It takes 288 tons of steel to shore up this Capitol Riverfront project

It takes 288 tons of steel to shore up this Capitol Riverfront project

72 steel beams weighing 4 tons steel rakers each

What will eventually be a 220,000-square-foot Class A office building at 99 M St. SE is now a four-story hole in the ground supported by 72 steel beams weighing 4 tons each.

That's how it goes when you are building on soft soil next to a Metro tunnel. Skanska USA Commercial Development invited the Washington Business Journal over for a look at what lies beneath.

This is a closer look at 4-ton steel rakers supporting 99 M St. SE.

"There is a lot of soil below-grade that is infill and it added to the complexity of the excavation," said Mark Carroll, Skanska's executive vice president of commercial development. "We added the steel rakers to support the excavation."

To go below grade, project engineers had to drill down 75 feet and add 360 concrete piles to support the building, Carroll said. As the underground parking levels rise, the vertical piles will be cut down.

The same goes for the steel rakers, which hold back the sides. There are 72 steel rakers overall, or 288 tons of steel beams on four levels. The rakers will be removed as the structure is secured, said Carroll.

"It's an unusual sequence as the building runs within 15 feet of Metro's tunnel," Carroll said of the location near the Navy Yard-Ballpark station. "When you look at a typical excavation in D.C., it takes about six months. This was much longer because of the complexity."

The $116 million project broke ground in August of 2015. When completed in 2018, the project, designed by Gensler, will be a LEED Gold, 11-story office building with a 4,750-square-foot roof deck and a fitness center. Retail tenants Circa and Open Road have signed on for the ground-floor, but no office tenants have been announced.

99 M is part of a square block just north of Nationals Park that includes Grosvenor Americas' F1rst, a 325-unit residential building set to open in April; a Residence Inn; a $150-milion mixed-use project on Half Street from Jair Lynch and MacFarlane Partners; and The JBG Cos.' Half Street project.

http://www.bizjournals.com/washington/news/2017/02/10/it-takes-288-tons-of-steel-to-shore-up-this.html

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