With the initial round of testing having proven successful, the plans to permanently stabilize the sinking Millennium Tower by adding a series of 52 new piles around the tower’s perimeter have been further refined, building permits for the fix have been requested, and a final round of pre-fix testing is about to get underway.
As envisioned, a 10-foot wide, 27-foot deep, trench would be excavated along the tower’s western perimeter fronting Fremont Street, and down 125 feet of Mission Street, allowing for the installation of the aforementioned new piles, each with a diameter of 24 inches, between the tower’s existing mat foundation and down to the bedrock below.
The retrofit would transfer around 20 percent of the tower’s weight to the new piles, “with intent of arresting [building] settlement and improving the foundation lateral capacity,” reducing stress on the tower’s mat foundation and Old Bay Clay below.
And while the stabilization project is currently expected to take around two years to complete, the perimeter approach will minimize the impact on the building’s residents and garage, neither of which will need to be vacated for the project to proceed.
The final round of testing has since been successfully completed and the plans refined, as newly detailed below:
The structural upgrade would involve the installation of 52 cast-in-place reinforced concrete piles beneath the sidewalk areas, within an approximately 8-foot-wide zone along the Mission (north) and Fremont (west) street sides of the Tower building.
Each of the piles would have a diameter of 36 inches (outer casings) through the Young Bay Mud and Colma Sands to a depth of approximately 70 to 90 feet, a diameter of 24 inches (shaft liners) through the Old Bay Clay to the Franciscan Complex bedrock at approximately 220 to 250 feet [below ground surface], and a diameter of 20 inches (rock sockets) extending 30 to 50 feet into the bedrock.
Once pile placement is complete, an 8-foot-wide, 10-foot-thick reinforced concrete extension of the existing concrete mat foundation would be constructed outward in the direction of the new piles.
The new piles would [then] be connected to the extended mat via a jack system that would transfer load from the existing foundation to the new piles. The jack system would be located in new vaults, one along Fremont Street and the other along Mission Street, located approximately 8 feet below the sidewalk.
In addition to preventing further settlement, the project’s geotechnical engineer has stated that, “this effort may allow for gradual tilt correction of the Tower building over time.”
Assuming the finalized plans are approved, the project team is aiming to commence work on the projected 22-month fix early next year (2020), with construction activities “staged along the perimeter of Fremont, Mission, and Beale streets, requiring the closure of one travel lane and sidewalks along Fremont and Mission streets and restricting pedestrian access on the sidewalk along Beale Street during portions of the construction period.”
And yes, a monitoring system to track any additional settlement, foundational stress or tower movement, both during and after the project’s completion, will be put in place as well.