WorldWide Drilling Resource

A Solution for San Francisco’s Leaning Tower? Compiled by Caleb Whitaker, Associate Editor WorldWide Drilling Resource ® The Millennium Tower in San Francisco sits on a 10-foot- thick mat foundation. The building is held in place by 950 rein- forced concrete piles driven 60-90 feet deep into clay and mud, but the piles do not reach bedrock. In WWDR ’s February 2017 issue, the article San Francisco's Leaning Tower dis- cussed the skyscraper’s worsening tilt since its construction in 2009. It is estimated the tower has now sunk about 17 inches, tilted 14 inches to the west, and 6 inches to the north at the roof line. Engineers have proposed an unconventional solution for the Millennium Tower ’s tilting troubles. Piles will be driven down to bedrock to stabilize one side of the 58-story tower, while the other side is allowed to continue sinking until the building straightens itself. Crews working inside the Millennium basement will drill about half of the 275-300 steel and concrete “micropiles” on the building’s west side. Then after the building stabilizes, the rest of the piles will be drilled to bedrock on the east side to stop the building from sinking and tilting. The entire process could take two to five years. At the start of the year, crews began an explorato- ry dri l l ing project at the Millennium Tower to eval- uate the planned fix. Holes were drilled 200-300 feet down to bedrock to see whether the method wi l l stabilize the troubled foun- dation. More drilling equip- ment is expected to be used outside the tower to bore additional holes deep into the ground to test the conditions of the soil and bedrock below. There are those who have questions concern- ing whether the proposed fix will solve the tower ’s problems. Some think at best the micropile strategy will stop further sinking and tilting, but will do nothing for the building’s current condition. Building occupants worry about boring micropiles into the foundation, particularly given the stress the building has already undergone. The building’s general contractor and engineers are hope- ful they have indeed found a solution. The micropile strategy is not new. It was used to shore up the Mandalay Bay Resort in Las Vegas, which sank some 18 inches during construction before being stabilized by more than 500 micropiles. With this in mind, contractors are already working with the city to obtain needed approvals and permits for the work. If soil tests are successful and the necessary permits are acquired, the lean- ing tower of San Francisco may finally find solid ground. 18 NOVEMBER 2018 WorldWide Drilling Resource ® Millenium Tower, courtesy of Victorgrigas. ENV