Engineers are proposing to drill piles down to bedrock to stabilize one side of a 58-story high rise condo in San Francisco, whose unexpected tilting has led to concern over its stability. The approach would allow the other side to continue to sink until the building straightens itself.
The San Francisco Chronicle newspaper says the job could cost from $200 million to $500 million, roughly as much as the $350 million it cost to build the tower in the first place.
Soil tests for the proposed retrofit already are under way, the newspaper says.
The Millennium Tower, which opened in 2009, has sunk about 17 inches and tilted 14 inches to the west and 6 inches to the north at the roof line.
The building sits on a 10-feet-thick mat foundation, held in place by 950 reinforced concrete piles sunk 60 to 90 feet deep into clay and mud. They do not, however, reach bedrock.
To retrofit the tower, engineers are proposing boring 275 to 300 steel and concrete micropiles — each 13.625 inches in diameter — down to bedrock. But only on one side of the building at a time. The first step would be for crews working inside the Millennium basement to drill about half the new piles on the building’s west side.
Once it’s stabilized, workers would allow the east side to sink long enough for the building to straighten itself. The rest of the piles would be drilled to bedrock on the east side to keep the structure from sinking even farther. The entire process could take two to five years, the paper says.
According to a report released in 2017 by Gregory Deierlein, Ph.D., Marko Schotanus, SE, Ph.D., and Craig Shields, PE, GE, the building remains safe even with the tilting.
The Structural Safety Review of the Millennium Tower at 301 Mission Street was ordered by the City’s 301 Mission Street Seismic Safety Study Committee to determine the building’s ability to meet structural and seismic safety standards of the San Francisco Building Code. The sinking has not compromised the building’s ability to resist strong earthquakes and has not had a "significant effect on the building’s safety," the report said.
The Millennium Tower complex consists of two structures, identified as the Tower and the Midrise on structural drawings. The drawings describe the Tower as a 58-story, 605-feet tall structure over a one story basement. The Midrise as a 12-story, 128-feet tall structure over five below-grade levels.
The Midrise structure includes a three-story-tall portion between the 12- and 58-story tall towers. The Tower and Midrise are structurally separated by a seismic joint.
Both structures are of cast-in-place concrete construction, using post-tensioned slabs for the floors above ground level.
The seismic force-resisting system of the tower consists of a “dual system,” which is comprised of a 36-inches-thick special reinforced concrete shear wall core with outriggers and concrete special moment-resisting frames. The Midrise relies on a special reinforced concrete shear wall system that includes the perimeter basement walls.
The two structures use different foundation systems. The Tower foundation consists of a 10-feet-thick pile cap supported by about 950 precast concrete piles, measuring approximately 80 feet in length. The Midrise structure rests on a mat foundation that varies between 6 and 8 feet in thickness.
Tie-downs resist hydrostatic uplift pressures under the three story portion of the Midrise building. The original design anticipated one inch of settlement under the Tower by the time of construction completion, and additional long-term settlement due to compression of the underlying clay layers of 5 inches. Settlements were expected to occur uniformly under the Tower foundation area.
The San Francisco Chronicle reports that drilling equipment is expected to be moved into place outside the building to bore a pair of holes deep to test the conditions of the soil and bedrock below.
Before the full repair job gets going, lawyers and insurance representatives for all the feuding parties — developer Millennium Partners, the Transbay Joint Powers Authority, the Salesforce Tower’s owners, the city, plus all their engineering and design consultants — will try to reach an agreement to pay for anchoring the building and settling multiple owner claims.