Settling causes unstable soil beneath the project.
The Oregon Department of Transportation is preparing to abandon an attempt to build an overpass on U.S. Highway 97 in La Pine, after having spent $12.4 million of the $17 million allocated for the project.
The Wickiup Junction project would have rerouted the highway to the east and over the BNSF Railway line, eliminating the last remaining at-grade rail crossing along U.S. Highway 97 between Washington and California. Instead, rapidly sinking soils have led project managers to conclude the overpass cannot be completed safely, and should be scrapped.
On Monday, ODOT officials appeared before the La Pine City Council to announce the news.
The project has been on hold since May, when crews working on the project discovered the ground beneath the embankments built to carry the highway over the rail line was sinking. Project manager Bob Townsend said the overpass itself sunk six inches over the winter, with portions of the embankments sinking as much as 18 inches.
Subsequent investigation determined the cause of the sinking — a roughly 200-foot-thick layer of unstable earth that was once the bottom of an ancient lake. Geologists working with ODOT believe a volcanic eruption more than 10,000 years ago dammed what would later become known as the Deschutes River, creating a lake of still-unknown size.
Della Mosier, area manager for ODOT, said her agency has never before encountered similar subterranean deposits.
Mosier said the deposit beneath La Pine is notable for the concentration of diatoms, single-celled creatures that leave behind a silica-based shell. The diatoms found in core samples taken from deep below the overpass are largely intact, she said, whereas in most locations a diatom layer will have been compressed into a sturdy layer of rock.
In La Pine, the diatom layer appears to be fracturing and compressing under the weight of the roughly 250,000 cubic yards of soil brought in for construction of the overpass, Mosier said.
Mosier said diatom-rich deposits typically include a great deal of clay and organic material mixed in, and engineers can reliably predict how such soil will settle. In La Pine, they’re operating in uncharted territory, she said.
Bob Bryant, ODOT district manager, said engineers working with his agency followed standard preconstruction processes to gauge the stability of the earth beneath, drilling 12 bore holes from 14 to 128 feet deep. That investigation found nothing to suggest excessive settlement was likely, he said.
Drilling done in the spring after the project was put on hold went deeper, well in to the diatom-rich layer.
Bryant said settling does not appear to be the cause of an earlier setback for the project, when two 130-ton beams were dropped while being put in to place in August 2016.
La Pine residents at Wednesday’s meeting questioned ODOT officials about what happens next. Resident Harold Johnson encouraged the agency to consider an underpass beneath the railroad tracks, while former city councilor Ken Mulenex said a paving project slated for next year between Sunriver and La Pine should be revisited to add intersection improvements that were to be a part of the overpass project.
City Councilor Don Greiner said ODOT should look seriously at a reroute that moves the highway east of La Pine, though he predicted such a move would be unpopular with La Pine businesses that depend on highway traffic.
Bryant said because the extent of the ancient lake bed is not known, additional drilling will have to be done to determine if it might be possible to construct a similar overpass near the one that was nearly completed. The site was chosen as the location where ODOT would be able to build the least amount of highway necessary to go up and over the railroad tracks, he said.
La Pine City Councilor Stu Martinez said he was disappointed by the discovery, but said unexpected developments are sometimes unavoidable.
“La Pine is unique,” Martinez said. “Unfortunately this isn’t the uniqueness that we want, but it’s here.”
Local ODOT officials plan to ask the state transportation commission to formally cancel the project at the commission’s meeting next month. Bryant said the remaining funds allocated for construction are likely to be used to start planning for an alternative rail crossing, and to restore land disturbed by construction.
Original Article published by Bend Bulletin - http://www.bendbulletin.com/localstate/5624728-151/odot-highway-97-overpass-in-la-pine-scrapped