01 July, 2023 By Thames Menteth
The latest phase of work to stabilise San Francisco’s troubled Millennium Tower has reversed some of the tower’s tilt, according to monitoring data – but an expert has said it is “too early” to tell if the fix is working.
The Millennium Tower in the city centre appears to have recovered around 19mm of tilt to the west after the final stages of an engineering upgrade to stabilise it were applied to the building in June.
The 58 storey luxury tower was tilting 736mm to the north west as measured from its roof in early June, since it was constructed in 2009. It is now tilting around 717mm.
The US$100M (£79M) engineering “fix” devised to strengthen the building’s foundations along its north and west sides is a pile perimeter upgrade (PPU). This has involved transferring a portion of the building’s weight from its existing foundation system onto 18 new perimeter piles socketed into bedrock.
The fix also involves extending the existing mat foundation to encompass the new piles. The original foundation is made up of a 3m thick reinforced concrete slab supported by 942 precast prestressed concrete piles extending approximately 24.3m below grade in the Colma Formation of dense clayey sand.
The upgrade aims to relieve stress on soils that have compressed beneath the building, which have caused its unanticipated movement.
The engineering upgrade was designed by Simpson Gumpertz and Heger consulting principal engineer Ronald Hamburger, who is the engineer of record for the fix, working on behalf of residential condominium association Millennium Tower. The main contractor for the pile driving work is Legacy Foundations, a division of Shimmick Construction.
But the PPU solution to stabilise the building has been hit by problems over the past two years.
A first attempt to stabilise the building in May 2021, which involved installing 52 piles spaced 1.8m apart to relieve a portion of the stress on the soils, caused the building to sink and tilt further. Work on the building was halted in August 2021 and the pile design was altered.
“We did not anticipate the pile installation would cause additional movement, but we were proactive about it,” said Hamburger.
Even after the pile driving method was revamped, the building continued to move and has tilted a total of 177mm north west during the pile upgrade work.
The latest data shows that the PPU could finally be working, the engineers involved in the fix have claimed.
It comes after the second stage of the load transfer was initiated on 10 June, which consisted of bringing the 12 piles along Fremont Street up to 500kips (2,224kN) each. On 15 June, Shimmick began the final stage of load transfer, which consisted of raising all 18 piles to 1,000kips (4,448kN) each.
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”, said Hamburger.
The Millennium Tower PPU design team expects the recovery to continue in the coming months and years.
However, geotechnical and earthquake consulting engineer Robert Pyke has warned that it is “too early to tell” whether the latest fix to San Francisco’s Millennium Tower has stopped its tilt.
He also raised questions about the integrity of the building’s new mat extension and what might happen in the event of an earthquake.
Analysing the latest monitoring report, Pyke identified that survey points on the mat extension have increased, which indicates that the mat extension and the edge of the mat have curled up relative to the rest of the building.
Explaining what this could mean, he said: “It is not so much differential settlement of the mat extensions, but differential uplift at present. That may decrease over time and the amount that has been observed so far is not a significant problem, but it does raise questions about the integrity of the mat and what might happen in an earthquake.
“The design includes ‘fuses’ to limit the loads applied to the mat extensions under rocking loads in an earthquake, but who knows what would actually happen.”
Another expert GE spoke to also pointed out that survey techniques are “notoriously prone to error and data should be treated with caution until a few rounds of survey have been carried out”.
The PPU programme is expected to be completed by late August or early September this year.
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