Unstable highway down to one lane

Part of State Route 169 near Black Diamond, including a bridge across the Green River, has been restricted to one lane because it could collapse under normal traffic.

The closure Tuesday of a bridge on State Route 169 between Black Diamond and Enumclaw while highway officials decided if it’s safe to drive on will be followed soon by more of the same for repairs.

The state Department of Transportation planned to close the Green River Bridge north of Enumclaw from 6 a.m. 10 p.m. to allow geotechnical experts to drill under its southbound lanes to assess the stability of the ground. A slope under the span is showing signs of weakness, according to DOT.

More closures are coming as early as next week, when emergency interim repairs could start. A DOT spokeswoman said preliminary estimates are that the road would be closed for up to two weeks during the repair work.

The work would include a short retaining wall in the area that’s considered most unstable. Long-term, a wall will need to be built for the entire unstable area adjacent to the highway, and it will need to be finished before heavy rainfall later this year, officials said.

The closures are “a big inconvenience to drivers who travel between Black Diamond and Enumclaw,” said Lorena Eng, a DOT regional administrator. “But we’re concerned about the stability of the slope under the south end of the bridge. We’ll keep traffic off the bridge until we’re sure drivers can use the road safely.”

Tuesday’s closure affected all traffic between Southeast 400th Street and Auburn-Black Diamond Road. A detour route passing west of SR-169 added seven miles and about 10 minutes of drive time to trips, DOT said.

At daybreak, crews were to close all lanes of the bridge and set up a drill rig to bore holes in the ground that’s causing concern. Data from the drilling will allow DOT engineers to analyze the condition of the bridge and roadway foundations. Once the drilling and analysis was finished, the bridge was expected to reopen to one lane of signalized, alternating traffic with a reduced speed limit.

“Our biggest concern is the vibration from big, fast trucks,” said Gary McKee, DOT project engineer. “We can minimize the vibration by reducing the speed limit.”

The limited access will last until further notice and should result in delays of up to five minutes for motorists, according to DOT.