Manual for Abandoned Underground Mine Inventory and Risk Assessment
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. Fortunately, no serious injuries were sustained although four vehicles drove through the sinkhole.
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 of the mine workings 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, U.S. Department of the Interior's Office of Surface Mining, the U.S. Geological Survey, the divisions of Mines and Reclamation and of Geological Survey of the Ohio Department of Natural Resources, 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 field investigated 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
Including the repair of the I-70 collapse site, three notable mine remediation projects on interstate highways have been undertaken by ODOT in response to roadway conditions suggesting mine-related subsidence activity.
I-70 in Guernsey County
The first of these mine remediation projects was undertaken in the area of I-70 in Guernsey County in eastern Ohio between March and July 1995. After investigations by ODOT revealed possible mine-related subsidence activity, a consultant was retained for the purposes of project investigation, design, and construction management assistance. Unfortunately, in spite of ODOT's monitoring of the site every four hours, a 4-meter-diameter by 3-meter-deep hole suddenly opened in the eastbound travel lane of I-70. Three cars and a truck were damaged when they drove through this hole. Fortunately, no critical injuries resulted from this incident. However the road was closed for more than three months.
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 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 second mine remediation project was undertaken in the interchange of I-70 and I-77 in Guernsey County between June and November 1995. The office and field investigations of this site revealed the need to perform remediation work on portions of all mainline lanes and all ramp lanes with the exception of one ramp. Conditions at one ramp necessitated the immediate closure and remediation of the ramp while project investigations and design were completed. 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
The third mine remediation project was undertaken on I-470 in Belmont County in extreme eastern Ohio between September and December 1996. This project area included 518 linear meters of interstate highway just west of the Ohio River bridge connecting Bellaire, Ohio, and Wheeling, W.Va. This project area had exhibited pothole subsidence in ditch and backslope locations. It was designated by ODOT as a study area for the field testing of investigative techniques and site evaluation techniques being drafted for the manual.
Through investigations, it was determined that the overburden 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:
1. Establishment of an inventory of all roadway sites beneath which abandoned underground mines may exist.
2. Assessment of the risk posed by each site to the safety of the traveling public.
3. Remediation of sites, if necessary.
4. Permanent monitoring of sites.
Establishment of an Inventory of Sites
The first step in this process is establishing an inventory of sites. An initial comprehensive site listing is based on: (1) a review of available records, (2) field report forms, (3) follow-up investigation of field report forms, and (4) field visits to all identified potential sites.
A large portion of the overall process is the risk assessment of each site on the inventory. The risk assessment portion of the process involves three levels of site evaluation: (1) initial site evaluation, (2) detailed site evaluation, and (3) priority site investigations and recommendations. The risk assessment criteria used for all levels of site evaluation take into account two basic factors: (1) the existing site conditions and (2) the level of the traveling public's exposure to those conditions.
Initial Site Evaluation
This initial site evaluation subdivides the entire inventory of sites into five risk-assessment groups. The groups, listed in order from the highest to lowest risk level, include: (1) surface deformation, (2) mine opening; (3) high rating, (4) low rating, and (5) eliminated sites groups. The first three groups proceed to the detailed site evaluation portion of the process. The low rating group is placed under a permanent monitoring program and remains as active files in the inventory program. The eliminated site group becomes inactive, but permanent, record files of the inventory program.
Interim periodic monitoring is initiated on all inventory sites following the initial site visits. The frequency and extent of required monitoring activities depend on site conditions as they can be identified at this point in the process. All confirmed inventory sites are subject to permanent site monitoring. Permanent site monitoring provides a feedback loop in the process to detect changed conditions that might warrant site reevaluation. This aspect of the process makes it a dynamic, responsive risk management system. The stability of abandoned underground mines and associated overburden strata will continue to deteriorate, at least in some cases, with increasing age.
Detailed Site Evaluation
The detailed site evaluation produces a prioritized listing of the sites for the surface-deformation, mine-opening, and high-rating groups. The inventory of sites at this point in the process are subdivided into risk-level groups with the prioritization of individual sites within each of the three highest risk groups.
Priority Site Investigations
Priority site investigations are performed on each site in the three highest risk groups. These site investigations are performed in order according to the prioritized listing of the sites within each of these groups. All sites within a particular high risk group are evaluated before the evaluation proceeds to sites in the next lower group.
Priority Site Recommendations
The priority site investigations result in priority site recommendations. These recommendations document the need to either remediate defined site conditions and periodically monitor the site following construction or to defer remediation and periodically monitor the site. Some recommendations may involve emergency action or temporary roadway closure.
Whether or not remediation is recommended, all sites remain on the roadway inventory of abandoned underground mines and are periodically monitored.
Development of Construction Documents
Guidance is included in the manual for the development of remedial construction contract documents. Regardless of the extent of investigations performed, the actual site conditions cannot be fully determined prior to construction. Therefore, the manual places emphasis on flexibility of methods, quantities, and project limits.
Existing conditions may change, or new conditions may develop on the site in the period required for contract document development. Guidance is included in the manual for continued site monitoring during the development of construction contract documents.
Guidance is included in the manual for remedial construction. General information is provided regarding the importance of close inspection of the work, monitoring of time and materials usage, and accurate record-keeping. Accurate construction records will be invaluable for post-construction monitoring and reference in case future subsidence conditions occur adjacent to the project area.
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
It is recognized that, at any point in the site investigation process, conditions may be discovered which warrant consideration of the need to close a roadway or portion of roadway to protect the traveling public from exposure to potentially hazardous conditions.
A comprehensive, statewide, abandoned underground mine inventory, using standard risk assessment site evaluation criteria applied to existing information, will generate an initial prioritized listing of the locations where abandoned underground mines may exist below roadways under ODOT's jurisdiction.
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:
- Work on the highest risk site at all times.
- Be as informed as possible before committing resources to a site.
- Be prepared to encounter "worst case" conditions for the nature of the site being investigated or remediated.
The benefits of this process are:
- Public Safety - The possibility of sudden abandoned underground mine subsidence in roadways, which could result in fatalities or bodily injuries, will be minimized.
- Reduced Liability - The process will identify and prioritize high risk sites permitting a systematic response.
- Budgetary Mechanism - The process, when combined with historic construction cost records, can be used to develop projected costs and budgets to reduce risks to a predetermined level.
Informational Resource - ODOT will benefit from this inventory process by creating a new database of information available to all staff. This database will be a tool used to avoid or anticipate potentially unstable underground conditions during project planning, design, construction, and maintenance.
Cooperation in the development of the manual and subsequent workshops has led to the formation of a 13-state technical working group to continue sharing information and to cooperate on related research. A Web site is being developed to assist in the sharing of information.
Douglas L. Crowell. History of the Coal Mining Industry in Ohio, Bulletin 72, Ohio Department of Natural Resources, Division of Geological Survey, 1995.
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 of work 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 has collateral duty responsibility for geotechnical matters. He has a bachelor's degree in civil engineering from the Pennsylvania State University, and he is a registered professional engineer.