Causes and solutions prove elusive on an Oregon bridge project where two bents moved out of plumb during construction.

The troubled was noticed in the winter on the $215 million U.S. 20: Pioneer Mountain-Eddyville highway near the Oregon coast.

Joe Squire, project manager for the Oregon Department of Transportation, says that a lateral load from adjacent fill and sub-surface 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, but bridges of 600, 600, 750 and 1,100 feet are on hold while the subsurface monitoring is completed because of the trouble.

The problem surfaced in February during routine surveys and crews from sub-contractor 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 since. Both companies failed to return repeated calls seeking comment.

The bents in question have moved just hundredths of an inch, but extrapolated out over the 75-year life of the bridge could compromise the rigid structure in less than 10 years. “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.”

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

The area located in the tricky landslide-risky Coast Mountain range has been in question since 2007 when the discovery of an ancient landslide changed the scope of the project. Work resumed in 2008 after a change order that specifically identified four areas that were apparently moving in “slide central” mitigated the issue with buttresses and shear-keys.

“We believe mitigation was designed to address movements to have a viable construction project,” Squire says. “The design-builder had to provide us a viable project when they went back to work.”

“Small movements in landslide massing” show otherwise. Solutions could rest with more buttressing, readdressing the amount of fill, creating longer bridge spans to replace bridge abutment fill material or different bridge construction techniques altogether.

The rest of the project continues. “We are well on track for wrapping up major construction activities,” Squire says. “Except for the bridge.”