The pace was 100 kips per day for five days, says Hamburger. On Saturday, June 10, Shimmick initiated the second stage of the load transfer, which consisted of bringing the 12 piles along Fremont up to 500 kips each. The purpose was to stabilize the building while crews excavated along Fremont to cast the mat extension.Ĭrews cast the two Fremont extension mats, one at the north end of the tower and the other at the south, on Saturday, June 3. In January, after casting the collar along Mission, Shimmick’s Legacy began the phased load transfer operation by synchronously jacking the six piles along Mission to 500 kips each. The piles, six along Mission on the north side of the tower and 12 along Fremont on the west side, support a new mat extension, known as a collar, that ties into the original mat. The fix, likened to putting a bumper jack next to a flat tire, relied on drilling and jacking the 18 perimeter piles-socketed more than 30 ft into rock that starts 220 ft below grade-under the sidewalks along Mission and Fremont streets. of Building Inspection issued a revised building permit for the 18-pile scheme. Last August, after more than six months of scrutiny, the San Francisco Dept. The engineer studied as few as six and as many as 52 piles and settled on 18, which accomplished the project objectives and allowed work to be completed within the available time slot, Hamburger explains. “The homeowners asked us to scale down the upgrade” to get back on track, he adds. The pile production problems, which were solved, nevertheless “burned a significant amount of schedule and money,” says Hamburger. “We did not anticipate the pile installation would cause additional movement, but we were proactive about it,” says Hamburger, who halted the repairs, voluntarily, in August 2021, so the team could develop a revamped pile-driving method that would not accelerate settlement. occurred during the pile upgrade work, says Hamburger. of tilt to the north and west, as measured at the roof horizontally, 7 in. The fix settlement and the overall settlement, which began during construction in 2008, is attributed to the densification of the Marine/Colma sands below the tower, says Hamburger. The redesign was a consequence of additional tower settlement triggered by the initial pile-driving work, executed by Legacy Foundations, a division of Shimmick Construction Co. There were 52 new piles in the original repair scheme.

The revised upgrade involved transferring a portion of the building weight, in stages-via 18 new perimeter piles socketed into bedrock-from the existing foundation system-a central-core reinforced concrete mat bearing on piles that do not go to bedrock.

“We expect to see the effect of the stressing on the settlement and tilt” over time, he adds. “Now that the foundation is in place and the piles are stressed, I think we’ll find the basic design of the retrofit was well-conceived,” says Deierlein, who was on site June 19 for the final phase of pile jacking to transfer loads to the new piles. of Building Inspection's engineering design review team (EDRT). The city’s independent team of engineers overseeing the fix since the beginning of design, concurs with Hamburger, says Gregory Deierlein, the chair of the Dept. of tilt to the northwest that had occurred over 15 years, adds Hamburger, who has been working on the voluntary foundation upgrade, intended to arrest significant future settlement, since 2014. Hamburger, the engineer-of-record for the fix and a consulting principal with Simpson Gumpertz & Heger (SGH), working for Millennium Tower, a residential condominium association.įurther, the building has begun slowly to recover the 30 in. As of June 19, “survey data reveals that settlement has indeed been arrested, as was the primary project objective, and in fact, the building has risen slightly out of the ground,” says Ronald O.

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