ITD is looking for highways that have a soft spot in their heart – or at least in the driving lanes.
Less affectionately known as sinkholes, or large potholes, soft spots like the one that surfaced recently in Chicago's South Side, usually result from water damage caused by rain or floods, but damage may be accelerated by man-made means or by rodents.
ITD maintenance crews in southwest Idaho try to stay ahead of those problems by being vigilant and recognizing 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.”
“There is little in the way of warning in most cases,” Bryant said. “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 usually are the 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 also can speed up the process and lead to a tragic conclusion. Last summer, a Melba woman commuting to work in morning darkness drove her car into a sinkhole and died. In that case, the culprit was a band of gophers that tunneled from a ditch bank under the road, undermining the surface. As water reached those holes, damage was expedited, causing a 15-foot by 40-foot sinkhole.
Burrowing animals near levees or ditch banks are constantly monitored because they can be the catalyst to a cave-in and floods.
However, water alone usually is responsible for the damage without any other contributor.
Although ITD remains vigilant, Bryant said crews can't always prevent sinkholes.
"We do watch for them, but we could get surprised," he explained. "About 20 years ago, I saw a car fall into a sinkhole at Cole Road and Ustick. There was no warning sign at all, until the car’s front wheels dropped through the pavement."
ITD crews repair hundreds of soft spots 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 highway and conditions. They also can help ITD address sink holes by calling the respective district office and providing the location of the problem area.