SF's sinking tower poses myriad challenges

SF's sinking tower poses myriad challenges

Not 16! Joe Montana is among condo owners.

by JOHN GREGERSON | September 30, 2016

Sixteen inches and counting... 

Soil testing began this week on the 58-story Millennium Tower, the height of luxury condominium living in San Francisco, but now more famous for sinking into its underlying footprint. Grabbing headlines this month for all the wrong reasons, the eight-year-old, 400-unit, $350-million high rise, the largest reinforced concrete structure on the West Coast, also has been found to be leaning two inches in the direction of a neighboring skyscraper.

Understandably, the building's high-profile owners, neighbors, designers, builders, and city officials are more than a little concerned.

Engineers reportedly had anticipated the tower would settle 4 to 6 inches over its lifecycle, though a recent report commissioned by condo owners indicates the structure could sink by as many as 31 inches before it stops, promoting additional tilt and potential failure of elevators and other internal systems. Of further concern is the very real likelihood that continued sinking will compromise sewage systems and other utilities that serve the building.

The building was designed by NYC-based Handel Architects LLP, and built by LA-based Webcor Builders. DeSimone Consulting Engineers was the structural engineer.

Interests ranging from former San Francisco mayor and current U.S. Sen. Dianne Feinstein (D-CA), building developer Millennium Partners and city politicians to top inspection officials and high-profile condo owners like retired NFL great Joe Montana, all want to know what went awry and why. Finding the answers to those questions may take awhile, as newly hired consulting engineers and geologists Cotton Shires Associates, Inc., has only just begun its investigative work.

Earlier this week, under the supervision of firm president and geotechnical engineer Patrick Shires, crews began to drill the first of three 6-in-wide holes at Millennium's entrance that will extend 260 ft below grade to bedrock for purposes of inserting an inclinometer, piezometer, and extensometer to investigate the building's tilt, groundwater and soil conditions, and the amount of soil that has settled.

“What [tests] will tell you is where the soil is compressing, where it is settling,” Shires told reporters.

Drilling has begun for soil testing, as explained by geologist-engineer Patrick Shires (below, left).  Photos by SFGATE. 

foundational Shifts, disputes

The predicament, unveiled just months ago, has unleashed a firestorm at a time when developers are pursuing new San Francisco high rises like gangbusters, raising questions about the ability of city authorities to keep up with proper reviews of building plans in the seismically challenged city.

“This is the first sentinel telling us maybe we should be a little more careful,” said earthquake specialist Nicholas Sitar, a civil engineering professor at University of California-Berkeley, speaking last week to the The New York Times. 

But the city's Transbay Joint Power Authority (TJPA) has placed blame squarely on the shoulders of Millennium Partners, citing failure to construct a structural system incorporating “end-bearing piles” that would have extended to bedrock. Instead, Millennium rises from a concrete slab supported by more than 900 “friction piles”, driven 80 feet into dense mud and landfill.

“The state of the art of foundation design in San Francisco has not been to go to bedrock”

— Christopher Jeffries, Millennium Partners

For its part, the owner alleges TJPA destabilized the foundation by dewatering the site for the agency's adjacent $4.5-billion Transbay City Center, softening and compressing soil surrounding the tower's support piles. It further contends that forgoing bedrock and employing friction piles was in accordance with city codes, which authorize both on-grade mats and friction piles.

“By the time TJPA started dewatering adjacent to Millennium Tower in May 2013, the tower [already] had settled... 12.1 inches and was tilting further to the North/Northwest,” TJPA countered in a statement. “The excessive settlement and tilt is due exclusively to the deficient foundation system of the tower.”

Despite TJPA's allegations, “The state of the art of foundation design in San Francisco has not been to go to bedrock,” Millennium Partners founding partner Christopher Jeffries told reporters.

In fact, examples of tall buildings resting on grade mats and friction piles abound in San Francisco, including the TransAmerica Pyramid. Only four projects under construction by TJPA will reach bedrock. “This represents a dramatic change in how buildings are designed in San Francisco and it's a standard that has all of sudden been imposed in the press as what everyone should be doing in San Francisco,” Jeffries added.

True enough. In a recent letter, Sen. Feinstein, the former mayor, wrote to current Mayor Edwin M. Lee expressing “alarm” that owners and developers aren't anchoring their buildings in bedrock. Referring to the ongoing mess at Millennium Tower, she noted, “I am left wondering if the city's building code played any role in allowing this to happen."

Lee said Millennium “was designed and constructed to the approved plans, building codes and standards of the time.” He added that he had authorized plans mandating that city officials perform evaluations of soil conditions and potential vulnerabilities to earthquakes during property sales.

Who knew what? When?

Following a September meeting among the city's Board of Supervisors, city officials indicated problems with Millennium are prompting them to reevaluate the manner in which buildings are assessed.

Meantime, the issue has opened a Pandora's Box that has cast unfavorable light on Millennium Partners and city building department officials, which city supervisor Aaron Peskin alleges were aware the building was sinking before it was occupied.

Earlier this month, Peskin said that Millennium building owners “had arguably been defrauded” and then, at a city hall conference, vowed to “get to the bottom of how this happened.” In hand was a 2009 document indicating that the city Dept. of Building Inspection was aware of the sinking problem in February of that year, two months before it had authorized buyers to begin occupancy.

But Stephen DeSimone, Millennium's structural engineer, told the San Francisco Chronicle that, at the city's behest, building plans underwent a pair of peer reviews before the condominium broke ground.

Whether problems with Millennium will prompt changes in local building codes, public policy and construction oversight remains to be seen. Also awaiting resolution is what measures to implement with the condominium itself. Experts say options will depend on the cause of the settlement, once it is determined, hopefully sooner than later.

One scenario could involve driving or sinking deep piles under the structure, with the building shored by hydraulic jacking, said Michael Lepech, an associate professor of civil and environmental engineering at Stanford University, speaking to reporters. Another potential approach calls for injecting the soil with stabilization materials or cement-based grout to bind soil beneath the tower, Lepech suggested.

• Two caveats: Both approaches are costly and not entirely predictable.

“These solutions will work, but it's not clear how challenging it would be,” he said. “Traditionally, (they) are not designed for a building of this size.” 

For now, the spectacle continues, as local news crews interview residents and gawkers gather for selfies. As interested as the local public may be, though, the engineering community across the U.S. and around the world is paying even more attention to the foundational lessons that may yet emerge.