Santa Clara County has decided to buy a colossal mechanical worm to dig a tunnel the size of a four-lane freeway underneath downtown San Jose.
On Thursday evening, the board of directors of the Valley Transportation Authority voted to spend $460 million to secure the enormous underground drill, known as a tunnel-boring machine, and other vital infrastructure.
The contract award is a major milestone in the decades-long effort to bring BART trains into downtown San Jose. It also commits the VTA to its contentious single-bore tunnel design that critics say is partly to blame for ballooning costs and missed deadlines on the long-delayed project.
With an outer diameter of 52 feet, the nearly five-mile-long VTA tunnel will be one of the world’s largest subway tunnels. It will trounce the recently opened 1.7-mile Central Subway in San Francisco in both height and length. VTA officials were inspired by similar massive subway tunnels pioneered in Barcelona, which were built deeper underground to minimize surface-level disruptions.
The machine, akin to a mechanical earthworm, will start eating through walls of dirt in about 14 months if the VTA’s plans go according to schedule. Workers will start by lowering the device into a chasm opened near the Mineta San Jose International Airport, and then the machine’s cutter head — a vast wheel with cheese grater-like teeth — will begin spinning through walls of muck.
Eventually, it will crawl southward and turn east, mining a tunnel underneath Santa Clara Street on a slow subterranean march of about 30 to 35 feet a day, according to a VTA estimate released this week.
It is still unclear who will build Santa Clara County’s vast tunnel-boring machine. Thursday’s $460 million allows the VTA to move forward with a confidential contract bidding process. The drill itself will account for about $145 million of that. The machines are typically imported from companies in a handful of tunneling powerhouse nations, including Japan, China and Germany.
How long could the digging take?
“It’ll be several years,” said Bernice Alaniz, a VTA spokesperson. “There are still lots of variables that are being evaluated.”
While the machine digs underneath your feet, one of the biggest impacts Santa Clara Country residents could see is the knock-on effect of finding a new home for all the excavated sludge. The machine will pull out about 2.9 million cubic yards of muck requiring nearly 150,000 freight truckloads to haul the ancient earth matter, according to environmental documents. The truckloads of dirt will result in about double the amount of waste and traffic the VTA estimated in its 2018 environmental report when it approved a smaller tunnel of 40 feet in diameter.
“The trucking of that material is quite significant,” said Palo Alto Mayor Pat Burt, a VTA board member who said he will be seeking more details on how the agency is mitigating the impact.
Alaniz said muck removal and traffic control are key parts of plans under development as Santa Clara County braces for the biggest infrastructure project in its history. The contractor is “still identifying where the muck will go,” said Alaniz. “We’re going to be working with the City of San Jose and working with the public.”
Ground zero for the mega-project will be Newhall Maintenance Facility, sandwiched between San Jose’s airport and Santa Clara University. Along with ordering the tunnel-boring machine, Thursday’s contract package authorizes hundreds of millions of dollars to build a cement facility at the site and establish a permanent high-voltage substation feeding 115 kilovolts of electricity to keep the mechanical worm whizzing.
VTA’s head contractor, Kiewit Shea Traylor, and a network of subcontractors will have their work cut out for them. Along with ensuring the proper mechanics of a machine, there are archeological finds, hidden utilities and other surprises that can mire a tunnel-boring machine into years of delays.
In Seattle, a slightly bigger tunnel machine dubbed Big Bertha using similar technology collided with a hidden steel pipe, sparking years of delays. Tunnel boring failures in Japan caused a series of massive sinkholes.