Rainier Square Tower
Wright Runstad & Company
The American Council of Engineering Companies awarded Hart Crowser a 2020 National Finalist Gold Award for this project in the Special Projects category.
The excavation for the new 850-foot Rainier Square Tower was immediately next to — and 50 feet deeper than — the foundation for the existing 41-story (similarly named) Rainier Tower, an iconic structure with an eye-catching tapered pedestal that has drawn attention since it was built in the 1970s.
The feat required advanced computer modeling — and creative thinking — to keep Rainier Tower from settling or leaning. Hart Crowser, the project’s geotechnical engineer, designed a support system to meet stringent deflection criteria, and designed and implemented monitoring to prove performance during construction.
The excavation for the new Rainier Square Tower’s parking garage had to extend more than 50 feet deeper than the existing tower’s heavily loaded, shallow foundation — and was set back only 18 feet from its edge. A quarter of the Rainier Tower’s soil support had to be replaced with a shoring system designed and built so the existing tower hardly settled and didn’t lean into the new excavation. Hart Crowser monitored the soil and shoring wall and predicted Rainier Tower’s behavior, testing the variables and different combinations of structure and strength and how they affected the existing building’s settlement and lean.
Hart Crowser’s shoring solution was a concrete and steel secant pile wall supported by a closely spaced network of 242 tieback anchors separated by 4 feet vertically and 5 feet horizontally and installed at an angle of 20 degrees down from horizontal.
Each of these individual tiebacks had to maintain a zone of undisturbed soil around it. The tiebacks had to be long — some in excess of 120 feet — to resist a bearing capacity failure of the existing tower. Because of this, the emphasis on precision was much more important and harder to maintain.
Poor soil conditions complicated the effort. Prevalent at the site was a soil type called lacustrine. In areas where past glaciers passed over this soil, they became fractured and weak. A thick zone of this low-strength clay extended across the construction site, adding significantly to the load the shoring system would have to resist.
The scope of work and the challenges presented made the project “the most challenging excavation we have ever done,” according to the contractor.
The end result is the second-tallest building in downtown Seattle, featuring 725,000 square feet of office space, 77,000 square feet of retail space, 191 luxury apartments and an 811-car below-grade parking garage.