The much-used Kummer Bridge over the Green River on State Route 169 will close Tuesday evening for about six months because a slowly moving landslide threatens the stability of the bridge, according to the state Department of Transportation.

The closure was to occur at 7 p.m.

The closure of the 80-year-old bridge knocks out an important route for travelers and commerce from Renton heading to Enumclaw and the Cascade mountains. State Route 169 begins in Renton.

According to the state, about 9,000 trips are made across the bridge each day, including police and fire crews.

The state has set up two detour routes for traffic, one for passenger vehicles and one for commercial trucks.

Drivers can download the maps at www.wsdot.wa.gov/Projects/SR169/GreenRiverRepair/Detour.htm.

The detour for commercial vehicles starts at Four Corners in Maple Valley, proceeding east on Kent-Kangley Road, then southeast on Retreat-Kanasket Road, then southwest on Southeast Veazie-Cumberland Road, to Southeast 392nd Street and finally to Southeast 400th Street at SR 169.

The detour route for passenger vehicles begins at Southeast 400th Street on the south and at Southeast Auburn-Black Diamond Road on the north.

Jamie Holter, a spokeswoman for the state transportation department, said local traffic can go all the way to the bridge from either direction, but vehicles cannot cross the bridge.

The state is posting people at the bridge to ensure no one crosses and then will put up concrete barriers, she said. Perhaps as early as Friday, the state will begin excavating roadway, she said.

“We are worried about people breaking through the barrier,” she said.

Drivers can follow project progress at www.wsdot.wa.gov/projects/sr169/greenriverrepair/.

After reviewing data gathered by electronic sensors in the landslide zone near the bridge in Black Diamond, state transportation geotechnical engineers determined the structure is vulnerable to significant land movement, according to the state.

So far, the ground has moved about an inch, Holter said.

The closure of up to six months will allow for significant long-term repairs, according to the state.

Last summer, the state completed repairs it thought would stabilize the ground, which is pushing against bridge piers.

“I can’t emphasize enough that this closure is about safety,” said Paul Hammond, the state transportation secretary. “We will do what is necessary to keep the public safe and preserve the bridge.”

State engineers believe the recent heavy rain could have aggravated the soil movement. They are concerned the soil may not be able to support one of the southern-most bridge piers. If the soil slides, the pier could move with it, according to officials.

The repair includes two steps, according to the state.

First, crews will remove excess water by clearing trenches, unplugging manholes and installing pumps to help drain water from the unstable hillside.

Second, crews will excavate the soil that is putting pressure on the bridge pier. They will dig out a section of roadbed 30 feet deep and 300 feet long just south of the bridge and replace it with lighter-weight material.

All the work will be done as an emergency contract and will cost about $10 million. This is a rough estimate as the details have not been worked out.