Below the surface, Nashville is known for limestone.

The hard bedrock has fueled what members of Mayor Megan Barry’s administration and engineers say is a myth — that building a transit tunnel underground isn’t feasible here, either financially or structurally.

In the face of such thinking, Barry has proposed a 1.8-mile, $936 million transit tunnel below downtown Nashville. It would serve as the central light rail and bus connector for a sweeping $5.2 billion transit plan, unveiled last week, that she wants voters to approve four tax increases in May to finance.

The city’s own planning document, NashvilleNext, says the city’s geology makes underground transit “cost-prohibitive.”

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Despite that warning — and examples of the cost of tunnel projects ballooning in some cities — Barry’s team of transit contractors talks confidently that the tunnel wouldn’t be a risk or take a remarkable engineering feat to execute.

They point to multiple cities undertaking tunnel projects. And they note there’s already a labyrinth of functioning utility tunnels — that few realize are there — under downtown Nashville.

Many were built years ago through a process called boring, which Metro would use for the transit tunnel. Think of it as taking a giant drill bit that’s made just for excavation.

“It’s just another underground condition that the industry at large has figured out a way to construct,” said James Czarnecky, vice president of HDR | ICA, a transportation infrastructure company that Metro hired to perform initial engineering work on the project. “It’s not really unusual. It's not atypical."

Double-track, 60-foot-wide tunnel the linchpin of proposed light rail system

Barry says the tunnel is needed under downtown logistically so light rail and new electric buses can avoid the more narrow and congested streets of the central business district.

It would be the linchpin of the light rail system, allowing lines along Murfreesboro Pike, Charlotte Avenue, Gallatin Pike and Nolensville Pike to feed into downtown. A rider could enter downtown on one corridor and leave on another under the plan.

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The tunnel would travel north-south below Fifth Avenue South from a revamped Music City Central, the city’s bus hub where a new boarding area would be built below. The tunnel would head to Broadway in front of Bridgestone Arena, where passengers could board or exit, and south to a new hub station at Fifth and Lafayette Street.

The plan calls for the tunnel to go 40 to 50 feet below the surface and stretch 60 feet wide, meaning it would extend beyond the width of the street. The tunnel would be a double-track, twin-bore version, with light rail and buses going on the same lanes. The tubes would be 21 feet wide each.

The mayor’s office has organized a community meeting for 5:30 p.m. Thursday at the Nashville Farmers' Market to discuss the downtown corridor.

Seattle, Los Angeles among cities with ongoing light rail tunnel projects

Project leaders cite tunnel projects in other cities that are similar to Nashville's proposal.

A light rail line in Pittsburgh uses a 1.2-mile, $523 million connection tunnel under the Allegheny River that opened in 2012, and overcame the area’s silt to be built over a five-year period.

But the cost of that project grew considerably, attracting critics along the way, according to the Pittsburgh Post-Gazette.

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Seattle’s light rail relies on multiple tunnels, including a 3.15-mile University Link tunnel that opened in 2016. The city is now working on a $1.9 billion Northgate Extension that will extend 4.3 miles and is set to open in 2021.

That same year, Los Angeles is scheduled to open a 1.9-mile regional connector tunnel that comes with a price tag of $1.75 billion, according to the Los Angeles Times.

Downtown Nashville has a labyrinth of existing tunnels

Above all, on the question of feasibility in Nashville, Barry's transit team says building a tunnel here is nothing new.

Nashville has a four-mile system of trenches and tunnels across downtown for Metro’s District Energy Systems, Metro Public Works Director Mark Sturtevant said.

Built in the 1970s and 1980s, DES consists of a steam and chilled water network that has provided heating and cooling for more than 40 downtown buildings — including the Tennessee state Capitol — for more than four decades.

In the late 2000s, Metro built a tunnel for Nashville Electric Service during the construction of Music City Center, which required an NES substation to move. The NES tunnel begins at Korean Veterans Boulevard and goes under the interstate along Fourth Avenue South.

“That’s a myth that you can’t tunnel in that limestone,” said Rick Ryan, operations manager for tunnel work at Reynolds Construction, formerly known as W.L. Hailey & Company, which led both projects. “That’s perfect tunneling rock in my opinion.”

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Nashville’s water department also has multiple tunnels, including from the old convention center to Bridgestone Arena.

There are also tunnels that date back much further. As part of its “Curious Nashville” series, WPLN radio explored long-forgotten tunnels for sewer and stormwater built as early as the 1880s.

Boring machines would be used to build transit tunnel

The city's utility tunnels were created by tunnel boring machines, which allow engineers and crews to essentially drill horizontally after building an initial pit underground.

None of those Nashville tunnels, some 8 feet in diameter, are anywhere close to the massive size of the proposed transit tunnel. And none are used for transportation either. But project leaders say the concept is the same nonetheless, but with larger machines.

For an interstate highway tunnel project, Seattle has used two massive machines that have the nicknames Brenda and Bertha. Bertha broke down drilling one of Seattle's tunnels, resulting in $80 million in repairs and years of delay, according to the Seattle Times. It was replaced by the smaller Brenda.

Nashville engineers say they would look to ensure that the impact on underground infrastructure is minimal.

“Further into design, we have to look at specific buildings, and specific intersections and specific utilities, and design ways to make it work without compromising the structure of any of the other adjacent structures,” Czarnecky said.

“In any big public works project, utilities are always an area that you have to pay special attention to. Tunnels are no different.”

Project leaders say cost estimates seek to account for ‘unknowns’

Czarnecky has been working with Butch Eley, president of HDR | ICA and a former aide during former Mayor Bill Boner’s administration, on the Nashville tunnel project.

If voters approve the mayor’s referendum, Metro officials want work on the Gallatin Pike light rail line to begin in 2022, with the tunnel construction starting in 2023.

The downtown tunnel was raised during a recent special council meeting on a separate proposal from Barry for a downtown flood wall and protection system. At-large Councilman Bob Mendes asked Metro Water Director Scott Potter whether he’s incorporated the tunnel into the city’s long-term flood plan.

“Nope,” Potter said. “But as a good engineer, I can solve that problem.”

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Perhaps the most notorious tunnel project in U.S. history was a plan to reroute the interstate in Boston that became known as “The Big Dig.” The 16-year endeavor was plagued by a series of cost overruns and delays and is still a punchline years after the $15 billion project opened in 2007.

It was a vastly larger project than Nashville would face.

To avoid potential cost overruns, Czarnecky said that, like any major project, the plan would recognize and identify risks before the undertaking.

Cost projections are based on Federal Transit Administration cost categories for items such as the track, guideways and facilities.

The estimate includes a 30 percent contingency to account for “unknowns.” According to the mayor’s office, the overall cost estimate is on the higher end to account for the possibility of a cut-and-cover method in case boring faces challenges.

“The feasibility of doing an underground tunnel is there either way,” Barry spokesman Sean Braisted said. “The tunnel boring is the preferred method.”

So far, Nashville’s tunnel engineers have performed only basic planning work. This includes analyzing the profile of the street, depth of the tunnel, grades and the geometry to see if it is feasible.

If the referendum passes, engineering planning would become more extensive before the drilling beings.

Reach Joey Garrison at jgarrison@tennessean.com or 615-259-8236 and on Twitter @joeygarrison.

Proposed downtown Nashville tunnel

Uses: Light rail and electric buses

Type: Double-rack

Projected cost: $936 million

Length: 1.8 miles

Width: About 60 feet

Depth: 40 to 50 feet underground

Access points: Music City Central, Lower Broadway, Fifth Avenue South/Lafayette Street.



Tunnel projects in other cities

Pittsburgh

Name: North Shore Connector

Built: 2006 to 2012

Cost: $523 million

Length: 1.2 miles under the Allegheny River

Source: Pittsburgh Post Gazette

Los Angeles

Name: Los Angeles Regional Connector Project

Scheduled to open: 2021

Projected cost: $1.75 billion

Length: 1.9-mile S-shaped connector project

Source: Los Angeles Times

Seattle

Name: Northgate Link

Scheduled to open: 2021

Projected cost: $1.9 billion

Length: 4.3 miles from Northgate neighborhood to downtown; includes both tunnel and elevated surface

Source: Sound Transit

If you go

What: Community meeting on Mayor Megan Barry's transit plan, specifically the downtown corridor

When: 6 p.m. Thursday, Oct. 26

Where: Nashville Farmers' Market