505 First Avenue with Historic Building Underpinning

Starbucks/The Seneca Real Estate Group

Seattle, Washington

Historic Office Building

This project won a silver award from the ACEC Engineering Excellence Awards. Hart Crowser provided geotechnical services for design of a new office building at 505 First Avenue in Pioneer Square. The new building, included four levels of underground parking into and below the water table. Construction dewatering and long term dewatering of the structure was not possible of the geology of the area, which has highly compressible organic wood waste and fine-grain soils. Dewatering of the site over the short term or long term would have resulted in unacceptable subsidence of the rights of way surrounding the site due to the consolidation of the highly compressible surrounding soils.

As a result, we designed the shoring for the excavation to be water-tight using a Cutter Soil Mixing trenching system, which was a relatively new technology to the Northwest (first commercial application in Seattle) that uses slurry trenches to create in-place lagging into which soldier piles can be placed. Because of the very high uplift pressures on the building, which is only six to seven stories above grade, the building has a net upward buoyancy, which required a series of tiedown anchors under the building’s structural mat to restrain the building.