Causes and solutions are proving elusive on an Oregon bridge project in which two bents moved out of plumb during construction.

Last winter, the trouble was noticed on the $215-million U.S. 20 Pioneer Mountain-Eddyville highway near the Oregon coast.

Joe Squire, project manager for the Oregon Dept. of Transportation, says a lateral load from adjacent fill and subsurface ground pressure may have caused the shifts in two of the 20 bents on the 10-bridge project.

The project consists of a six-and-a-half mile section of new road that bypasses a 10-mile stretch of substandard highway. Six of the 10 bridges are essentially complete; however, because of the finding, a 750-ft bridge, a 1,100-ft bridge and two 600-foot bridges are on hold while the subsurface monitoring is completed.

The problem surfaced in February during routine surveys. Since then, crews from subcontractor URS Corp.’s Portland office and design-build contractor Yaquina River Constructors, Eddyville, Ore., have been monitoring ground forces at the remaining four bridge locations. Both companies failed to return calls for comment.

“We have a very classic bent with a drilled shaft column on top of the bent cap,” Squire says. “The beam sheets rest on top of the bent caps.”

The bents in question have moved just hundredths of an inch, but that much movement could compromise the rigid structure in less than 10 years.

With columns 100 to 120 ft tall, one inch of movement at the bottom of the column translates to three feet at the top. The 8- to 10-ft -dia. reinforced concrete shafts are drilled 55 to 95 ft below the original soil surface. Questions remain about the fills placed against the shafts.

Located in the landslide-prone Coast Mountain range, the project has been in question since 2007, when the discovery of an ancient landslide changed the scope. Work resumed in 2008 after a change order specifically identified four areas that were apparently moving. The change order mitigated the issue with buttresses and shear-keys, but a lasting solution may take more analysis.