Oregon DOT Slip plane beneath bridge abutments causes builder to ponder new solutions.

Ancient underground landslides at a $150-million, seven-mile highway realignment on U.S. 20 in Oregon have led to at least a nine-month temporary suspension of work.

Design-builder Granite Construction Inc., Watsonville, Calif., agreed to the Sept. 1 suspension in mid-July. During the suspension, Granite and the Oregon Dept. of Transportation will examine ways to minimize costs yet satisfy environmental permit requirements, says ODOT spokesman Joe Harwood.

Construction began in summer 2006, a year after ODOT awarded the $130-million design-build contract to Yaquina River Constructors, a joint venture of Granite and subsidiary Wilder Construction Co., Blaine, Wash. Granite began negotiating with ODOT to get out of the contract in March after determining that geological conditions under the abutments of eight planned bridges could lead to an extra $61 million in remediation.

At the time, Granite had completed 98% of the design. “After looking at everything going on, the ODOT director [Matthew Garrett] decided it would be in the best interest of Oregon taxpayers to keep the contract and use Granite to finish the job,” says Harwood.

Teams discovered more landslides, reaching as deep as 100 ft beneath the surface, says Granite project manager Darryl Goodson. “The more you looked, the more you found,” he says. The slides, invisible from the surface, became evident in core samples, showing a slip plane similar to those found in an earthquake zone. “Oregon is on a seismic faulting area,” Goodson explains. “Any potentially un-stable pile of dirt is not a good thing.”

The worst slide runs directly beneath a 1,000-ft bridge. It spans a waterway that feeds a state-regulated salmon stream. Granite’s $61-million solution included driving some 500 4-ft-diameter caissons as deep as 120 ft, placed an arm’s width apart in a “pincushion” arrangement. Granite estimated $28 million of the cost to be overhead for keeping a full staff on site for an additional two years.

“We said $61 million is too expensive,” says Harwood. While negotiating the temporary suspension, ODOT devised a best-case scenario that would put the contractor back to work in just one year at an undetermined lower cost.

Goodson says potential alternatives to the pincushion could involve minor shifts in road alignment, different bridge types, or different use of the soil embankments. How the parties will split the cost of the delay and any new mitigation measures remains to be determined.