When a January 2012 landslide along a major Washington highway caused closures and damage to residential properties, the city of Bellevue needed a fast solution that would stabilize the slope and get traffic moving again.
Officials were unable to determine what caused a water main break during a heavy rain and ice event which resulted in a significant landslide. The slide deposited debris in the yards and basements of several high end homes that lie between West Lake Sammamish Parkway and Lake Sammamish.
Prior to the slide, the city of Bellevue had been considering Redi-Rock retaining walls for 3-4 phases of planned improvements along the highway. When the slide happened, engineers and city officials contacted Redi-Rock within 48 hours because “repairing the slope needed to happen quickly,” said Torger Erickson, P.E., and project manager for TetraTech. Other engineers involved in the project included the City of Bellevue Transportation Department, GeoEngineers Inc. and Development Engineering PLLC.
“We didn’t have soil conditions that would allow us to drill tiebacks or lay geogrid. Other types of retaining walls higher than 6 ft require tie backs, but we were able to achieve a 12-ft-tall wall by utilizing the weight of these blocks,” explained Bill Cross, senior construction inspector for the city.
The project actually required two walls; the lower wall was 9 ft tall with 3 ft of freestanding walls on top, and the upper wall was 12 ft tall. Both were built as gravity structures, which rely on the massive size of each block to create tall walls that require no reinforcement at all.
Engineers designed the lower wall using Redi-Rock 41-in. gravity blocks in the bottom courses, 28-in. blocks in the middle courses, and freestanding blocks on top. The lower wall also included a 24-in. pipe at the base of the lower wall which allowed a stream to flow under the highway.
The design of the upper wall called for 60-in. base blocks in the bottom six courses (9 ft) of the wall to increase its stability, topped with one course of 41-in. middle blocks and one course of 28-in. top blocks.
“We were holding back a lot of soil; this was a 30-ft embankment with a highway on top that we were holding back with a 12-ft-high wall, so that’s a lot of load,” Erickson added.
In total, the solution included two walls and 1,365-sq-ft of blocks—all in a Ledgestone texture that was stained to match the natural rock tones in the area.
“It was quick,” explained Doug Neal of Westwater Construction Inc. “We built the wall in two stages; the lower wall took 4-5 days and the upper wall took 7-8 days,” Neal said. He added: “The best thing about the system is that you don’t have to lay the blocks at a batter because it’s already built in. As a contractor, that makes a huge difference.”
The road was open again four months after the slide occurred.