Millennium Tower retrofit: DBI, EDRT and independent experts debate 52‑pile plan vs. reduced 18‑pile proposal

Committee members on Feb. 6 received technical updates and contested analyses about the proposed perimeter‑pile underpinning for 301 Mission (Millennium Tower). The hearing featured the structural engineer of record, geotechnical leads, DBI’s permit staff, the Department of Building Inspection’s Engineering Design Review Team, and at least one independent critic.

Supervisor (Paskin) opened the item noting that the originally submitted perimeter plan included roughly 52 piles intended to bear into bedrock; presenters said that sequence and the contractor’s initial means and methods contributed to a period of accelerated settlement and tilt last summer. Ron Hamburger (structural engineer of record, Simpson Gumpertz & Heger) said the team instituted a voluntary moratorium on construction from Aug. 22 through Oct. 12, ran a pilot program (three 36‑inch casings and three 24‑inch piles) using revised techniques, and reported the pilot piles did not cause additional settlement. He told the committee the project team has now submitted a reduced‑scope plan that would install as few as 18 perimeter piles with higher load per pile and said, "The building remains safe," while urging the work resume so tilt can be arrested.

Geotechnical lead John Egan and testing partners described indicator‑pile programs and said revised cased‑socket methods and revised monitoring increased confidence in per‑pile capacities; Egan said the pilot and test data support higher average pile capacity (about 30% above earlier measure), enabling a design that concentrates piles at the mission/fremont corner where the settlement is greatest. Egan and Ron Hamburger said analyses show the revised approach will arrest settlement at the northwest corner and that structural modeling indicates the existing mat and foundation can accept the higher per‑pile jacking loads with an adequate factor of safety.

DBI deputy director Neville Pereira described heightened DBI oversight since August (daily senior inspector presence on production days, quarterly habitability checks, and EDRT engagement). He said DBI had just received the revised 18‑pile submission and expected to review a detailed projection of settlement and the team’s FLAC and finite‑element modeling; DBI expected a four‑week technical review of outstanding analyses.

Independent reviewers and critics urged caution. Ben Turner (Dan Brown Associates) and Robert Pike (independent geotechnical engineer) said monitoring and logging improved but raised questions about whether pilot testing was sufficient (Pike noted only one indicator pile reached ultimate capacity in tests) and whether asymmetric, reduced‑pile strategies would leave ongoing secondary consolidation that could drive future tilt. The Engineering Design Review Team (Professor Greg Dierlein) said the EDRT is reviewing the analyses; it noted that while current tilt (~24 inches westward) is significant, initial checks show the building remains within seismic safety margins under current code‑level earthquake scenarios, and that numerical modeling is under active review.

Supervisor Peskin (Paskin) and others asked why the team did not explore optimization earlier; Ron Hamburger answered that the original 52‑pile design grew out of prior comparisons and that optimization for fewer piles was only pursued once field behavior and pilot work showed different installation performance and pile capacity. DBI and the EDRT said they will continue detailed analysis of mat behavior, pile‑to‑mat load transfer, anticipated tilt recovery under different pile layouts, and seismic performance under combined gravity and maximum‑considered earthquake cases.

After oral presentations and discussion the committee voted to continue the item to the call of the chair to allow DBI, the EDRT and the design team time to complete the outstanding technical reviews.