Monitoring the threat of sinkhole formation under a portion of US 18 in Cerro Gordo County, Iowa.
Sinkhole formation is a common occurrence in northeast Iowa and U.S. 18 in Cerro Gordo County was constructed over an area where sinkhole formation had only been locally known and had not been recorded or identified in the Iowa DNR database at the time. Since 2004, sinkholes have developed along the right of way but it has not been possible to definitively identify the cause based on geophysical surveys. Detailed information obtained with soil borings indicates that shale overlying the carbonate rock has been eroded and allowed karst dissolution of the carbonate rock. Without removing the structural fill and site soils to expose the rock, it will not be possible to arrest the natural processes occurring. An alternative approach was adopted and consisted of: removing the existing pavement, installing coaxial cables in trenches excavated within the subgrade, replacing the pavement as double reinforced pavement (including shoulders), and monitoring the cables using Time Domain Reflectometry (TDR). The cables are interrogated several times a day and data is transmitted via cellular modem to Iowa DOT facilities. Among the data transmitted is a log file of deformation activity along each of the cables which is evaluated and an action plan is initiated based on information in the activity file and updated plots for each cable.