running nonstop for approximately 600 ft of tunneling.

After successful dewatering using the pumps, excavation continued using two shifts. One crew quickly mined ahead during the day, while a second crew installed the liner plate during the night. The 16-in. plates were grouted in place using cement and water grout, mixed on site. This process of dual shifts allowed for much better production rates for both the mining crew and the liner plate crew and brought daily averages to more than 20 ft.

“The machine did a pretty great job — it averaged 18 to 20 ft per 10-hour shift for the 900 ft run in dry conditions, and at least 20 ft per shift on the 500-ft run. We lost some time in wet ground when locomotives hindered advance rates, but that could not have been helped,” said Ryon.

The Robbins Rockhead completed its first 900-ft long drive in three months, holing through on Sept. 14, 2011, into the 36-ft deep intermediate shaft. After the first drive, the crew found almost no perceptible wear on the cutterhead. From the second shaft, the tunnel took an approximate 90 degree turn and drove another 500 ft to complete the project.

Ground became drier on the second, 500-ft run and there were few problems to hinder advance rates. The second tunnel was completed on Nov. 1, 2011, also with minimal cutterhead wear. “We did both runs with the same set of cutters. At the end there was only one worn cutter after 1,400 ft,” said Ryon. 

Final carrier pipe installation was completed in winter 2011, with crews back-filling the annular space between the pipe and liner plate using cellular grout. Rock from the excavated tunnels was then used as backfill in the shafts after the pipes were installed. Construction on the water main is scheduled to wrap up in late February 2012.

As for Layne Heavy Civil, the company is busy working on projects in and around Nashville, and has plans to complete its high voltage tunnel using a veteran Robbins/Jarva TBM by June 2012.

Desiree Willis is a technical writer for The Robbins Co.