The vast space directly underneath Fourth Street is called the "launch box." That's where two tunnel boring machines will take aim at a wall and excavate their way 1.5 miles north to North Beach, re-surfacing where the Pagoda Palace now stands on Powell Street.
The machines have cutter heads more than 20 feet in diameter and trailing gear 300-feet long. They're arriving disassembled from China and will be dropped piece by piece through a hole into the launch box, then put back together. That assembly will take four to six weeks.
"They're called an 'earth pressure balance machine' and they have a rotating cutter head for soft ground, and we're anticipating drilling into soft ground," said Program Director John Funghi.
A teeny-tiny road header is preparing the wall face for the arrival of the boring machines. They'll push downward working between 60 and 90 feet below ground. "We're going to be below a lot of the old infrastructure and things that have been built up in the city over the years," resident engineer Sarah Wilson said. "We're going to be below that. There's been a lot of planning that's gone into this project."
In fact, there's been 20 years of planning. The Central Subway is a continuation of the T-Third Rail Line, diving underground on Fourth below Highway 80, with one above-ground station there and three underground at Moscone Center, Union square, and Chinatown.
Construction will be going on for another six years. The SFMTA says the underground work should not be heard or felt by anyone above-ground. "I think anytime you're working on a major construction project in an urban area, you have to keep in mind that you're working with your neighbors. So, that's always a challenge every day," Wilson said.
The first tunnel boring machine was sitting at the Port of San Francisco Wednesday, slated to be trucked over to the construction site overnight then lowered down into a 40x40 foot hole piece by piece into the launch box where it will be reassembled. The second boring machine is set to arrive in June and it will begin boring in August.