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14TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 5 Background In 2004, several sinkholes formed along US 18 between STA 248+00 and 250+00 (distance of 200 m or 600 feet). A couple of sinkholes formed on the shoulders of the eastbound lane (Figure 1 and Figure 2). In addition, several small sinkholes and one rather large sinkhole formed in the ditch of the eastbound lane. A geophysical investigation, to determine the cause and extent of the problem (and propose options for remediation) included a Ground Penetrating Radar Survey, a Resistivity Survey, and a Soil Survey (drilling program). No voids were identified beneath or within a couple meters of the pavement surface. It was decided to continue to monitor the area for sinkhole formation and to investigate the integrity of the roadway should additional sinkholes form within the limits of the area of concern (Figure 3 STA 248+00 to 253+00, 500 m or 1500 feet), as determined by a geologic review and analysis of the drilling data. In addition, as a remediation to the problem, a double reinforced inlay or overlay was proposed for the area of concern, as soon as it became practical. Abstract Sinkhole formation is a common occurrence in northeast Iowa, and US 18 in Cerro Gordo County was constructed over an area where sinkhole formation had only been locally known. It had not been recorded or identified in the Iowa DNR database at the time. Since 2004, sinkholes have developed along the right of way. Geophysical surveys contributed very little in the identifying the cause. However a Soil Survey (drilling program) identified numerous voids within carbonate bedrock. The soil borings indicated that shale overlying the carbonate rock has been removed/ eroded, and resulted in the development of a karst subsurface through the dissolution of the carbonate rock. Without removing the structural fill and site soils to expose the rock, it will not be possible to impede the natural processes occurring. An alternative approach was adopted and consisted of: (a) removing the existing pavement, (b) installing coaxial cables in trenches excavated within the subgrade, (c) replacing the pavement as double reinforced pavement (including shoulders), and (d) 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: (a) information in the activity file, and (b) updated plots for each cable. Unexpected behavior has been observed, with activity occurring annually between the months of September and March. Although several explanations have been proposed, there is no definitive correlation between locations of the activity detected by TDR, sinkhole locations, or geophysical anomaly locations. In spite of this uncertainty, real time remote monitoring for ground movement is continuing. MONITORING THE THREAT OF SINKHOLE FORMATION UNDER A PORTION OF US 18 IN CERRO GORDO COUNTY, IOWA USING TDR MEASUREMENTS

Monitoring the Threat of Sinkhole Formation Under a Portion of US 18 in Cerro Gordo County, Iowa Using TDR Measurements