Scott Swanson
Sheep Creek Bridge on Highway 20 is sliding, and the fix will be costly in time, money and impact on travel.
That’s the gist of what an Oregon Department of Transportation official told Sweet Home City Council members last week during a report on the project.
A landslide has been pushing the easternmost span of the bridge out of position for decades, slowly shearing bolts, sensors and “bearing pads.”
For years, it moved about an inch, but since 2010, it’s been moving at upward of 7 inches annually. The Oregon Department of Transportation made repairs to the bridge in 1993, 2006 and 2011, but it has no room left to adjust it any more.
Project Leader Jerry Wolcott visited the Sweet Home City Council during its regular meeting on Sept. 8 to provide an update on the project and explain the complete closure of Highway 20 at the bridge next year.
Sheep Creek Bridge is located at milepost 56.6, about four miles east of the old Mountain House and roughly 25 miles east of Sweet Home on Highway 20.
The repair project started Aug. 24 as ODOT began preparing for drilling along north edge of the bridge and, beginning the first of the month, it commenced drilling and driving 50-foot steel piles to be embedded in concrete columns 4 feet in diameter through the landslide into bedrock in hopes of diverting the flow of earth and rock from the landslide. Installed in a row, the piles form an underground “picket fence.”
The first phase of the project is expected to coast $770,000, with Phase 2 coming in at about $2.3 million.
In the first phase, ODOT will install three sensors on three of the piles, said Bill Martin, coordinator.
“We’ll be monitoring all winter,” said spokesman Rick Little. The sensors “will tell us if anything happens.”
ODOT officials anticipate that Phase 1 of the project, this year, will continue through the end of October. They warned of potential delays of up to 45 minutes. Although the delays have not occurred yet, they advise travelers to allow extra time when traveling. So far, 20-minute delays have been the norm.
The bridge was built with its eastern abutment on the landslide in the 1960s, Wolcott said. In the 1990s, ODOT redesigned it to move with the slide and be adjusted as the earth around it moved.
Next year, ODOT will close the road, demolish the east end of the bridge, remove the dirt under the east end and refill it with rock, Wolcott said. Then ODOT will rebuild the eastern end of the bridge, pave, stripe and reopen the road.
“They’re planning on removing the first two spans (sections of the bridge on the eastern end) and putting them back in there, squared with the bridge,” Martin said.
The dirt will be excavated and rebuilt in “slices,” Little said. “In the meantime, the picket fence is handling the load of the slide.”
The new abutment will be able to move and be adjusted back into place. The project won’t change the need for the bridge to accommodate earth movement, but it is designed to redirect and reduce the activity so that the impact to the bridge will be consistent and predictable and therefore more manageable.
The closure will last six to eight weeks, Wolcott said. It will begin during the construction weather window of August and September, after the Oregon Jamboree.
ODOT is planning an open house to answer questions from the public from 5 p.m. to 7:30 p.m. on Tuesday, Oct. 6, at the Jim Riggs Community Center, 890 18th Ave.
The project is still under development, Wolcott said, and officials are looking for ways to reduce the length of the closure.
ODOT could build a temporary bridge at a cost of $2 to $4 million, Wolcott said, but it would add another year to a project that has already been moved forward a year due to slide activity.
Rather, ODOT is looking at other methods to speed up the process, he said. First, it can be designed using “accelerated bridge construction methods.” ODOT also will hold pre-qualification meetings with contractors who want to bid on the project.
ODOT officials will look for qualified and experienced contractors who have worked under pressure like this before, Wolcott said, and ODOT may offer incentives – bonuses – to finish early and disincentives – penalties – for finishing late.
A nearby disposal site will also help shorten the closure, he said.
Fire and fire danger could delay Phases 1 and 2, Wolcott said. They have been and will work with area emergency responders in planning for the closure.
“We don’t want a fire named after our project,” Wolcott said. El Nino-caused rainfall could delay the start of the second phase or activate the landslide before fixes can be completed, potentially making the bridge unsafe and inoperable.
The project will not permanently fix the problem, Wolcott said. The slide is too big, and the cost is high to stop it, build a larger bridge to cross it, or reroute the highway.
Rerouting the highway may just take the highway into different, undiscovered slides, he said. “You could reroute it to a place that’s just as active.”
The slide is not the only one in the area, Wolcott said. That stretch of Highway 20 has several smaller slides, and ODOT is applying for grants to help address the smaller ones.
The bridge is safe to use right now, Wolcott said, and ODOT will monitor and inspect it through the next year to assess whether it continues to be safe.