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Managing Car-Crunching Sinkholes
A 4-meter section of the eastbound driving lane of Interstate 70 in Guernsey County, Ohio, suddenly collapsed on March 4, 1995. The collapse was caused by the caving in of an abandoned underground mine. Although no vehicle fell into the sinkhole, four vehicles swerved around, dipping some wheels in the hole. Fortunately, no serious injuries were sustained. The Ohio Department of Transportation (ODOT) recognized that other mines may exist beneath interstate highways and other roadways and that other significant settling or sinking could occur due to the increasing age of the abandoned mines. Consequently, an effort was initiated to develop and implement an Abandoned Underground Mine Inventory and Risk Assessment process.
This process, as documented in a recently adopted ODOT manual, is a proactive response to the need to locate and assess the risk of all roadway sites beneath which abandoned underground mines exist. The overall purpose of this inventory and risk assessment effort is to enhance the safety of the traveling public by minimizing the possibility of a sudden roadway collapse that could result in fatalities or bodily injuries. The first reported production of coal in Ohio was in 1800, and since then, various types of recoverable resource mining have occurred in the state.1 Most of the abandoned underground mines in Ohio range in age from 50 to 150 years. Detailed abandonment maps are available for approximately 4,600 underground mines. However, for an estimated 2,000 mines in Ohio, no detailed maps are available. The development of an Abandoned Underground Mine Inventory and Risk Assessment process for all state roadways in Ohio is a formidable task. Hundreds of roadway sites may lay atop underground mines. These roadway sites represent an existing, undefined, and yet possibly significant risk to the safety of the traveling public. Counties in 10 of the 12 ODOT districts contain identified mines for which there are available maps. The Ohio manual was developed with input from the Federal Highway Administration, the U.S. Department of the Interior's Office of Surface Mining, the U.S. Geological Survey, two divisions of the Ohio Department of Natural Resources (Division of Mines and Reclamation and Division of Geological Survey), the Ohio Environmental Protection Agency, the Ohio Mine Subsidence Insurance Underwriting Association, West Virginia University's Department of Mining Engineering, and nine state transportation agencies. During development of the manual, ODOT examined several roadway sites. These investigations were made in the interest of preventing subsidence (or sinkage) similar to the I-70 collapse from occurring again while the inventory and risk assessment process was being developed. Remedial
Response During Process Development I-70 in Guernsey
County The length of the project area was approximately 610 linear meters. The remedial effort involved the air rotary drilling of approximately 1,800 boreholes down to the mined coal seam interval - a depth of approximately 20 meters. Approximately 13,800 cubic meters of flyash grout were pumped into subsurface void areas. Two land bridges with lengths of 213 meters and 34 meters, respectively, were constructed over areas where the drilling and grouting program encountered high concentrations of caved and broken materials at the mined level. The cost of this project, including land bridge installations and pavement replacement, was approximately $3.6 million. I-70 and I-77
Interchange in Guernsey County The project entailed approximately 9.2 lane-kilometers of roadway, including the work on all mainline lanes and ramps. The remedial effort involved the air rotary drilling of approximately 2,600 boreholes to the mined coal seam interval at a depth ranging from 3 to 30 meters. Approximately 61,000 cubic meters of flyash grout were pumped into subsurface void areas. The cost of this project was approximately $4.7 million. I-470 in Belmont
County Through investigations, it was determined that the overburdened rock was extremely fractured. A large void was found to be migrating toward the surface. Consequently, it was decided to immediately close the roadway and remediate the site by excavating to the base of the mined coal seam and then backfilling. The resulting mine
remediation project involved approximately 306,000 cubic meters of roadway
excavation, new pavement, signs, lights, guardrail, striping, and revegetation.
The project was completed in 14 weeks at a cost of approximately $3 million.
Long-Term
Process Approach
The ODOT Abandoned Underground Mine Inventory and Risk Assessment process is the most logical and practical approach to responsibly monitor the safety of the roadways in areas with abandoned underground mines. Due to the large number of potentially problematic sites, it is not logistically or financially responsible to commit limited resources and funding to random investigation and remediation of sites. The process comprises four basic activities, as shown in figure 1:
Establishment of an Inventory
of Sites
Risk Assessment Initial Site Evaluation
Site Monitoring All confirmed inventory sites are subject to permanent site monitoring. Permanent site monitoring provides a feedback loop in the process to detect changes in conditions that might warrant site re-evaluation. This aspect of the process makes it a dynamic, responsive risk management system. The stability of abandoned underground mines and associated overburdened strata will continue to deteriorate, at least in some cases, with increasing age.
Detailed Site Evaluation Priority Site Investigations Whether or not remediation is recommended, all sites remain on the roadway inventory of abandoned underground mines and are periodically monitored. Remediation
Existing conditions may change, or new conditions may develop on the site during the period required for contract document development. Guidance is included in the manual for continued site monitoring during the development of construction contract documents. Remedial Construction Existing conditions may change, or new conditions may develop on the site during remedial construction. Certain forms of remediation may unintentionally induce additional mine-related settlement. Therefore, the manual provides guidance for site monitoring to detect possible changes during remedial construction. Emergency Action/Road Closure
The proposed risk assessment techniques will direct the prioritized investigations and prioritized site remediation where necessary. The higher risk sites will be the first sites where field work and associated expenses will be incurred. This type of site will more likely require immediate emergency construction and/or top priority, non-emergency construction than other sites. Some basic principles of the Abandoned Underground Mine Inventory and Risk Assessment are:
The Future
Reference
L. Rick Ruegsegger is the special projects coordinator for the Geotechnical Design Section of the Ohio Department of Transportation's (ODOT) Office of Materials Management. His current primary responsibility is to define and establish a statewide inventory and risk assessment process for roadways above abandoned underground mines. His work for the state of Ohio since 1974 has included 3½ years with the Ohio Environmental Protection Agency, 15 years with the Ohio Department of Natural Resources (ODNR), and six years with ODOT. His 15 years with ODNR involved project design and construction in the Abandoned Mine Lands Program of the Division of Mines and Reclamation. He has bachelor's degrees in civil engineering and biological sciences from The Ohio State University, and he is a registered professional engineer. Thomas E. Lefchik is an assistant bridge engineer with the Ohio Division Office of the Federal Highway Administration. He is also responsible for geotechnical matters. He has a bachelor's degree in civil engineering from the Pennsylvania State University, and he is a registered professional engineer.
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