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Acid Mine Discharge Adversely Impacts Community Development
In Maryland, Abandoned Coal Mine Stabilization Is Made Economical with GIS
By Joseph F. Giacinto, ERM, Inc., Annapolis, Maryland
The Maryland Department of Natural Resources Power Plant Research Project (PPRP) was formed in 1971 to ensure that Maryland meets electricity demands at reasonable costs while protecting the state's valuable natural resources. A component of ensuring reasonable costs is the cost-efficient and environmentally friendly management of coal combustion by-products (CCPs) generated by coal-fired power plants in and around the state. As a legacy of coal mining, western Maryland is associated with detailed but often nongeoreferenced maps. The threat of land subsidence from mine tunnel (void) collapse and acid mine discharge (AMD) from these abandoned mines has adversely impacted development in and around increasingly sprawling communities.
When mixed in the proper proportions with water, CCP grout is a viable and much cheaper alternative to conventional concrete and can be used to effectively fill abandoned mine voids and restore structural integrity to the subsurface, as well as mitigate AMD. The tunnel voids are filled by pumping CCP grout through boreholes that are drilled specifically to penetrate the mine voids. Therefore, an accurately georeferenced mine map for optimal borehole placement and spacing over mine voids is a prerequisite for efficient planning and implementation of mine void stabilization operations.
In western Maryland, pilot projects have been undertaken to demonstrate the effectiveness of CCP grouting in abandoned underground coal mines. The projects require georeferencing historic (circa early 1900s) mine maps typically available only in paper copies. From the initial work of scanning, digitizing, and georeferencing to production of the final maps and associated raster data, ArcGIS Desktop (ArcEditor) and ArcGIS Spatial Analyst are key components of the mine map and raster development process.
Although most of the old commercial mines were originally surveyed in a nongeoreferenced coordinate system, many of the survey reference points are inside inaccessible mines, and many former survey mine monuments have been destroyed or are overgrown. In these cases, field investigations using geophysical surveys and downhole video cameras to inspect borehole positioning with respect to tunnel voids may be required to adjust and translate the maps to a georeferenced coordinate system. A rotational adjustment to the maps is often necessary, as magnetic declinations depicted on old maps have changed since map production. Mine void grouting operations require a significant field effort of drilling anywhere from 50 to 100 or more boreholes to serve as grout conduits. The boreholes are relatively expensive to drill, and each borehole that does not hit a mine void must be redrilled. In addition to the requirements of an accurately georeferenced mine map, the internal mine geometry must be known, all shafts and entries must be located, and the regional hydrogeology must be understood before grouting operations can commence. The mine geometry includes parameters, such as mine floor elevation and slope, and the width, height, and direction of tunnels.
For developing mine floor elevation surfaces and slopes, ArcGIS Spatial Analyst is used in the associated raster development. Once the mine map and geometry (i.e., mine floor slope) is finalized, ArcGIS Spatial Analyst and ModelBuilder may be used to simulate the path of CCP grout through the tunnel network. This modeling becomes especially useful for planning field operations and optimizing grout injection points.
Developing and georeferencing the historical mine maps and associated data within ArcGIS involve an iterative multistep process. The initial step is to develop the maps based on the information available on the historical maps, the next step involves field surveys to uncover and survey any available mine monuments or reference points, and the final step involves integrating data from field surveys into the spatial adjustment of the mine maps.
With an extensive arsenal of ArcGIS tools and expertise, the Geospatial Research Group of Frostburg State University, Maryland, develops the mine map layers and provides iterative spatial adjustment and digitizing of the associated mine map layers for the western Maryland project areas.
A final refinement of the mine maps involves the integration of spatial data collected from field investigations. Under direction from PPRP, Esri Business Partner Environmental Resources Management (ERM)the environmental engineering integrator for PPRPand the U.S. Department of Energy's National Energy Technology Laboratory have conducted field studies using equipment, such as video cameras inserted into exploratory boreholes that penetrate tunnel voids, and electromagnetic geophysical surveys to refine the georeferencing and mine geometry. Downhole cameras provide valuable insight into the exact position of the borehole with respect to a particular tunnel. Although dependent on subsurface conditions, electrical geophysical surveys may be particularly valuable in assessing the general accuracy of the tunnel system and configuration on the mine maps. These geophysical studies can generate multimillion-record databases that have typically been processed, analyzed, and displayed with the power of ArcGIS. Although the tunnel networks are shown on the historical mine maps, not all tunneling activity was recorded, particularly during the end of the mine life cycle. Therefore, field surveys and investigations are a vital component of confirming the tunnel configuration and mine geometry.
Once the mine maps are finalized, GIS can be used to create a grid of optimally spaced boreholes based on the mine floor slope (calculated with ArcGIS Spatial Analyst) and the flow (rheologic) distance characteristics of the CCP grout. The CCP grout can typically flow approximately 400 feet given a four- to seven-degree mine floor slope. Borehole locations determined in ArcGIS may be extracted and distributed to field crews for pinpointing drilling locations that will penetrate the mine tunnel network. Typically, a few exploratory boreholes will be drilled for iterative map adjustment prior to the drilling of the entire borehole grid.
Attributable in part to the efficiency of grouting operations, approximately 10 years after CCP mine void grouting, favorable results of the PPRP Winding Ridge (Frazee Mine) Project include a substantial reduction in acidity and harmful metals in the mine water with cured grout strengths equal to or greater than the typical surrounding rock. The fact that the grout maintained strength and low permeability demonstrated that CCP grout is adequate for controlling mine subsidence and mitigating AMD. The CCP grouting approach has the added benefit of reducing the stress on existing landfills to continually stockpile an ever-growing inventory of CCPs from coal-fired power plants.
While the Winding Ridge project demonstrated considerable success for one abandoned mine, abandoned subsurface mines number in the thousands across the Mid-Atlantic Highlands and are often overlaid by ever-expanding communities, towns, and cities. Emergency repairs to key community roads and buildings affected by mine void subsidence are an expensive proposition and may cripple a community disaster response plan. Using ArcGIS tools, many states in the Mid-Atlantic Highlands are developing coal mine mapping projects and repositories. The next step is to prioritize the risk associated with subsidence to key community infrastructures in a manner similar to the work done for interstate highways by the Federal Highway Administration (www.fhwa.dot.gov/engineering/geotech/hazards/mine/index.cfm). Once identified and prioritized, these risks can be mitigated with proactive measures, including the use of CCP grout, which has been demonstrated by PPRP to cost an average of 50 percent less than conventional concrete grouting applications.
For more information, contact Paul Petzrick, Maryland Department of Natural Resources (tel.: 410-260-8669, e-mail: email@example.com).