And the review determined that the two affected beams, both over 60 feet long, barely moved an inch due to the fractures. The redundancies in design guaranteed the beams’ stability. The overall safety of the building was never compromised. If those workers hadn’t discovered the cracks by chance, we still might not know about them.
Source: Salesforce Transit Center: San Francisco’s $2.2 Billion Cracks
The problem is that the 60-to-90-foot-long friction piles underpinning the building were driven into sandy soils rather than bedrock at 200 feet down. While no concrete explanation has been given for continued sinkage, developer Millennium Partners has blamed construction of the neighboring Salesforce Tower for pumping out too much groundwater and causing the soil to settle.
Source: San Francisco threatens to block access to Millennium Tower – Archpaper.com