DPR sets soil nail record at tall tech building in Bellevue, WA

DPR will nail at least two firsts in the construction of Bellevue Technology Tower: the 20-story high rise will be DPR’s tallest building to date, and the 850,000-sq.-ft. tower will be the deepest vertical structural soil nailed wall in the US. “It’s the deepest in the nation using this system,” said John Tessem, of Cary Kopczynski & Co., the tower’s structural engineer. The soil-nailed wall will extend down 86.5 ft. from the street surface to the bottom of the footings.

The project team, after looking at the physical characteristics of the Bellevue, WA site-it’s surrounded by NE 4th Street, 108th Avenue NE, an office building to the south and a bank to the west-determined the top-down soil nailing approach would fit the site conditions better than the more standard soldier pile and lagging system. By using the soil nailing method engineered by Golder Associates, DPR will save owner E&H Properties time and money, because the shoring and placement of the structural wall will be done in a single step (i.e. soil nailing/reinforced concrete).

“It is unique because when we get to the bottom of the hole, the perimeter structural wall will be done,” said DPR Project Manager Ron Montoya. “By combining a normally two step process into one step, the soil nailed wall saves time and money by reducing schedule and materials.”

“The wall we will put in place is the permanent structural wall of the building; plus, it serves the second purpose of retaining the adjacent soils and safeguarding us against cave-ins during construction,” says DPR Superintendent Bruce Remmen. These soil nailed walls remain in place to be the permanent basement perimeter walls. When crews get to the bottom, they will have created a 12-in. thick structural concrete perimeter wall. Coming up, they will pour slabs that tie into the shotcrete (sprayed concrete) wall.

Soil nailing on the Bellevue project means drilling through the soil under adjacent streets and buildings at a 15-degree angle, installing a 1¼-in. x 70-ft. bar, filling one end with concrete and embedding the other with a headed stud plate that sticks out beyond the dirt. Two layers of reinforcing bars will be added and sprayed with 12 inches of shotcrete. “The process will be performed in six-ft. sections. We’ll excavate six feet, shotcrete, excavate another six feet, shotcrete, until we get to the bottom, about 86.5 feet down,” says Remmen. When soil nailing is complete, the deepest spot will have 14 lifts, and about 1,800 70-ft. soil nails will have been installed.