Infrastructure Technology Institute
The Ohio Department of Transportation (ODOT) is deploying a continuous remote monitoring system developed with ITI support to safeguard a section of Interstate 70 in Muskingum County that is threatened by collapse of an abandoned coal mine beneath the highway. This deployment, a cooperative effort between ODOT, ITI, and Geotechnical Consultants, Inc. of Westerville, Ohio, represents the continuation of ITI’s previous work in geotechnical applications of time-domain reflectometry (TDR) technology pioneered by ITI researcher Prof. Charles Dowding, and the successful transfer of TDR monitoring technology from university research to practice.
TDR may be thought of as “cable radar”: a radio-frequency pulse is sent down a coaxial cable and the reflected signals are analyzed. In geotechnical applications, TDR cables are typically grouted into boreholes in soil and rock; the pulse will be partially or completely reflected by localized shearing of the soil or rock, complete severing of the TDR cable, or upon contact with the ground water table. In their book, Geomeasurements by Pulsing TDR and Probes, Prof. Dowding and Dr. Kevin O’Connor cited a coal mining case study in which TDR detected subsurface movement four days before movement was detected at the surface1. Thus, TDR data may be used both to provide early warning of sub-surface failures and to evaluate the performance of mitigation efforts.
ITI’s TDR applications to highway subsidence in Ohio began in 1996, when a 10x12-foot section of I-70 near the city of Cambridge in Guernsey County suddenly dropped into a sinkhole formed by the collapse of an abandoned coal mine. After the highway was repaired and the mine was grouted, TDR cables were installed underneath the highway to monitor for any additional sinking of the roadway. ITI provided the TDR data acquisition system, and ODOT personnel interrogated the cables manually for several years. In 2001, ITI began autonomous remote monitoring of the site: each night, data were downloaded and posted on a web site – without human intervention – for easy comparison with historical norms. Continuous Internet-enabled remote monitoring continued through May 2009, when the Guernsey County site was determined to be stable and remote monitoring equipment was removed.
The new Muskingum County site will feature an expanded instrumentation suite with five TDR cables installed in horizontal boreholes under the highway. The instruments will be remotely monitored before, during, and after the sealing and grouting of the mine to provide increased confidence in the safety and functionality of the highway.