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Home / Commentary / A CONCRETE POUR FOR THE AGES: How the foundation was laid for the BMO Harris Tower

A CONCRETE POUR FOR THE AGES: How the foundation was laid for the BMO Harris Tower

(Rendering courtesy of Kahler Slater)

(Rendering courtesy of Kahler Slater)

GZA GeoEnvironmental recently took on the opportunity to provide geotechnical engineering and related services for the $132.6 million BMO Tower that’s now under construction in downtown Milwaukee, right across from Milwaukee City Hall.

As the Daily Reporter reported in early June, GZA and Irgens, recently reached a milestone in the project as crews from J.H. Findorff & Son and Ozinga Brothers poured 6,700 cubic yards of concrete—nearly 700 mixer loads arriving over the course of 21 hours—to form the new building’s foundation.

The successful foundation pour ranks as one of the largest continuous concrete pours in the history of the state of Wisconsin.

With the giant pour behind us and construction progressing rapidly every day, we’d love to now tell a little more of the story behind the story of one of the biggest projects to be taken up recently by the Wisconsin construction industry.

John Siwula

John Siwula

As consultant to Irgens, GZA has provided geotechnical engineering design, bearing capacity-testing for the soil supporting the project’s foundation, and related services, including developing a vibration monitoring and mitigation protocol to ensure construction would never disrupt BMO Harris Bank’s data center or a nearby high-pressure steam tunnel. GZA also worked separately with Findorff and Pierce Engineers to design the foundation base for a 280-foot-high tower crane being used to build the headquarters.

The construction site at 790 North Water Street sits on a once-marshy mix of glacial till and alluvial deposits, close to the Milwaukee River. Because of the composition and limited strength of the soil mix, many downtown Milwaukee buildings in this neighborhood have been put up on deep-foundation driven piles or drilled shafts.

GZA’s team–composed of the senior geotechnical engineer Pat Harrison, geotechnical engineer Jesse Graham, geotechnical engineer Ian Mosbrucker, senior geotechnical consultant Scott Breen, and me–developed an innovative deployment of pressure-meter testing to design a shallow-mat foundation for the new office tower. Using pressuremeters to test the soil strength in-situ rather than extracting core samples left the soil considerably less disturbed, and therefore better able to model the support of a shallow-mat foundation. Compared with alternative methods, such as driving piles or drilling shafts, the shallow-mat foundation presented a reliable and economical solution.

GZA also tested and carried out a vibration protocol to ensure demolition and construction activities would not disrupt the existing BMO Harris Bank’s data center directly adjacent to the construction site. Keeping this computer center up and running 24/7 was critical to BMO Harris Bank’s operations.

Running compactor equipment inside the 5-story parking garage before it was razed, GZA staff established criteria and alarm levels for construction-activity vibration. In the end, they successfully ensured work on the bank’s new headquarters would never disrupt business for the bank’s customers. (There’ve been no complaints from Mayor Tom Barrett or his staff at Milwaukee City Hall, either.)

For everyone connected with this project, seeing the completion of this extraordinary 21-hour operation to pour the foundation of the new BMO Tower was a moment of deep professional satisfaction and a chance to celebrate a job well done. Construction is proceeding briskly, and we’ll be among those most looking forward to celebrating the grand opening of the new tower in December 2019.

– John Siwula is associate principal at GZA GeoEnvironmental

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