Mine subsidence is the most likely culprit behind further work on Illinois 15 between Illinois 158 and Illinois 159, and the reason for closed westbound lanes Friday.
"We first discovered (a problem) about three weeks ago, with the milling of the old pavement. A dip developed. ... Then a couple weeks later we discovered another void," said Jeff Church, engineer with the Illinois Department of Transportation.
Westbound traffic is being detoured to Illinois 13.
IDOT is working with the Illinois Department of Natural Resources to determine the "limits of the void", Church said.
An engineer with the Department of Natural Resources was at the site Friday afternoon to take core samples of the road.
Bruce Schottel, with IDNR, said mine subsidence issues are pretty common in the area, but "not necessarily under highways."
Drivers in the area might not notice the damage because the fresh pavement acts "as a bridge" over the void, Church said.
IDOT says the repaving of Illinois 15 is still on schedule, with work yet to be done on shoulders and ramps.
For the problem area, "the goal is one lane open this afternoon (Friday) and the other open tomorrow afternoon (Saturday)," Church said.
IDOT announced earlier this week that all non-emergency work on Illinois highways would cease over the holiday weekend. Construction zone speed limits remain in effect even if workers are not present.