Tom Gurtowski is a vice president with Shannon & Wilson, the geotechnical and environmental engineering firm responsible for the projects. Gurtowski tells GlobeSt the ground will be excavated to between 40 and 50 feet.

The sides will be lined with three-foot-thick concrete, and a slab anchored to the firmer ground at the bottom, creating a watertight foundation -- no easy task. "You have to dewater as you're excavating," says Gurtowski, "putting in vertical wells with pumps that keep pumping until the slab is complete."

While bathtubs have been built elsewhere, such as Boston, Chicago and San Francisco, Martin Smith will be the first to use them in Seattle. The proposed projects, still in planning phases, will utilize their bathtub areas for underground parking. They are a 200,000-sf office tower at 505 First Ave., and a 170,000-sf office building on Occidental Ave. South.

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