The glassy blue Millennium Tower on Mission Street cuts a sleek figure on San Francisco’s downtown skyline. But these days the 58-story building is noteworthy not so much for its $10 million penthouses but for something much less glamorous: its foundation.

The dramatic sinking and tilting of one of the most exclusive residential towers on the West Coast is turning San Francisco into a city of armchair foundation experts. City politicians and residents are delving into such esoteric topics as dewatering, differential settlement, slab-mat foundations, Colma sand, and the benefits of friction piles versus end bearing piles.

The Millennium Tower, which opened at 301 Mission St. in 2009, has sunk 16 inches and tilted 2 inches to the northwest. Engineers expected it to settle 4 to 6 inches over the life of the building, but a report prepared for its homeowners says it could eventually sink as much as 31 inches, worsening the tilt.

The Transbay Joint Powers Authority, which is building the $4.5 billion Transbay Transit Center next door to the Millennium, blames the building’s structural shortcomings on the developer not using what are called “end bearing piles,” which would have reached down 200 feet into the bedrock. Instead, it rests on more than 900 “friction piles” between 60 and 91 feet in length driven into the dense mud that lies beneath fill dumped into the bay more than a century ago.

Millennium Partners, the developer, says the foundation is fine. It argues that the transbay authority caused the damage, pinning the blame on its dewatering — a process that provides a dry construction environment — of the transit center site. Pumping out millions of gallons of groundwater has softened and compressed the soil into which Millennium’s piles were driven, Millennium Partners says.

At the heart of the fight is the question of the building’s foundation and whether it is adequate to support the tallest — and probably heaviest — reinforced concrete building on the West Coast.

In choosing to drive friction piles, rather than go to bedrock, Millennium Partners says it was simply following the city’s building code. For the past 50 years, developers of downtown San Francisco towers have picked between two options when it came to foundations: on-grade mat or friction piles.

“The state of the art of foundation design in San Francisco has not been to go to bedrock,” Millennium Partners founding partner Christopher Jeffries said.

Sites that were close to bedrock — 555 California St. or the Transamerica Pyramid at 600 Montgomery St., the city’s two tallest buildings, are examples — called for a mat foundation, a thick slab of concrete reinforced with steel. Those on bay fill, such as buildings on the southern edge of the Financial District or in Mission Bay, used friction piles made of concrete, timber or steel driven through layers of softer, newer sand into dense and stiff bay mud.

The transit center project is surrounded by buildings that use both types of construction. Just to the west of the transit center, the 27-story office building at 100 First St. used friction piles, as did 123 Mission St., a 29-story office tower just to the east. The new office building at 350 Mission, directly across the street from the Millennium Tower, is on a slab-mat foundation.

Before the Millennium project, no major building in downtown San Francisco had piles driven into bedrock. But since then, four projects under construction are being built with piles that reach bedrock — all are developments on Transbay Joint Powers Authority land: 181 Fremont, Park Tower at 250 Howard St., the Transbay Transit Center itself and the Salesforce Tower, soon to be the city’s tallest building. While transbay authority officials haven’t said the Millennium’s problems are the reason these buildings go to bedrock, they knew about the excess settling at the Millennium as early as 2009.

“The only buildings built to bedrock are these four,” Jeffries said. “This represents a dramatic change in how buildings are designed in San Francisco, and it’s a standard that has all of a sudden been imposed in the press as what everyone should be doing in San Francisco.”

The soils and rocks that lie beneath downtown San Francisco’s streets change block by block, but generally things get denser the farther down you go. At the eastern end of Mission Street, which was once part of the bay, there is fill over softer young bay mud. Beneath that is Colma sand or marine sand, which starts out dense and get denser as it deepens. Beneath that is the old bay clay, which is even stiffer and less compressible. Finally, at the bottom, there is bedrock, called the Franciscan Assemblage — the reddish-green, twisted-looking rock seen throughout coastal California.

John Egan, a partner at Sage Engineers who has been hired by Millennium Partners to investigate the building’s issues, said most of the new construction on nearby Rincon Hill, like the Lumina and Jasper towers, is built on slab foundations because the bedrock is so close to the surface.

“We have a long history of foundation practice in San Francisco that tells us that buildings on a mat or piles pounded in the dense to very dense sand perform quite well,” said Egan, who has been the geotechnical consultant on dozens of city buildings. “We have a lot of historical evidence to suggest that buildings founded like that do just fine over many decades.”

But San Francisco’s other tall buildings that use friction piles have steel frames, rather than concrete, and therefore are far lighter than the Millennium Tower, according to Transbay Joint Powers Authority.

On Friday the transbay authority released a report by the engineering firm ARUP, showing that the Millennium Tower is four to five times heavier than any other building in the area with a similar foundation, including 100 First St., 199 Fremont St. and 555 Mission St.

Building weights are measured in kips — 1,000 pounds — per square foot of pressure on the soil below. The tower at 555 Mission, for example, is 487 feet tall compared with 645 for the Millennium, but it exerts 2.4 kips per square foot while the Millennium exerts 11.4 kips.

“While a foundation with shallow piles may have been appropriate for other, much lighter high-rises, it was clearly not appropriate for the Millennium Tower,” the TJPA report stated.

The issue of whether the Transbay district’s squishy fill could support a heavy residential building came up in 2004 when the city fought developer Jack Myers’ plan to build a 53-story tower at 80 Natoma, which also abuts the Transbay Transit Center.

Myers wanted to use a combination of mat slab and friction piles, which he said was more than adequate to support the building. But Maria Ayerdi-Kaplan, who was then the director of the TJPA, objected to the project because, she said, the foundation was inadequate and would have interfered with the underground train route coming into the transit center. At the time, Ayerdi-Kaplan said she pushed the developer to go to bedrock, but he refused.

“You have a 50-story building built on short piles,” she said at the time. “There’s no way we could come in and excavate under a building that’s built on short piles.”

Myers refused to go down 200 feet to bedrock, she said, because of “his financial documents that precluded any kind of delays.”

Eventually the transbay authority paid Myers $80 million to acquire the site by eminent domain.

Myers says that the foundation would have been fine. He said the choice wasn’t based on cost. “Both systems were expensive, but this took less time.

“It was state of the art, foolproof and over-engineered for the site,” Myers said. “Friction piles and mat foundations are simply time-tested and they simply don’t independently sink into the ground. They just don’t do that.”

Gerald Cauthen, an engineer who worked at the TJPA in 2003 and 2004, said the questions about 80 Natoma were good ones, and that the city would have done well to make similar inquiries about the Millennium Tower, which was peer reviewed by two engineers for its seismic safety but not its foundation.

He said the San Francisco Department of Building Inspection has long lacked the geotechnical expertise to evaluate the taller skyscrapers being constructed South of Market, especially the part of the building that sits underground.

“If you are not looking carefully at a building’s proposed foundation you are overlooking the very biggest risk,” Cauthen said.

“We didn’t have anything in place from a regulatory perspective on how to deal with buildings of this nature,” DBI Deputy Director Ron Tom said at a hearing Thursday called by Supervisor Aaron Peskin to investigate the foundation problem. DBI relied on reports from the developer in its approval process.

Howard Johnson, a retired architect, said that the Millennium issue is similar to a project in the late 1960s at 140 New Montgomery St., the Art Deco Pacific Bell building that is now home to Yelp. Johnson had been hired to design an underground parking structure and 22-story tower to the rear of the building that would have been built on friction piles.

But the project was halted when a soils engineer concluded that the site would have required dewatering, which he said would have had a devastating impact on 140 New Montgomery.

“He said, ‘If you dewater the site without maintaining the water table equilibrium of the adjacent site, 140 New Montgomery would end up like the Leaning Tower of Pisa,’” Johnson recalled.

“Nobody thought about going down to bedrock during that period,” Johnson said. “I think it’s striking that with all these high-rises nobody has taken into account the effect of high-rises building next door to each other on mud. The DBI, they should have been aware of the problem of dewatering and the effect on adjacent sites and be very careful in allowing buildings to be built.”

Whatever is causing the Millennium Tower to sink — an inadequate foundation, its sheer weight, the dewatering at the transbay site or some combination of those factors — the answer to that question could determine who pays the hundreds of millions of dollars to solve the problem.

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