Interstate Technical Group on Abandoned Underground Mines
Fourth Biennial Abandoned Underground Mine Workshop
Abstract: Real Time Monitoring of Subsidence I-77, Summit County, Ohio
The Ohio Department of Transportation (Ohio DOT) encountered abandoned underground coal mine subsidence during an investigation conducted as part of a project to widen I-77 in Summit County, Ohio. A decision was made to stabilize 2000 feet of four-lane highway by backfilling the mine voids with cement grout while traffic remained on the interstate. Due to the potential for additional mine subsidence during remediation work, Ohio DOT required the installation of a real-time monitoring system to activate an alarm when movement or settlement of the road base exceeded threshold values. Innovative monitoring of ground deformation was accomplished using time domain reflectometry (TDR). This technology is a form of RADAR in which voltage pulses are transmitted along coaxial cable and reflections are created at every location where the cable is being deformed. The distance to each location is determined by the pulse travel time, and the magnitude of deformation is determined by the magnitude of the TDR reflection. This allows for real time monitoring of all locations and the rate of deformation where ground movement is causing deformation of the cable. The cables were installed by a directional drilling contractor in horizontal holes drilled 5 feet or more beneath each lane of the entire section of highway. The cables were connected to a central remote data acquisition system that automatically recorded and stored measurements. The initial record for each cable was stored as the baseline measurement. When the magnitude of any TDR reflection differed from the baseline value by a specified amount, the system initiated a phone call to GeoTDR personnel who could then analyze TDR measurements in real time via a phone line connection to verify that the alarm was associated with cable deformation. If it was determined that actual movement of soil had occurred, Ohio DOT personnel were alerted to intensify visual reconnaissance and determine if lane closures were necessary. This project demonstrated the capability of real time monitoring of ground movement over a wide area utilizing a horizontal application of TDR technology.
* President, GeoTDR, Inc., 720 Greencrest Drive, Westerville, Ohio 43081-4902
** Special Projects Coordinator, Ohio Department of Transportation, Office of Geotechnical Engineering, 1600 West Broad Street, Room 2033, Columbus, Ohio 43223
*** Senior Geologist, Ohio Department of Transportation, Office of Geotechnical Engineering