Let the digging begin.

Construction on the biggest public works project ever in Silicon Valley is about to take off. Groundbreaking for the BART to San Jose extension within Santa Clara County is Thursday, rain or shine, and work that began in Fremont in 2009 is steaming ahead.

Combined, the two segments will cost $3.2 billion and run nearly 16 miles from Fremont to East San Jose. Key streets carrying thousands of drivers a day — Mission Boulevard, Warren Avenue, Kato Road, Dixon Landing Road, Sierra Road — will be torn up and replaced to run over or under BART. Three train stations will be built.

Construction should end by 2015, with the full extension opening a year later. Until then, hundreds of workers in orange hard hats, cranes looming several stories in the sky and a multiplicity of lane-closed signs will be an everyday part of the landscape.

“To do all this means a lot of dirt being moved out,” said Carolyn Gonot, Valley Transportation Authority’s chief BART Silicon Valley program officer. “That is what the public will see. Trucks with dirt.”

A lot of trucks. More than 33,300 truckloads will be needed to move 850,000 cubic yards of dirt. And 21,570 tons of steel will be put down, along with 200,000 cubic yards of concrete and 100 miles of wire for power and communications equipment — all just within Santa Clara County.

Crews are now nearing the end of work on the 50-foot-deep subway in Fremont’s Central Park and at Lake Elizabeth. Building demolition also is under way near Warm Springs Court to make way for BART tracks.

Later this month, Kato Road will be closed for nine months so that it can be sunk 16 feet into the ground. This summer, buildings will be torn down at Montague Expressway and Berryessa Road, where stations will be built. Around Labor Day, work to run Mission Boulevard and Warren Avenue under the future BART alignment should be well under way.

Gone will be those nasty, street-level crossings that Union Pacific freight trains once used. While most work will be along that old rail corridor and out of sight, there will be headaches.

“This is very complicated work, like a mini-subway,” said Jim Pierson, the Fremont public works director, referring to upcoming work at Kato and Warren. “The difficulty with both of these crossings is there is no room for detour roads.

“Each of them will have lane reductions and each road will be completely closed for approximately nine months,” he said. “Naturally, closure of these major roads will impact a lot of drivers.”

The Milpitas station at Montague Expressway and Capitol Avenue will be among the toughest challenges because of all the utilities that need to be moved and installed, including a major PG&E gas pipeline that will be moved at a cost of $25 million. A Chevron pipeline from Richmond needs to be rerouted, and so does Penitencia Creek.

BART will run mostly at street level, but will go underground north of Montague. A station there will serve as the transfer point for riders who want to take light rail to the many high-tech companies along Tasman Drive.

North of Berryessa Road, BART will rise 35 feet above ground level and remain elevated past Mabury Road, before the line ends near Highway 101.

Someday, BART boosters hope, the line will tunnel under downtown San Jose, ending at the Caltrain depot in Santa Clara. But that final extension would cost $4 billion, and the VTA is $2 billion short. No timetable has been set for that work.

Utility work will extend onto nearby streets and traffic patterns will change, especially in South Fremont.

“Mission Boulevard backs up already,” said the VTA’s Gonot. “But it will have even more of a constricting feel because we will be out there trying to build bridges and other activities in the middle of Mission.

“We’re not taking away any lanes, but it’s not going to get any better until we get done.”

That’s three-plus years. Until then, watch out for the trucks, give the hard hats a break and hold onto that steering wheel tight.

Contact Gary Richards at 408-920-5335.