4/19/2013
Reed Hollinshead
ITD Communication
208-334-8881
reed.hollinshead@itd.idaho.gov
ITD on lookout for highway soft spots and sinkholes, usually precipitation undermining road
BOISE - Highway soft spots or sinkholes, like the one in Chicago's South Side recently, usually are the result of water damage from rain or flooding, but damage may be accelerated by man-made means or by rodents.
Idaho Transportation Department crews in southwest Idaho try to stay ahead of these problems by being vigilant and knowing the warning signs.
“ITD crews know their road sections well enough that they will immediately notice an indentation or pothole that may be an indication of a developing soft spot,” said Maintenance Coordinator Dan Bryant. “They can initiate an immediate response.”
Click here for Picture 1, Picture 2 and Picture 3 of a soft-spot repair on Idaho 78 last June.
Bryant said sinkholes can develop slowly over time, or seemingly overnight.
“There is little in the way of warning in most cases,” said Bryant. “Occasionally we see the surface start to indent, as the asphalt tries to act as a bridge for a short time. Other times, there's suddenly a pothole in the road, and when you look into it, you can’t see the bottom.”
Bryant said soft spots or sinkholes are almost always a result of uncontrolled water moving below the surface.
“A broken pipe, worn-out culvert, a spring or subsurface flows from heavy precipitation or fast-melting ice and snow are common culprits. The water moves below the highway or other surface and as it flows, it carries away the fine sand and silt, which leaves spaces between the larger materials. If the flow is heavy enough, the larger materials move too. As the materials move out, they leave a void at the depth of the flow.
"The roof of the void will gradually work its way higher, as material from the top falls into the flow and is washed away. When the roof moves high enough, the surface is unsupported and caves in, especially if a heavy weight travels across it, which is why cars and trucks accelerate the cave-in and are often caught in sinkholes.”
Animals can speed up the process too, and lead to a tragic conclusion. Last summer, a Melba woman commuting to work in the dark of the morning drove her car into a sinkhole and died. In that case, the culprit was a band of gophers that had tunneled from a ditch bank under the road, undermining the surface. As water got into those holes, damage was expedited, causing a 15-foot x 40-foot sinkhole.
A lookout is often kept for burrowing animals near levees or ditch banks, as they can be the catalyst to a cave-in and flooding.
However, water usually does the damage all by itself without an accelerant.
Bryant said that although ITD remains vigilant, crews can't always stop sinkholes from happening.
"We do watch for them, but we could get surprised," said Bryant. "About 20 years ago I saw a car fall into a sinkhole at Cole Road and Ustick one day. There was no warning sign at all, until the car’s front wheels dropped through the pavement."
ITD crews do hundreds of soft-spot repairs each spring to combat potential undermining. As Bryant said, “we know what to look for and treat it quickly, trying to address it before it becomes a more serious problem.”
Drivers are asked to help by driving cautiously and being aware of the road and conditions.