In 1971, with passage of the Power Plant Siting Act, Maryland established the Power Plant Research Project (PPRP) and made the Department of Natural Resources responsible for its administration. The PPRP, which is funded by an environmental surcharge on all electricity consumed in the state, has two broad objectives: ensuring the availability of affordable electricity in Maryland in the future and protecting the state's natural resources. To help reach those goals, the project expends most of its own resources on three activities: power plant licensing, long-range energy supply planning, and environmental assessments.
One of the more interesting PPRP programs under the environmental assessment umbrella is an ongoing evaluation of the use of coal combustion products/by-products (CCPs) as backfill to reduce or eliminate drainage of acids and harmful metals from abandoned coal mines into soil and water. The evaluation is part of a larger plan to manage CCPs produced by Maryland power plants in cost-effective yet "green" fashion.
Left unstabilized, CCPs are capable of leaching soluble metals into the environment and significantly contaminating ground and surface water. Stabilized and engineered to take advantage of their pozzolanic (cementitious) properties, CCP mixtures have proven to be an environmentally benign and affordable alternative to conventional cement grouts for use as roadbase, highway embankments, and mine fill.
Two birds, one stone
As engineering consultants to the PPRP, Environmental Resources Management (ERM) Inc. has conducted several studies and pilot projects to evaluate and demonstrate the beneficial reuse of CCPs for deep mine stabilization. There are several hundred abandoned underground mines in western Maryland and several thousand across the Mid-Atlantic Highlands (Figure 1).
1. Back to the source. This map shows abandoned underground mine lands within the Mid-Atlantic Highlands. Railroads and interstate highways provide a readily accessible transportation network for moving coal combustion products/by-products (CCPs) to mine reclamation sites. Mines and power plants are typically located along rail and/or highway transportation corridors that are ideal for economical movement of massive volumes of CCP. Source: Environmental Resources Management Inc.
Stabilized CCP materials, in the form of grout, offer an attractive means of permanently restoring to productive use land and waterways that have been rendered uninhabitable by acid mine discharge (AMD). Typically, AMD-impacted streams are dosed with lime to raise their pH toward neutral—an unattractive solution because the dosing systems must be maintained in perpetuity. In contrast, permanent solutions such as mine void grouting and mine pavement sealing eliminate the source of AMD.
In Maryland and the Mid-Atlantic Highlands, the proximity of abandoned mines to coal-fired plants makes it cost-effective to transport massive volumes of engineered CCPs via the existing network of railroads and interstate highways. The voids in the Maryland mines alone have an estimated total volume of one billion cubic yards, or enough capacity to permanently (and beneficially) dispose of hundreds of years worth of CCPs generated in the state. At present, about 27 million tons of CCPs are produced annually in the Mid-Atlantic Highlands, with Maryland contributing approximately 2 million tons. The rule of thumb is that one ton of CCPs can be turned into one cubic yard of grout.
One major CCP is flyash, which is primarily a mixture of silicon dioxide, aluminum oxide, and iron oxide. In combination with an alkali activator and moisture, flyash becomes a cementitious material with excellent structural and engineering properties. Under PPRP direction, Atlanta-based Hemmings & Associates LLC has developed various CCP mix designs for grout and mine backfill that maintain long-term strength and cohesion suitable for a variety of mine stabilization requirements.
Because the chemistry and physics of abandoned mines are complex, CCP-based grout must be adaptable to a variety of underground conditions. It should be:
- Fluid enough to be pumpable, for optimal penetration of fissures and shafts.
- Strong enough to abate surface ground subsidence.
- Able to reduce acid formation.
CCP grouts also must be compatible with the often-acidic water in flooded underground mines (Figure 2). They should simultaneously be strong enough to provide control of subsidence and retain the physical and chemical integrity needed to resist dissolution and release of soluble metals. Optimizing the balance between a grout's fluidity and cementitious properties is a key challenge in the engineering of a CCP for reuse.
2. Down periscope. A panorama from a remote borehole camera shows the underground structures and conditions of the abandoned Frazee Mine in western Maryland. Mine pools often contain acidic water that could ultimately enter surface waterways through seepage or mine wall ruptures. Courtesy: Environmental Resources Management Inc.
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