Foundation Repairs
Fix for Settling Millennium Tower's Troubled Pile Upgrade Inches Ahead
Pilot pile tests new installation methods after repair itself caused accelerated settlement
Outer casing installation (above) has stopped during the pilot pile installation.
Photo by Ronald O. Hamburger
After three successful pilot casing installations using revised procedures intended to fix the ailing perimeter foundation upgrade at the troubled 645-ft-tall Millennium Tower in San Francisco, Shimmick Construction Co. began installing a pilot pile inside the first casing—also using modified methods. The foundation team developed the new procedures after the remedial work, intended to correct some 17 in. of tower settlement, had instead accelerated settling and tilting by 1 in.
Casing work stopped during the work on the pile, which required many moving parts “to come together,” including repositioning the equipment on site and installing more vibration monitors, says Ronald O. Hamburger, a senior principal with Simpson Gumpertz & Heger and the engineer-of-record for the $100-million fix, likened to putting a bumper jack next to a flat tire.
After reports of accelerated settlement, Hamburger halted the upgrade work on Aug. 20. The remedial pilot program, begun last month, involves testing two construction activities: 1) installation of a 36-in.-dia steel outer casing to a depth of about 106 ft below grade; and 2) installation of a 24-in.-dia steel-encased, cast-in-place concrete pile down about 270 ft.
Estimates of Probable Future Settlement
“The purpose of this program is to confirm the effectiveness of modified installation procedures and to permit us to develop reliable estimates of probable future settlement during the remaining construction effort,” said Hamburger, in a Nov. 12 letter to Patrick O’Riordan, the interim director of the San Francisco Dept. of Building Inspection (DBI).
DBI gave its nod to the pilot pile based on the success of the pilot casings, with similar stipulations about monitoring and inspection. The pilot work showed that each 36-in.-dia casing produces about 1/50 in. of settlement at the center of the tower’s mat and 1/16 in. at the northwest corner. This resulted in about 0.1 in. of additional roof tilt toward the north, and about 3/16 in. of tilt to the west, according to Hamburger.
“This is within the range we anticipated,” he wrote in his letter.
Millennium Tower Remains Occupied
The residential condominium, which remains occupied, has sunk a total of about 18 in. since its completion in 2008. The cause is consolidation of Old Bay Clay, according to Hamburger.
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The pile upgrade calls for 52 concrete-filled steel-pipe piles within the steel casings, along two sides of the building. Currently, Shimmick has installed thirty-six 36-in.-dia casings and six 24-in.-dia piles.
DBI has charged Hamburger with projecting the likely settlement through construction completion and to assess potential impacts on building safety and functionality, based on the pilot program’s data.
After that, DBI, the engineering design review team and geotechnical consultant Dan Brown Associates “will work with Mr. Hamburger and the Millennium Tower Association’s project team to understand the potential impact of additional pile and casing installation and potentially review his proposal for installing additional 24-in. piles,” said O’Riordan, in a Nov. 12 letter to the Millennium Tower Association.
Hamburger says the pilot pile work, which began Nov. 15, will take most of this week.
