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REMOTE MONITORING OF MINE SUBSIDENCE WITH TIME DOMAIN REFLECTOMETRY
Charles Dowding, Department of Civil
Engineering
Table of Contents
As part of our pioneering effort to develop practical remote monitoring tools for the infrastructure, the Infrastructure Technology Institute (ITI) at Northwestern University has installed a mine subsidence monitoring system in Cambridge, Ohio. The experimental system was placed at the site on Interstate Highway 70 where a portion of the highway had collapsed due to a sinkhole created by coal mine subsidence (for more on the highway collapse, see the Akron Beacon Journal article). The system is being evaluated in a one-year cooperative effort with the Ohio Department of Transportation (ODOT). This effort involves both ITI staff and Geotechnical Engineering Faculty from Northwestern's McCormick School of Engineering and Applied Science. Professor Charles Dowding of Northwestern developed the technology that uses cable sensors to measure soil and rock stability. This report addresses the process of insitu instrumentation in general and the parallel relationship of the TDR test facility to ongoing surveillance by ODOT. There are three main phases to an instrumentation: 1) installation, 2) observation, and 3) interpretation and response. ODOT has completed these phases, and ITI's experimental facility represents a fourth phase, automation of the data acquisition. This fourth phase is separate from, and operates in parallel with, ODOT's conduct of phases one through three. The test facility is designed to allow ODOT and other DOT's to determine the economies of remotely operable systems for surveillance of critical facilities. Phase One: Installation and Placement of TDR cables The cable itself is a common coaxial cable TV distribution line. The purpose of the system is o detect subsidence indirectly by monitoring the cables for distortion using time domain reflectometry (TDR). In TDR, a cable tester sends a voltage pulse down the cable. This pulse will reflect off of any changes in impedance (crimp, shear, break, etc.). The cable tester then records the reflected voltage versus time. This data can then be displayed in any software with plotting capability. The plot gives information about the type of distortion, relative magnitude of distortion, and distance from the cable tester. An undisturbed cable would appear as a flat line with a positive spike at the end. An open circuit appears as a positive spike, and a short circuit appears as a negative spike. Any connectors in the circuit also introduce spikes. The soil and rock between the mine ceiling and the interstate highway are being monitored with two cables approximately 500 feet apart (FIGURE 1 shows the electronics with Interstate 70 visible in the background). Fifty-foot-long coaxial cables were placed in vertical bore holes in the road shoulder and then grouted into place (FIGURE 2 shows one of the cable heads at the bore hole site). Placement of the cables by ODOT presented a challenge, as unlike above-ground bridges and pavement, the subsurface is not visible. FIGURE 3 shows the roadway passing over an unverified and historical map of the mined-out area; the exact location of abandoned mine rooms is unknown, and cannot be seen. The material and condition of the mine roof was observed only at the location of the two TDR holes; even that observation was made by the driller and as such, conveys only the grossest of properties. FIGURE 4 shows the tan roof rock overlying the black coal seam at an abandoned opening (gray) in a near-by road cut. Finally, Ground Probing Radar (GPR) experiments revealed no subsurface cavity information, despite their known existence. As with strain gages on a bridge, the ability of TDR cables to monitor critical behavior depends upon their placement at a critical location. All such contact instruments only measure response in their vicinity. This proximity restriction is as true for strain gages placed on bridges as it is for settlement points, extensometers, TDR cables and slope indicators for underground structures. Adding to the complexity of the situation is the unknown state of the rock and caverns away from the cable locations. Since the cables were placed vertically they can respond most accurately to the stability of the nearest opening(s). Phase Two: Observation through Monitoring Manual monitoring The least complex method of monitoring involves recording TDR data at each cable with a single, field portable instrument. This approach allows ODOT to obtain data directly with no involvement or other parties in order to guarantee the safety and stability of I-70. It remains important that data be acquired by ODOT often so as to be aware of any indicators of movement as soon as they manifest themselves in the cable response. Remote monitoring The key to success of this technology is the use of remote monitoring. Ohio DOT is currently monitoring the cables manually. The primary drawback to this approach is the need for a DOT employee to visit the test site regularly and gather data. The current practice is to gather data at weekly time intervals. It is important to increase this frequency since the failure or collapse may occur in a few days or hours. The remote monitoring system will allow ODOT to access the test at their convenience and gather data daily or whenever needed. There are two types of remote monitoring systems being tested simultaneously in Ohio. Both are powered by a solar panel and 12 volt storage battery and use a cellular modem for communication. A hard wired phone line and 110V power can also be used if necessary. The first type is referred to as the "remote intelligence" system. In this setup the Tektronix cable tester is connected directly to the cellular phone/modem by a null modem RS-232 serial cable. A personal computer at another/or remote site is then used to call the cable tester. The cable ester software and any data interpretation programs are run on this remote PC. A low power time turns on the cable tester and call phone/modem for a fixed call in a window of fifteen minutes each day to conserve power. The second type is referred to as the "local intelligence" system. In this setup, a field rugged personal computer is placed on site. This computer is running PC-Anywhere remote control software and is connected to the cell phone/modem and the Hyperlabs cable tester via RS-232 serial cables. The cable tester software and any data interpretation programs are also run on this local PC. The PC remains in a very low power sleep mode, waking itself at programmable intervals to test the cables. Power is switched on and off by the PC. The data is then stored locally on its hard drive. The data can be retrieved over the modem or an interpretation program can be set to call out when an alarm condition is detected. Latching microwave A-B switches actuated by a 12 volt signal are used to switch between the two cable testers and cables. A-B switches were used in this installation because of the small number of cables, but large switching matrices are also available. A RS-232 serial port controlled bank of four relays is used to actuate the microwave switches. This device can be used to remotely choose which cable and which cable tester are selected in a remote or local intelligence based system. The remote intelligence configuration places less equipment in the field and has a lower cost, but is limited in flexibility. The local intelligence configuration places more equipment in the field and has a higher cost, but is extremely flexible. Call in windows and testing schedules can be changed remotely. Any type of additional PC based sensors or control devices can be utilized. All hardware and software used has been off the shelf. Phase Three: Interpretation of TDR Signals With successful completion of Phases One and Two, it is then necessary to interpret, and act upon, the data obtained from the TDR cables. Dr. Kevin O'Connor, of GeoTDR, Inc., a consulting firm specializing in the interpretation of TDR, signals has been engaged to verify the interpretation of ODOT personnel. Phase Four: Automation of Data Acquisition The experimental test bed at the I-70 site was installed to field test various equipment necessary to automate TDR data acquisition. The project is being supported through funds provided by ITI by US DOT for the commercialization of new instrumentation schemes. The purpose of this experiment is to:
Future plans include development and evaluation of automated data acquisition and analysis software that will result in a system that alerts the customer when changes in the cable sensors occur. Other potential applications include monitoring of the stability of bridge foundations in scour susceptible locations and structural integrity following a seismic event.
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