The 58-story Millennium Tower cost an estimated $350 million to build in 2008, but the cost to fix the sinking and tilting skyscraper may climb even higher, reaching even to $500 million.
San Francisco Chronicle’s Mattier and Ross—who first broke the story of the Millennium Tower’s meandering foundations in 2016—now report that a potential engineering fix for the building involves securing only half of the structure to bedrock with hundreds of long “micropiles” in the foundation and then “letting the other side continue to sink until the building straightens itself.”
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After the structure is level again, further piles will attach the entire building base to bedrock, and all will be righted in the world. The process could take as long as five years—and come with a hefty price tag.
Pennsylvania-based construction company Nicholson explains that micropiles are:
High-performance, high-capacity drilled deep foundation elements typically between 5–12 inches in diameter that can extend to depths of 200 feet and achieve working loads of over 200 tons. Micropiles are comprised of high-strength steel casing, rebar and grout.
[…] Micropiles are generally used when there are difficult ground conditions, such as natural or man-made obstructions, sensitive ground with adjacent structures, limited access/low headroom and/or karstic geology. They are commonly used to replace deteriorating foundation systems […] or reinforcement including embankment, slope and landslide stabilization.
Mattier and Ross go on to report that some of the building’s well-heeled inhabitants aren’t fond of this fix; there are other competing proposals for how to get the tower and its ritzy business back on an even keel.
A similar 2017 proposal called for up to 100 larger additional piles—at least ten inches in diameter—to secure the building, estimated at the time to run up to roughly $150 million.
Other proposals have included reinforcing the slippery soil beneath the building with concrete, constructing a larger building next door and connecting the two so that the counterweight will eventually even everything out, or doing nothing at all while concentrating on the legal fight.