100 ft. down, 1.3 miles long — A look at the largest underground tunnel project in GVL history

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Renewable Water Resources will soon be diggin’ Greenville – or more precisely, digging under Greenville. ReWa is planning the largest-ever underground tunnel project in Greenville’s history, boring a 1.3 mile-long tunnel through granite rock 100 feet underground for a new sewer line that will run from Cleveland Park near the Greenville Zoo to Hudson Street near downtown.

The $46 million project will meet sewer needs in the Reedy River basin for the next century, said Graham W. Rich, ReWa’s executive director. The underground tunnel will be 10 feet in diameter and house a 7-foot diameter pipe.

Construction is not expected to begin until January 2018 and is expected to take 30 months to complete.

Without the additional sewer line, the current sewer system for the Reedy River basin, which affects downtown Greenville all the way to Travelers Rest, will run out of capacity and development would grind to a halt, Rich said.

In addition to providing capacity for future growth, the project will meet immediate needs by providing an additional buffer against sewer surcharges due to inflow and infiltration during rainy weather.

Rich said the tunnel is not the least expensive alternative but the best long-term solution for the community.

“Greenville has developed so much that doing an open-cut project would tear up the heart of downtown,” he said. “The tunnel is not the cheapest alternative but we selected it because it won’t be disruptive. When the tunnel is being drilled, people will never know what’s going on or where it is.”

Trenchless technology is becoming more common in urban and suburban areas where infrastructure is buried well below ground and where open-cut projects would mean months or years of disruptions and detours.

The gravity sewer tunnel conveys wastewater without the use of mechanical equipment. The wastewater upstream of downtown Greenville will drop down a shaft approximately 100 feet to the pipe in the tunnel, and then flow by gravity to the existing sewer system in Cleveland Park.

This is what a similar project in Charleston looked like during construction .
This is what a similar project in Charleston looked like during construction .

 

This is what it will look like after construction is complete.
This is what it will look like after construction is complete.

 

Access shafts will have to be constructed at each end of the tunnel. The blasting of rock will occur at each end of the tunnel. Officials say the blasting will occur during the first 12 months of the project. People and businesses near the blasting sites will be given plenty of warning, Rich said.

The tunnel itself is constructed without blasting by utilizing a tunnel-boring machine that cuts the rock as it moves along the tunnel alignment.

Part of the parking lot off Cleveland Park Drive in Cleveland Park will be used as a construction staging area during the project. ReWa will add parking spots on the Washington Street side of the Cleveland Park playground to make up for the loss of those spaces during construction. After the 30-month construction project is completed, ReWa will restore the parking area used for construction staging, resulting in more parking for Cleveland Park/Greenville Zoo.

“There’s some benefit to the city in the long term but inconvenience in the short term,” Rich said.

Once the boring starts, work will go on 24 hours a day, seven days a week. Wright said it should take about a year to bore the tunnel. The 20,000 to 25,000 cubic yards of rock will be removed by mine rails and dumped in the construction staging area. Trucks will remove the rock. ReWa is working with the city on the truck routes, likely on Washington Street and Laurens Road.

A drill like this will be used to bore a tunnel under downtown Greenville.
A drill like this will be used to bore a tunnel under downtown Greenville.

 

Once the access shafts are constructed, Rich said residents won’t feel any vibrations. “They won’t feel it because it’s so far down,” he said. ReWa will use state-of-the-art equipment for geotechnical monitoring during the project.

ReWa Engineering Director Greg Wright said granite is the perfect type of rock for the tunnel. He said the tunnel would be completely encased in rock. ReWa used historical data from borings made over the years and did about 50 of their own to determine the topography, depth and character of the rock. Samples were sent off to the Colorado School of Mines for testing, Wright said.

“We’ve got pretty solid rock,” he said.

The contractor likely will advance 20 feet to 30 feet per day during the boring. The tunnel will be 10 feet in diameter, a size dictated by the equipment. A 7-foot reinforced fiberglass resin pipe will be installed and the space in between the pipe and the tunnel wall will be filled with grout.

“It won’t be able to shift,” Wright said.

The trenchless technology will require the least amount of maintenance of all of the alternatives studied, Wright said.

Perhaps the most famous downtown tunnel constructed was the Big Dig in Boston. The Big Dig, officially called the Central Artery and Tunnel and the most expensive highway in U.S. history, moves traffic through 1.5 miles in downtown underground. The project ballooned from $2.6 billion to nearly $15 billion and was eight years behind schedule by the time it was completed in 2006.

But Rich said trenchless technology is becoming more common when building infrastructure in urban, developed areas. Charleston, for instance, has many tunnels on the peninsula.

“As this project unfolds, we want all of Greenville to be excited,” Rich said. “Our first priority is letting the people of Greenville know what’s happening.”

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