Millennium Tower upgrade: engineers report pilot casing methods reduced ground loss; DBI, EDRT and HOA seek more tests

Supervisor Aaron Peskin convened a hearing on Nov. 4, 2021 to receive progress updates on the voluntary perimeter pile upgrade (PPU) for 301 Mission (Millennium Tower). The homeowners association, lead design engineers and contractor described the retrofit plan — installing perimeter piles tied into a mat extension to transfer loads to bedrock — and reviewed the monitoring and recent construction experience.

Charles Litt, counsel for the HOA, summarized the mediation-originated design process and peer reviews that led to the PPU. Ron Hamburger, the structural engineer of record for the retrofit, reviewed geotechnical conditions (bay mud, Colma sand and deeper bedrock), explained the PPU method (36" steel casings followed by 24" piles to rock and a mat extension), and said that construction coincided with roughly 1.25 inches of additional settlement during installation. Hamburger identified ground loss during casing and pile work and vibration compaction of sands as likely causes and said the contractor paused operations in August to revise means and methods.

EDRT members and outside deep-foundation experts (Dan Brown Associates) reviewed the revised approach and reported successful pilot installations using tighter controls on casing splices, measured soil-plug thickness, water levels in casings, and isolation of vibration sources; those pilot installs produced negligible settlement when prisms were monitored hourly. DBI said it set a per-installation threshold (0.25" settlement) that would require a pause and further analysis. Engineers emphasized that further testing of the 24" piles is necessary before resuming broad installation; the EDRT and DBI will continue weekly data reviews. Public comment urged caution and additional non-destructive investigation. The committee voted to continue the item to allow outstanding tests and further review.