Courtesy Oregon Department of Transporation
SALEM — Engineers with the Oregon Department of Transportation plan to recommend tearing down a nearly completed $12 million overpass on Highway 97 in La Pine after a geotechnical investigation found the underlying soil is unstable.
The Oregon Transportation Commission is scheduled to hear and decide on the engineers’ recommendation at a meeting in Silverton in late October.
Project managers ordered routine soil tests of the project area before construction of the Wickiup Junction overpass began, but the tests failed to detect rare diatoms in the subsurface of the soil, said Della Mosier, an engineer and interim area manager for ODOT projects in Central Oregon.
The diatoms are silica-based skeletons of algae that lived in an ancient lake in the area and sank, forming a thick bed on the bottom thousands of years ago. The lake had been identified in geological maps, but initial tests gave no indication the soil differed from other parts of Central Oregon, where ODOT has successfully built other bridges, Mosier said.
Furthermore, past surveys of the ancient lake had limited information about the characteristics of the soil in the area, said Chris Carpenter, senior associate engineer with Cornforth Consultants. ODOT contracted with Cornforth and used expertise from the Federal Highway Administration to conduct the subsurface soil analysis at the Wickiup Junction project, using boreholes and soil samples from as deep as 280 feet below ground.
In contrast, ODOT’s routine pre-construction soil tests involve penetrating and testing soil 125-feet down into the earth, during which the agency identified only a 50-foot layer of volcanic crust deposit and 75 feet of dirt.
“We have been building bridges in Central Oregon for decades and have never encountered this,” said Paul Mather, administrator for the ODOT Highway Division.
The advanced subsurface tests were conducted after engineers noticed two earthen embankments constructed for the overpass were settling faster than expected.
The weight of the overpass crushes and shifts the diatoms, causing unpredictable settlement of the structure, Mosier said.
By the time the conditions were detected, project managers had already completed 80 percent of the project and spent more than $12 million of the $17 million budget.
The cost of tearing down the bridge is estimated at another $2 million, said Robert Townsend, a project manager in the ODOT construction office in Bend and The Dalles.
The diatoms are common in parts of Oregon’s Klamath Falls and Mexico City, where engineers design “floating” structures with big basements that remove most of the structure’s weight, Carpenter said.
Engineers, however, don’t know enough about characteristics of the soil to determine whether such as design option would work and be safe for motorists on the Wickiup Junction overpass, and any such project changes would be expensive, Mosier said.
The project was intended to enhance safety and mobility at the intersection of Highway 97 by allowing vehicles to bypass a Burlington Northern Railroad crossing at the location. School buses and vehicles carrying hazardous materials are required to stop regardless of whether a train is present at the crossing. The frequent stops created potentially dangerous situations for heavy freight traffic and other vehicles passing through the area.
The intersection is the last “at grade” highway crossing of railroad tracks in Central Oregon, said ODOT spokesman Tom Fuller.
The overpass spans a segment of the highway that intersects with a road that leads to a fishermen’s paradise at Oregon’s second largest reservoir, Wickiup Reservoir, in the Deschutes National Forest. The overpass is located about a 30-minute drive south of Bend.
Highway 97 is considered an emergency northbound arterial route through the state in the event of a catastrophic earthquake that destroys bridges connecting Interstate 5.
Engineers on the Wickiup Junction project plan to ask the commission for direction in October on whether to use leftover money from the project to conduct more tests of the soil in the area or investigate alternatives to make the Wickiup Junction safer.
ODOT Director Matt Garrett also plan to convene a panel of experts through the Federal Highway Administration’s Resource Center to analyze problem soils in other parts of the state.
The obstacle to the Wickup Junction project comes as ODOT is under the spotlight for how it might manage a $5.3 billion transportation package, approved by state lawmakers in early July.
“Obviously, our eyes are open, but we don’t anticipate having this problem in future projects, with such a unique situation,” Mather said. “We wouldn’t anticipate to encounter this again.”