505 First1

Bathtubs and Floating Buildings

You wouldn’t think that a building built on solid ground would be in danger of floating upwards. In fact, building where the water table is close to the surface can make this a concern. Dewatering the soil can keep floating buildings from being a problem, but some areas can’t be dewatered.

Here’s one example. Back when Seattle was just a pioneer town, the 505 First Avenue Building property was part of the waterfront. It had a wharf on pilings for timber mill-related businesses.  In the late 1880s and early 1890s, the area was filled in with, among other things, sawdust and wood debris from sawmills, wood planks and pilings, ship ballast, and burn debris from the Great Seattle Fire of 1889. This all meant that the soil where Starbucks now wanted to put up an office building was very soft and contained wood debris, abandoned timber piling foundations, and many other obstructions.

For this and other reasons, engineers couldn’t pump out the groundwater during or after construction. The four-level deep excavation had to be water-tight (think bathtub, but with the water on the outside). And this could lead to “an unbalanced hydrostatic uplift force in excess of the building weight acting on the foundation.” In other words, the seven-story building would float upwards.

How do you restrain such a building? In this case, engineers tied it down with 360 micropiles below the five-foot thick structural mat foundation.