SF mayor gets peek at new Warriors arena as it emerges at...

The Chase Center, the Golden State Warriors future home in San Francisco’s Mission Bay, was 25 percent complete as of Tuesday.

As a construction milestone, 25 percent isn’t something that merits popping the Champagne, and it won’t stop traffic on Terry Francois Boulevard.

But for San Francisco Mayor Ed Lee, who counts the Warriors coming move across the bay from Oakland among his biggest achievements, it was as good an opportunity as any to take a look at the arena’s progress.

The mayor joined Golden State Warriors President Rick Welts on a tour of the job site, where after seven months of underground work — drilling, digging, pile-driving — the $1 billion Chase Center is starting to poke its head above the dirt.

“Wonderful. Inspiring,” Lee said. “They are further along then I thought they’d be. Getting stuff out of the ground takes such a long time in the city. Private developers have answers we ain’t got in the public sector.”

Lee learned about the six-month excavation job, which included the removal of 300,000 cubic yards of dirt, creating a hole that goes down 25 feet. The foundation is supported by more than 1,300 piles, which because of unusual soil conditions includes some piles that go to bedrock and some to mud.

He toured the visitors center next door, where the Warriors have created a sales center for prospective season-ticket holders and corporate sponsors. He watched a video touting the Chase Center’s future role as a home not just for Stephen Curry and Draymond Green, but as an entertainment destination capable of hosting stars like Bruce Springsteen and Rihanna.

In a neighborhood full of tower cranes, the Chase arena project has the distinction of having more cranes than any other project: five of them. And there are about 450 workers on site daily.

Welts said the project is at the point where even frequent visitors are met with startling change.

“I was here yesterday, and all those cement trucks weren’t pouring concrete in that corner,” said Welts, pointing to the northeast corner of the property where four cement trucks were lined up. “At this stage, it’s daily change. A year from now, the arena will be fully formed, with a roof on top. It will be watertight, and we’ll be working on the inside of the arena. And 10 months later, we’ll be open with a whole series of events leading up to the Warriors 2020 season.”

The 10,000 tons of steel that will provide the arena’s structural bones is currently being shipped from Salt Lake City to Sacramento. It will start arriving at Mission Bay by truck in late November. That’s when the casual observer will start to see dramatic change, said Jim McLamb, project director for Mortenson/Clark, the joint venture building the arena.

“Then the whole landscape will be different. You will see this place explode,” he said. “And that is when everyone will say, ‘Wow, you’re really building something.’”

For a mayor who’s dealing with a steady stream of complaints about everything from homeless camps to tenant evictions to traffic, a trip to the future Warriors arena can’t be bad for morale.

“You don’t get someone to spend $1 billion of their own money if they don’t have confidence in the city,” Lee said of the Warriors, who are paying for their own building.

In addition to the 18,000-seat arena and event space, the project will include 580,000 square feet of office and lab space, 100,000 square feet of retail space, and a 35,000-square-foot public plaza.

Twenty-nine retail spaces, mostly spilling out onto a public plaza, will be food and beverage heavy. Uber has committed to taking the project’s office space, and the Warriors have said that retailers will include Bay Area locals like Bakesale Betty’s, Tacolicious, Sam’s Chowder House, and Nate’s BBQ.

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com Twitter: @sfjkdineen