With the toss of some dirt from gold shovels, a project decades in the making was officially underway.
"Today is really an historic day, we've been working on this project for many, many, many years," Caltrans Director Randell Iwasaki said.
The new fourth bore of the tunnel on Highway 24 will go in to the north of the existing three bores. It will be two lanes and 3,300 feet long.
"Four years, four bores and the inevitable slowdown on 24 will be eliminated," Rep. John Garamendi, D-Walnut Creek, said.
Two bores of the current Caldcott Tunnel first opened to traffic in 1937. In 1964, a third bore was completed. Today, on average 160,000 cars use the tunnel each day. The middle bore is switched from east to westbound traffic, depending on the greatest need.
"It's going to make travel reliable; people will know going out what the conditions at the tunnel will be," Orinda City Council member Amy Rein Worth said.
The fourth bore will be constructed using the "New Austrian" tunneling method, which involves a series of sequential excavations made by a piece of equipment called a roadheader.
"We'll excavate two meters or so, a little less, then we'll put temporary support in, then we'll continue on like that," Caltrans engineer Peter Strykers said.
The total cost of the project is $420 million. Of that, nearly half came from the Amerian Recovery and Reinvestment Act. The nearly $200 million of Recovery Act money for the fourth bore of the Caldecott Tunnel is the largest amount granted so far for a single project in the United States.
"The very first thing the Obama administration did was the stimulus and here we're seeing 5,000 jobs as a direct result," Garamendi said.
Caltrans hopes to have the fourth bore open and running by mid-2014.