Infrastructure

BART to San Jose: Is One Subway Tunnel Cheaper, Less Disruptive To Build?

San Jose's most recent Bay Area Rapid Transit project is causing traffic snarls, but would one tunnel or two be less disruptive for the city?

by Gary Richards, The Mercury News / October 2, 2017

(TNS) -- SAN JOSE -- Empty Bay Area Rapid Transit trains have been cruising along the 10-mile extension from Fremont to the Berryessa station in San Jose, testing equipment before opening next year.

But as that project winds down, the focus is shifting to the immense challenges to tunnel under downtown and finish the next six miles to central San Jose sometime in the next decade. The tunnel project could snarl traffic for four years on busy Santa Clara Street, keep customers away from nearby businesses, and put the Valley Transportation Authority at risk of nearly $500 million in extra costs. But the impact may vary depending on which tunneling method is chosen.

The key question: One bore, or two?

One approach means a longer walk to the next boarding area, more stairs, and dealing with security concerns on a known design versus a new approach.

The VTA, which is building the tunnel, has recommended a single 45-foot diameter subway bore. It would carry two tracks within the same tunnel, using a dividing wall between trackways. VTA argues the design will be cheaper and less disruptive during construction.

But the so-called single-bore option has not been widely tested. And design studies are only 20 percent complete, meaning full costs are a guessing game.

BART, which will operate the trains, prefers two single-track, 21-foot tunnels. That would be similar to BART’s 19th Street stop in Oakland, and what it uses on its existing 112-mile system. The twin-bore design is used globally by transit systems and design studies on the VTA project are 65 percent complete, meaning likely costs are better known.

“Sure you can design and build a one-bore tunnel but is that the right project?” said BART Deputy General Manager Bob Powers, who believes safety issues should get more consideration as well as potential costs that could add millions to the $4.7 billion price tag. “BART has a completely different perspective.

“It’s a 100-year decision and we want to make sure we get that right.”

VTA’s Dennis Ratcliffe said his agency has another view: “In every category of consideration at least with regard to VTA staff opinion, the single bore is either equal to or superior to the twin bore.”

The VTA had planned to vote on its options at its board meeting Thursday. But it canceled that vote until a later date to meet with BART and iron out issues.

But a quick vote is imperative, said VTA general manager Nuria Fernandez, citing looming deadlines to seek $1.5 billion in federal aid.

If the twin bore wins out, there will be major impacts to Santa Clara Street between Market and Fourth streets during utility relocation and station excavation. Once the station is excavated, the VTA will use what is called “cut and cover” — digging an opening, moving a massive 45-foot drill under the street and then covering it with metal plates to keep traffic on Santa Clara Street moving.

It’ll dig one to two blocks and one intersection at a time below ground, and then move on.

Under the single-bore alternative, work often will be done on side streets adjacent to Santa Clara Street. VTA says this choice could shave 10 months off the four-year tunneling phase, trim $50 million in costs and be less intrusive to pedestrians and traffic.

“There are some factors that are important to our city on costs that are not being thought about, like having our streets torn up for an extra 240 days,” said VTA board member and City Councilman Johnny Khamis of San Jose. “Time is money. We’re talking about our businesses not having access to roads, parking, all kinds of stuff for a year.

“For me, it’s a clear decision.”

Sharon Fredlund, the executive director of the Building Owners and Managers’ Association in Silicon Valley, said her buildings surround the whole construction area.

“No access for my tenants, no access to parking and transportation disruption for not just months but years,” she said.

Faster is far better, added Alfredo Diaz, owner of Diaz’s men’s clothing store on Santa Clara Street. His business fell 40 percent in the 1980s when light rail track tore up the downtown area.

The city and transit agency helped defray those losses, but so far there’s no discussion of a similar help for the BART work.

“Am I worried?” Diaz asked. “Correct. The city and VTA should help us. BART might bring new customers, but this work will affect us a lot.”

By comparison, the Fremont to Berryessa line has been a breeze. It’s all at ground level on a path where Union Pacific freight trains once ran.

Now, BART trains are being used to test the communication systems, tracks and software. This is one of the milestones VTA and BART must achieve before opening for passenger service by June.

The downtown tunneling extension is far more complicated. It involves moving the mammoth drilling device, trucks needed to haul away dirt, care taken for proper ventilation, monitoring underground water tables and shifting soil near Highway 101-Berryessa.

Plus, stations must be designed so riders can get from an underground platform to Santa Clara Street in six seconds during an emergency.

From a tunnel with one bore or two.

©2017 the San Jose Mercury News (San Jose, Calif.) Distributed by Tribune Content Agency, LLC.