Searching for answers

By CHANSE WATSON

Managing Editor

KELLOGG — After nearly two months of investigation, officials have yet to determine the exact cause for the “dips” in Interstate 90 just west of Kellogg that have plagued motorists.

A collaborative effort consisting of personnel from the Environmental Protection Agency, Idaho Department of Environmental Quality, the U.S. Army Corps of Engineers and the Idaho Transportation Department have been looking into the matter since the first dips were reported in early February.

These dips, which have affected both westbound and eastbound lanes near milepost 48.6, and have put the suspension of any vehicle going over the dips to the test.

Since the collaborative team first got on scene, work began to ascertain the cause and initiate a temporary fix to keep the interstate open. On Feb. 14, a temporary solution was put into motion that involved ITD adding layers of asphalt to the depressions to create a smoother driving surface. While this did help reduce its severity, the dips returned in the same areas and still currently exist.

In a public meeting with representatives from all involved agencies on March 22 in Kellogg, it was revealed that while the exact cause for the dips has yet to be determined, the problem may be sorting itself out.

“The subsidence in the road has really slowed down right now,” ITD District 1 Manager Marvin Fenn said. “So our plan of action is to continue monitoring the settlement with the professional surveyors.”

EPA representatives at the meeting explained that they have eliminated several possible causes that would be associated with the nearby Bunker Hill Central Treatment Plant Upgrade and Groundwater Collection System Project.

The Groundwater Collection System Project has been in progress on primarily the south side of I-90 since the summer of 2018. As part of the project, work crews have been installing a 20-30-foot soil/bentonite underground wall between I-90 and the Central Impoundment Area (CIA, or locally referred to as the “slag pile”) to catch contaminated runoff that attempts to flow into the Coeur d’Alene River. The project has also included installing various wells and pipes for water extraction and collection.

At this time, the working theory is that the depressions in the road are the result of silt and clay collapsing in the aquitard (a zone within the Earth that restricts the flow of groundwater from one aquifer to another) beneath the interstate. This silt/clay layer is possibly being pushed down into a void or loose zone below made of sand. What is causing this process though remains a mystery.

Work crews have investigated and ruled out three possible causes for the collapse including water flowing under the underground wall in a localized area, water flowing through a gap or flaw in the wall, and water flowing around the east end of the wall.

The two remaining possible causes are an increased upward water-flow gradient or buried channel relics in the vicinity of the seeps and dips.

The first option involves the wall causing underground water levels to rise on its north side and effect the aquitard in a (possibly weakened) area that doesn’t normally see those water levels. Since the wall has been built, tests have shown the water gradient on the south side has been consistently higher than the north side (i.e. not allowing the contaminated water to get past) — so if this is the cause, it would be an isolated problem relegated to where the dips are.

The second option involves underground water making its way through the aquitard and under the interstate through the use of old water channels. Undetected old channels comprised of more permeable soils could theoretically give the water a new path to flow.

Whatever the case may be, the seeping has slowed as of late, so the EPA (and other assisting agencies) plans to continue its work on the Groundwater Collection System Project. They expect that the completion of the project will further lower the north side underground water gradient.

As for the road itself, ITD will continue to survey the area and track further changes if they occur. Work crews are expected to initiate another temporary fix in mid-April, which will cause intermittent lane closures and speed reductions.

Depending on the rate of degradation, a more permanent fix could be a ways off.

“If this does really slow down like it is doing now and tapers off, you may not see a permanent fix out there till next year — or may not see one at all,” Fenn said.