The iconic 58-storey residential building known as Millennium Tower at the city of San Francisco continues to sink despite a 100 million dollars fixing program has been implemented to fix it.
The luxury tower, considered as the higher residential building at San Francisco, has a history of excessive settlement nearly from 2009, when its construction had finished. In 2014 it had sunk around 46cm and was leaning 36cm to the West, while the expected settlements for the whole design life of the tower was around 10 to 15cm.
The tower, is sitting over a rafted pile foundation, consisted of a 3m thick concrete mat with 942 prestressed concrete piles. The piles extend around 25m below the ground, into Colma Geological Formation, a dense clayey and silty sand. Beneath the Colma Formation, there are approximately 50m of marine and alluvial deposits, the upper layers of which are a stiff clay material know locally as Old Bay Clay.
In May 2020, the $100m upgrade plan started with the construction of a perimeter pile upgrade (PPU) of 52 concrete piles with 1.8m spacing, to anchor the building to a stable bedrock layer 75 meters below the ground. Also, an extension of the existing mat foundation was decided, to encompass the new piles together with jacking of 800kips of load from the existing building onto each of the new piles.
The design goal was to relieve a portion of the stress on the Old Bay Clay soils along the building’s north and west sides, to stop further settlement, and to recover a portion of the building’s tilt.
Shortly after the works begun, the tilting and the sinking of the tower accelerating.
In May 2021 crews began to dig down to install 91cm diameter pile casings along Fremont Street, and by June 2021 the building had tilted approximately 6.3cm more to the west.
In early July the contractor started to install 91cm diameter pile casings along Mission Street, which resulted in a small increase in the rate of tilting to the north.
According to Ron Hamburger, senior principal director and engineer of record for the foundation retrofit, “the tower’s settlement is primarily due to the compression of the Old Bay Clay soils under the influence of the building’s weight, and prolonged dewatering of soils to accommodate adjacent construction projects”.
Meanwhile, the tilting is a result of the “non-uniform stratigraphy and strength of soils across the site and the influence of adjacent construction”, he added in the statement to the San Francisco Department of Building Inspection
From a technical standpoint, the opinions of different specialists do not seem to converge.
Ron Hamburger said that the building is continuing to settle “at the rate of about 0.5in (1.27cm) per year and to tilt at 3in (7.62cm) per year. It is doing this whether we are conducting work at the site or not”.
He also claimed that the building “can tilt more without becoming a safety issue or a serviceability issue” while geotechnical and earthquake engineering consultant Robert Pyke believes that even a moderate earthquake can be a serious threat for the safety of the building.
According to Pyke, “the PPU will not solve the settlement problem and that the south and east sides of the building will continue to sink as a result of secondary consolidation/creep bearing capacity failure for the foreseeable future.”
In a presentation to the San Francisco Board of Supervisors in January this year, Pyke said he believes that the “optimal course of action” is to “reduce the number of piles that are connected to the tower to zero”.
In mid-July, the contractor started to install 61cm diameter piles along Fremont Street, which again resulted in an increase in the rate of tilt rate to the west.
After three more 61cm diameter piles were installed in August, all piling work was halted.
Hamburger determined that the increased building settlement during pile installation was caused by over excavation of soil and vibration of granular material.
The engineer and contractor then developed a revised design comprising just 18 piles as opposed to the original 52 pile design and improved techniques for the over excavation and vibrations.
After successfully completing a pilot pile installation without additional settlement, crews installed 18 piles in mid-March this year. This stopped the settlement at the northwest corner, according to NBC Bay Area.
But the local news station reports that monitoring data shows the sinking resumed when crews started digging along the northwest edge of the foundation at Mission and Fremont Street to make room for the foundation extension.
The latest data means that the tower has now tilted around 29.2cm northwest since July 2020, before work on the "fix" started
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