An abandoned rock quarry a few miles west of metro Atlanta has served as a backdrop for apocalyptic fictional worlds such as “The Walking Dead” and “The Hunger Games.” But the quarry will soon function as a reservoir safeguarding against a very real potential disaster—drought.

The $300-million project, including a five-mile-long tunnel, is being built to connect the 400-ft-deep quarry with two water treatment facilities and the Chattahoochee River. The project is indicative of the growth of water tunnels across the nation to solve myriad water problems, from drought to pollution, flooding or storm surge.

In Atlanta, a severe drought in 2009 made the city realize that it needed a longer-term source of water, so planning began for a tunnel solution to provide an alternative water source for the metro area of 4 million people.

In cities including Washington, D.C., and Fort Wayne, Ind., tunnels are being used to keep stormwater and wastewater from polluting natural bodies of water. In Charleston, S.C., tunnels are being used to alleviate flooding during major tidal events called king tides. And in Houston, a tunnel could be part of the solution to alleviate flooding in the flat, overbuilt region.

“From my perch, quite frankly, it’s exploding,” David Egger, senior vice president and managing director for infrastructure systems for Black & Veatch’s water business, says of the tunnel market. Egger estimates water tunneling is a $6-billion-a-year business in North America. “More contractors are entering the space, and owners left and right are seeing the advantages of tunnels.”

“The momentum on water tunnels has reached critical mass.”

– Brian Gettinger, Black & Veatch

Most of the attention in other cities may go to the multibillion-dollar transportation tunnel projects, like the Second Avenue Subway in New York City or the Gateway tunnel between New York and New Jersey. But the sheer number of water tunnels being built or planned exceeds the sexier transportation tunnels. “There are more water tunnels than transportation tunnels,” says Mike Schultz, technical strategy leader for the geotechnical-structural group at CDM Smith.

Egger at Black & Veatch estimates there are about 200 miles of water tunneling in various stages of planning and design in North America, 50 miles of which directly involve Black & Veatch. Jacobs alone is involved in the development of  more 150 miles of water, wastewater and CSO tunnels around the world.

Technology has reduced the price of tunnels enough so that mid-size cities like Fort Wayne and Alexandria, Va., can use a tunnel option to handle their excess water. Technology also has made tunneling a viable option in some cities with soft ground, such as Houston, which couldn’t have considered building tunnels just a decade ago.

A premium on land in urban areas also has elevated the cost-benefit ratio for tunnels. “The value of the surface land should be considered a major asset by modern cities,” says Mark Johnson, global solutions director for tunnel and ground engineering at Jacobs. “For example, it could be of much greater societal value to enable retail or commercial development than to give land over to a wastewater treatment plant.”

In short, the era of the water tunnel has arrived. “The momentum on water tunnels has reached critical mass,” says Brian Gettinger, a Houston-based project manager and tunnel practice leader within Black & Veatch’s water business.

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Atlanta’s Tunnel Solution

The growth of Atlanta makes the possibility of a major water reservoir in the city itself unthinkable. In 2006, Atlanta purchased the rock quarry, in an industrial area a few miles west of metro Atlanta, with the idea that it could serve as a raw water storage facility. It took some tunnel engineering, however, to design a system that could bring the water to the people of Atlanta. The Atlanta Water Supply Program, launched in 2015, is scheduled for completion next year.

When complete, the Bellwood quarry will become a 2.4-billion-gallon raw water storage facility that can provide emergency drinking water for up to 90 days for the city. The water supply will be a backstop for Atlanta’s sole source of water, Lake Lanier, which could be drained dry in 90 to 121 days. “There is new development everywhere. We can’t afford not to have safe, clean drinking water all the time. If we lose water for a day, it would be a devastating impact to our 1.2 million customers,” says Ade Abon, senior watershed director.

The Atlanta project demonstrates how a tunnel can provide a vital link for water projects of all kinds. “A single tunnel makes everything else work,” says Bill Hansmire, technical director of tunneling for WSP.

Stantec, with team members PRAD Group Inc. and River 2 Tap, designed the hard-rock tunnel. Don Del Nero, vice president of Stantec, says it is the first project in the U.S. where a tunnel-boring machine (TBM) was assembled on site, using an Onsite First-Time Assembly method created by manufacturer Robbins.

Driller Mike

The 400-ft-long, 12.5-ft-dia TBM, dubbed Driller Mike, has completed about three of five miles, says Bob Huie, senior project manager with PC Construction Co., construction manager at-risk (CMAR) on the project in a joint venture with H.J. Russell & Co. This may be the longest tunnel of its kind and only the third in the U.S. to be delivered under CMAR, adds Del Nero.

Using CMAR shaved about a year off the project schedule, says Atlanta’s Abon. “We didn’t have to complete the design entirely or have the tunneling contractor on board before procuring the TBM. So we received the TBM a year in advance of a typical delivery date .”

Driller Mike is mining through mostly good rock at about 80 ft per day, says Huie. Approximately 40% of the bored tunnel will receive 10-ft-dia cast-in-place lining, adds Gevan McCoy, vice president with Guy F. Atkinson Construction. Atkinson and Technique Concrete Construction are the joint venture tunneling subcontractor. At the quarry site, crews are building a 253-mgd pump station with four vertical turbine pumps and three submersible turbine pumps. Crews blasted two pump shafts, 20 ft and 35 ft in diameter, as deep as 300 ft, along with a power substation.

Preparatory work included stabilizing the quarry walls with rock anchors and netting, says McCoy. The TBM began mining from the quarry bottom at 550 ft above sea level. “The lower tunnel will only be used when there’s an extreme situation and water in the quarry dips below that elevation,” he says. An adit located at elevation 636 above the tunnel will be the primary conductor of water to the treatment plants, says McCoy. The tunnel alignment, which reaches as deep as 450 ft, heads northeast from the quarry to the Hemphill Water Treatment Plant. There, North American Drilling excavated five 11-ft-dia blind-bore shafts, a first in Georgia.

The blind bores were an alternative to drill-and-blast, which would have impacted the surrounding community and adjacent reservoir, says Huie. “As you bore down, water fills in. [Crews] use air to lift the water, rocks and chips out of the hole.”

“We see this as a model other cities could follow.”

– Jay Fayette, PC Construction

A system of pipes, a weir and a pond filters out the solids and returns the water back into the tunnel. The bores, nearly complete, will eventually connect the main water tunnel via short drill-and-blast tunnels to a new Hemphill pump station, which crews will begin building this year, says Huie.

The main tunnel will head northwest to the Chattahoochee Water Treatment Plant, where crews are also gearing up to build a new pump station and a 30-ft-dia, 250-ft connecting shaft. That requires a 2,000-ft tunnel extension to the river intake. Because the city ultimately decided to build a new pump station at the river rather than renovate the existing one, the CMAR team decided to come up with a way to introduce water into the tunnel early while it finishes testing the new station that crews will begin building this year, says Huie. 

The team is considering ways to divert the current water flow between the existing pump station and existing old pipes into the new water tunnel to feed the quarry before the new pump station is completed.  “We can introduce a tunnel plug at some strategic upstream location,” says Huie.

Once the new pump station is complete, the rest of the tunnel can be put into service and all the water flow will be diverted from the old system. The new system is designed for a 100-year life, and the city also envisions a 300-acre park and recreational area around the quarry. “We don’t know that any other city has undertaken this,” says Jay Fayette, COO with PC Construction. “But they are taking note of this. There are lots of abandoned quarries throughout the U.S. We see this as a model other cities could follow.”

Smaller and Deeper

Atlanta’s quarry project demonstrates the complexities that can make water tunnels more difficult to build in some respects than transportation tunnels.

Water tunnels don’t need ventilation and exits like highways or subway lines, but they do usually incorporate intake shafts and tie into pumping stations or wastewater treatment plants. And water tunnels must be sealed to prevent infiltration of contaminants from the outside and exfiltration of wastewater or sewage to the outside. “We don’t design hydrologic structures that leak!” says Schultz.

Often, a cast-in-place secondary liner is required to meet a project’s specified design-life. Other solutions for providing adequate durability include the provision of precast concrete segments with membrane liners or, in the case of DC Water’s Blue Plains Tunnel, the addition of ½ in. to 1 in. thickness to the interior of the precast concrete segmental liner. The additional concrete can be sacrificed to the acidic environment of a combined sewer or wastewater tunnel to maintain the tunnel’s integrity, says Johnson of Jacobs.

Water tunnels are typically smaller in diameter, from about 20 to 30 ft, but they are usually deeper than transportation tunnels, making issues like worker safety and water tables more challenging.

Like transportation tunnels, improving technology is making construction of tunnels easier, safer and cheaper. Advances in TBMs are a big part of the improvements. TBMs can be refurbished and moved to a new job. The TBMs and computer systems monitor and measure everything, allowing work to be completed uniformly. “The technology has so advanced in the last 15 years, it’s great, it’s dramatically different,” says Schultz. 

Pollution Solution

Historically, water tunnels were used to move drinking water from point A to point B. But that changed after the Clean Water Act of 1972, which aimed to eliminate raw sewage discharges into rivers, lakes and other water bodies during heavy rains or combined sewer overflows (CSOs) from the century-old combined sewer systems mainly in the Northeast and Midwest. Work in Chicago to eliminate the CSOs that were polluting Lake Michigan began almost immediately. Construction on Chicago’s first tunnels began in 1975 and were completed in 1985. Chicago is still working on its tunnel and reservoir system, and when up to 3 in. of rain fell on the city Feb. 19-20, the 3.5-billion-gallon McCook Reservoir, a former hard-rock quarry that came on line late last year, filled for the first time, greatly reducing the amount of combined sewer output into the Des Plaines River system, according to a spokeswoman for the Metropolitan Water Reclamation District of Greater Chicago. Other large cities, including Cleveland and Milwaukee, also have built tunnels to prevent the release of wastewater.

Most others, though, lagged behind. In the 1990s, EPA began taking enforcement actions against cities that didn’t address the issue, resulting in more than 50 consent decrees with cities requiring them to act to prevent overflows, including New York City, Atlanta and Washington, D.C. As of last year, the Environmental Protection Agency still listed more than 850 municipalities, primarily in the Northeast and Midwest, that were releasing some sewage into water during heavy rain. Projects to remedy the CSOs, in the works since those enforcement actions began, are now being built.

“The Clean Water Act set in motion a lot of things. … We are seeing the build-out of things that were mandated years ago.”

– Bill Hansmire, WSP

“The Clean Water Act set in motion a lot of things for cities and municipalities to clean up their systems,” says Hansmire of WSP. “We are seeing the build-out of things that were mandated years ago.”

Similar environmental edicts in Europe have led to the construction of tunnels to manage CSOs in the U.K., including London’s Lee Tunnel along the Thames River (ENR 9/19/16 p. 45) and Glasgow, Scotland’s Shieldhall tunnel.

In most CSO tunnel systems, stormwater reaches the tunnels through a series of shafts. Once in the tunnels, the water is fed by gravity toward a treatment plant. The water stays in the tunnel until the plant is able to treat and release the water. Each mile of an 18-ft to 20-ft-dia tunnel can hold roughly 10 million gallons of water, says Chris Ranck, a principal water engineer for Arcadis.

Tanks, reservoirs, open aqueducts and green infrastructure are also used to handle CSOs. Tunnels typically form the backbone of a CSO program that is augmented by these other solutions. For larger cities, tunnels are the only reasonable way to move water through a congested city.

“A lot of these tunnels are for municipalities that have dense areas. To try to put in new infrastructure, like an open-cut solution, would be terribly disruptive,” said Carlton Ray, director of Washington, D.C.’s $2.6-billion Clean Rivers program, a CSO program that will include four tunnels when complete, including the Blue Plains Tunnel (ENR 3/6/17 p. 26).

Ray also led tunnel efforts in Indianapolis to address CSOs. “I can’t emphasize enough the benefits associated with tunnels. It’s being able to pick up the flow without disrupting the fabric of the community.” In addition to tunnels being out of sight, the tunnels can move water outside of population centers, eliminating the need to build a wastewater treatment plant in the middle of a city. That was especially critical in Washington, D.C., where the tunnel system is just a “stone’s throw” from the Capitol, says Ray.

“Putting some types of facilities underground can free up surface space for a higher and better use for society. Adopting a tunneled solution can also reduce surface impacts and therefore plays into a more sustainable solution,” says Johnson.

Increasingly, water tunnels are being used or considered to address CSOs in smaller, midsize cities. “We’re looking at bigger communities right now, but smaller communities will see the benefit” of the technologies advanced through tunnel solutions in larger cities, says Ray. The Clean Rivers program, for instance, demonstrated the success of TBMs in soft soil with almost no ground movement.

Smaller Cities, Big Tunnels

In Fort Wayne, Salini Impregilo and S.A. Healy are constructing a $188-million, five-mile tunnel that, when complete in 2021, will eliminate CSO discharge by 90%, or 800 million gallons a year.

In Alexandria, which is under a state mandate to reduce its CSOs at four spots by 2025, stakeholders appear to favor a solution to build two interconnected tunnels to manage the overflows. The city and its water utility, Alexandria Renew, proposed four solutions. But the connected tunnel solution appears to best meet all five of its criteria: lowest life-cycle cost, adaptability, schedule, operations and maintenance and community benefits and impact. The community strongly opposed an option to construct tanks to hold excess water.

“It’s difficult to say that one thing trumps another, but there was a strong sentiment that the disruptions associated with the tanks—residents didn’t feel like they were appropriate,” says Liliana Maldonado, director of the wet weather program for Alexandria Renew.

“There was a very significant sentiment that the tanks weren’t consistent with the aspirations of the city or the harmony of the sustainability goals and historic nature of the city,” says Bill Skrabak, deputy director of infrastructure for the city. While the plan has not yet been finalized, it is expected to cost between $350 million to $550 million to complete.

Advances in technology are driving down cost and reducing the risks associated with tunneling in cities. For some cities, says Johnson of Jacobs, it’s simply more cost effective and less disruptive to build a single storage or conveyance tunnel along the length of a river, as is being done in the 20-mile long London Thames Tideway Tunnel Program, rather than building a series of independent storage tanks and conveyance conduits.

The 100-plus-year lifespan of water tunnels also makes them attractive to communities that want their investments to last beyond a standard 50-year design life. “If they are going to build something, they want to make sure it’s going to last for a long time,” says Schultz of CDM.
Like other communities looking at tunneling solutions, Alexandria will save money by designing its tunnels around which tunnel-boring machines are available in the time frame the city needs to build the tunnel. “We’re seeing cities and utilities trying to time the construction market,” says Arcadis’ Ranck. “The whole industry is getting smarter.”

Flooding Remedy

As technology expands the potential for water tunnel construction, tunnels also are increasingly being considered to solve historic and new flooding problems.

Black & Veatch is championing the idea of building tunnels in Houston as one way to alleviate the flooding experienced in several city floods, including Hurricane Harvey. For decades, tunneling couldn’t be considered in Houston because of its high water table and its clayey and sandy soil. But TBMs have made such a solution technically possible, says Gettinger of Black & Veatch. Gettinger says the surface water solutions of bayous and reservoirs that are now used to handle water in the region are at capacity.

“They want to get rid of floodwaters, but they don’t want their bayous to look like the Los Angeles aqueduct,” Gettinger says. “They don’t want to pave over these natural assets.”

Talks about Houston tunnels are still in the embryonic stage, however, as the region grapples with a long-term infrastructure plan that will likely cost billions, he says.

Dallas and Austin are currently building tunnels specifically to alleviate flooding. Austin chose a tunnel instead of an open reservoir to free up park space. “I think that it’s a trending solution, there’s been enough documented success, and [tunneling] provides major environmental benefits,” Gettinger says.

“I can only see the demand for tunneled solutions increasing as we move forward.”

– Mark Johnson, Jacobs

Water tunnels can also be used to handle coastal flooding. Black & Veatch has designed a tunnel system in Charleston, being built by Jay Dee Construction Inc., that will help alleviate flooding during king tides. The $135-million project will use a 12-ft-dia tunnel, 120 ft to 150 ft deep, and a pump station to help clear the low-lying areas, including a hurricane evacuation route, a hospital and the Corps of Engineers office, which have been inundated during king tides.

Most utilities are adjusting water modeling based on climate-change-driven droughts and heavy rainfalls when constructing CSOs and crafting plans to manage water in the future.

“For a utility overall, it’s a double whammy. You don’t want to see your total precipitation go down, and then when it rains, it comes all at once,” Ranck says, summarizing the problem.

 There’s also an increasing conversation about stored water reuse, says Kevin Shafer, general manager of the Milwaukee Metropolitan Sewerage District and board chairman of the US Water Alliance. Shafer says that on a recent trip to China he learned the country was considering just such a proposition.

The water tunnel business appears to have no likely end. “A situation like Cape Town [which has almost run out of water] could happen in the United States. We are beginning to realize that and think about that,” says Ranck. China is designing a water tunnel to irrigate its desert. California is planning a tunnel to take water from the Sacramento River to Los Angeles.

“Technology is going to improve further, and it’s going to bring more and more projects into the realm of the feasible,” adds Johnson. “I can only see the demand for tunneled solutions increasing as we move forward.”