After the successful installation of a 24-in.-dia permanent pilot pile at the troubled foundation upgrade of the settling Millennium Tower in San Francisco, the Dept. of Building Inspection (DBI) has given Shimmick Construction Co. permission to install a second pilot pile, beginning Dec. 1.
“Our position is based on the minimal overall settlement and tilt produced by the three 36-in. pilot casings and one 24-in. pilot pile installed…,” said Patrick O’Riordan, interim director of the San Francisco DBI, in a Nov. 30 letter to the Millennium Tower Association. He added that if the second pile installation is successful, DBI would "support installation of one additional 24-in. pilot pile along Fremont Street."
Shimmick completed the installation of a 24-in.-dia pile at the 645-ft-tall residential condominium using a revised method designed to stem accelerated settlement that had been caused by the perimeter pile upgrade. The operation, which followed the successful installation of three pilot pile casings, also using revised techniques, was completed Nov. 19.
The foundation team developed the new procedures after the remedial foundation work, intended to correct some 17 in. of tower settlement, had instead accelerated settling and tilting by 1 in.
No Further Measurable Settlement or Tilt
No further measurable settlement or tilt occurred during the installation of the pile and there has been no movement detected since, beyond the movement that was occurring naturally, says Ronald O. Hamburger, a senior principal with Simpson Gumpertz & Heger and the engineer-of-record for the $100-million fix, which has been likened to putting a bumper jack next to a flat tire.
Hamburger, who halted the pile work on Aug. 20 after reports of accelerated tower settlement, hopes to get a nod for the third pile “to confirm that the pile installation results are repeatable,” he says.
The combined results of the three pile installations and the three 36-in.-dia casings installed in October will enable the foundation team to predict building settlement and tilt through the completion of the project and will demonstrate that the foundation upgrade can be finished without endangering the tower's functionality or safety, says Hamburger.
For the first pilot pile work, which began on Nov. 15, drilling took a little over two days. Crews placed reinforcing on Nov. 18 and concrete on Nov. 19.
The remedial pilot program, begun in October, 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.
The pile upgrade calls for 52 concrete-filled steel-pipe piles within the steel casings, along two sides of the occupied building. Currently, Shimmick has installed thirty-six 36-in.-dia casings and seven 24-in.-dia piles.