What Really Happened at the Millennium Tower?

Summary

The Millennium Tower in San Francisco, the city's tallest residential building, began sinking and tilting soon after construction. Despite using deep foundation piles, the soft mud beneath caused excessive settlement, leading to the building sinking 16 inches by 2016. The media frenzy and lawsuits followed, with residents and organizations seeking compensation. Engineers devised an innovative retrofit plan involving hydraulic jacks and perimeter piles to slow the sinking and stabilize the tower. However, drilling issues caused further complications, highlighting the challenges of predicting soil behavior and the impact of adjacent construction in dense urban areas.

Takeaways

• 😀 The Millennium Tower in San Francisco is the tallest residential building in the city, with 58 stories and 419 luxury condominium units.
• 😀 Despite the tower's high-tech foundation design, engineers noticed it was sinking and tilting even before construction was finished.
• 😀 The foundation includes 950 concrete friction piles driven 80 feet into the ground, allowing the building to rest on stronger soils rather than weak, surface-level layers.
• 😀 The main issue arose from the underlying Old Bay Clay, a layer of Ice Age mud that was not anticipated to cause as much settlement as it did.
• 😀 Settlement happens when soil compresses under increased load, a process accelerated by dewatering and nearby construction projects.
• 😀 By 2016, the Millennium Tower had sunk 16 inches, three times more than expected, and began tilting, drawing significant media attention.
• 😀 The building's tilt, while considered safe by engineering experts, caused a drastic drop in condominium values and led to lawsuits from various parties.
• 😀 In 2017, the city commissioned a panel to assess the safety of the tower, which confirmed its earthquake safety but warned of further movement.
• 😀 After lengthy litigation, a settlement was reached, with compensation for condo owners and a plan to fix the building's foundation.
• 😀 The retrofit involves installing 52 piles around the building's perimeter, with hydraulic jacks lifting the foundation to relieve stress on the clay layer.
• 😀 Despite challenges during the retrofit process, including accelerated settlement during drilling, engineers continue to work on solutions to ensure the tower is properly supported.
• 😀 The Millennium Tower serves as a case study in geotechnical engineering, highlighting the challenges of predicting soil behavior and the complex relationship between engineering, media, and public perception.

Q & A

What is the Millennium Tower, and why is it significant?

The Millennium Tower is the tallest residential building in San Francisco, with 58 stories and 419 luxury condominium units. It is significant because of its foundation issues, as the building began sinking and tilting shortly after construction, which became a major engineering problem and legal issue.

What caused the Millennium Tower to sink and tilt?

The building's foundation was designed to bear an immense load, but the ground beneath it, including a weak layer of mud and rubble, could not handle the pressure. The soil consolidation process, where water is squeezed out of the soil under pressure, accelerated due to the construction methods, including dewatering and nearby construction projects like the Transbay Transit Center.

How did engineers predict soil behavior before the construction of the Millennium Tower?

Geotechnical engineers predicted that the Millennium Tower would settle by 4 to 6 inches over its lifetime. They made these predictions based on the properties of the soil, including the Old Bay Clay layer, which was expected to compress under the building's weight over time.

What is soil consolidation, and how does it contribute to settlement?

Soil consolidation is a process in which soil compresses over time when subjected to increased load, causing water within the soil to be squeezed out. This leads to the particles within the soil compacting together, which results in gradual settlement of the structure above.

How did the construction of the Millennium Tower's basement contribute to the building's sinking?

During the construction of the basement, the contractor used vertical wells to pump water out of the subsurface to keep the site dry. This dewatering process caused the settlement of the foundation to accelerate, as it disturbed the soil beneath the building.

What was the extent of the settlement in the Millennium Tower by 2016?

By 2016, the building had sunk more than 16 inches (41 centimeters), which was three times more than initially anticipated for its entire lifetime. This settlement was uneven, causing the building to tilt to the northwest.

How did the media and public respond to the Millennium Tower's issues?

The media sensationalized the situation, dubbing it the 'Leaning Tower of San Francisco.' This led to a significant drop in condominium prices, as potential buyers were hesitant to invest in a building that appeared unstable. Public scrutiny led to lawsuits and investigations into the cause of the building's issues.

What was the conclusion of the safety assessment commissioned by the city in 2017?

The panel of experts concluded that the Millennium Tower was still safe to occupy, even with the settlement and tilting. However, they warned that continued movement could compromise the building's safety in the future, especially in the event of a strong earthquake.

What steps were taken to fix the Millennium Tower's foundation?

The solution to fix the building involved installing 52 new piles around the tower’s perimeter down to bedrock. Hydraulic jacks would be used to lift the building and transfer some of the load onto these new piles, slowing the sinking and hopefully straightening the tower over time.

Why was it difficult and expensive to repair the Millennium Tower's foundation?

Repairing the foundation was expensive due to the complexity of drilling piles after the building had already been constructed. The space constraints, vibrations, and noise from drilling around the tower made the process challenging, with initial estimates of the cost exceeding $500 million.