Millennium Partners’ proposed 1.6-million-square-foot tower could compromise the structural integrity of two neighboring office high-rises in Boston’s Financial District, the buildings’ owners claim.

The 1-million-square-foot 100 Summer St. could settle as much as 1.7 inches, leading to potential structural damage, owner Equity Office Properties said, while Rockpoint Group says its 75-101 Federal St. tower also is at risk.

Equity Office cites geotechnical studies by Haley & Aldrich that indicate excavation and dewatering work during construction of Millennium’s 5-story underground garage could cause up to 1.7 inches of settlement and potentially cause significant structural damage to its building. The two buildings’ foundations would be eight feet apart, and Millennium plans to excavate to a depth of 75 feet for the garage.

Equity Office has hired its own team of engineers to study potential impacts, according to a letter by Paul Filtzer, the company’s local director of portfolio management, to the Boston Planning and Development Agency.

Rockpoint Capital, owner of the neighboring 75-101 Federal St. towers, took issue with Millennium’s plans to use an unusual construction technique for the first time on a high-rise in Boston. The “top-down” method involves simultaneous foundation work and vertical construction.

“As direct abutters, we are very concerned about the prospect of facing a building settlement situation similar to that which Millennium Partners is currently facing in San Francisco,” Rockpoint General Counsel Ron Hoyl wrote in a March 19 letter.

Millennium Partners is facing lawsuits from condo owners including Hall of Fame quarterback Joe Montana over who’s to blame after its luxury tower in San Fransisco sank 17 inches and tilted 14 inches since completion in 2008. A potential fix is estimated at $150 million.

Millennium Principal Joseph Larkin said the Boston tower at 115 Federal St. “will serve as a source of pride and inspiration” for the city and downtown neighborhood, in a statement issued Thursday.

“The issues identified in the public comment period are emblematic of the typical civic dialogue for projects of this size.  We’re very excited by the progress of this transformative project and look forward to working with all stakeholders as this process moves forward,” it stated.

Millennium has asked Haley & Aldrich for additional geotechnical studies.

The BPDA is reviewing Millennium’s plans after picking the firm in 2016 to redevelop the former municipal garage property.

The latest proposal calls for a 55-story tower including 500 condos totaling 760,000 square feet, 750,000 square feet of office space, 65,000 square feet of amenity space for the condos and offices, 40,000 square feet of public space and retail and 40,000 square feet of shared meeting space.

Tower Owners Say Millennium Winthrop Square Gives Them a Sinking Feeling

by Steve Adams time to read: 2 min