Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
6 Limiting Potential Failures A coal waste impoundment can be viewed as a system composed of a series of elements that include the embankment, principal and emergency spillways, runoff diversion structures, the basin, barriers, slurry delivery and water recovery systems, and any additional drainage systems installed to protect the integrity of the structure. Analysis of the ways individual elements can fail when coupled with analysis of the impacts a specific failure or set of failures will have on the entire system allows a designer to evaluate ways to mitigate effects of individual failures. The objective is to design and assess facilities so as to avoid failures that compromise the impoundment system integrity. An embankment or basin can each fail in a number of ways. It is essential to understand these failure modes and take appropriate measures to mitigate them. This chapter examines embankment and basin failure modes and mitigative measures. Particular emphasis is placed on basin failure modes and mitigation activities, because the largest remaining uncertainties for impoundments lie in the characterization of the basin area and in the mitigation of risks associated with the breakthrough potential (see Chapter 3~. Risk assessment of new and existing impoundments is the first step toward risk reduction. Once the impoundments with the highest risk are identified, various methods can be explored to manage or reduce the level of risk. Failure can be initiated through faulty construction and operation of coal waste impoundments. The role of the impoundment operators is discussed, including the establishment of best practices and management systems and emergency planning and risk communication. Creating an impoundment, particularly near the head of a valley, which is usually a groundwater discharge zone, can also have significant conse- quences for the local hydrogeological regime. In the worst case, changes in the local hydrogeologic flow setting caused by an impoundment, could contribute to mine-associated blow-outs that can cause flooding or other environmental damage. Although a detailed analysis and discussion of blow- 111
112 COAL WASTEIMPOUNDMENTS outs is not within this committee's Statement of Task, the hydrogeological implications for impoundment siting should not be neglected in design. A comprehensive systems approach should be applied to the entire impoundment structure when analyzing potential failure modes. While it is important to view the entire impoundment as a system, it is also useful to review the modes of failure that can occur for major components of an impoundment the embankment and the basin. The next sections provide that review. EMBANKMENT FAILURE MODES An embankment to contain coal mining waste is similar to an embank- ment to store and contain mine waste in other extractive industries, where they are usually called tailings dams. While the nomenclature is different in the coal business refuse impoundment versus tailings dam, fine refuse versus tailings, and coarse refuse versus waste rock the concept of impounding slurry behind an engineered embankment is the same. Coal refuse impoundments are a subset of a larger group of mine slurry impound- ment systems called tailings dams. It is common for tailings dams to be of the upstream, centerline, or downstream types (see Chapter 3~. While the design and construction of tailings dams draws on the technology used for water-storage dams, tailings dams differ from water-storage dams in a number of important ways. They store primarily mine waste and only secondarily water. Tailings dams are often constructed from components of the mine waste stream and are usually built by mine operators over the life of the mine. Finally, the allowable seepage may be more restricted than with water storage because of environmental concerns. There have been a number of worldwide surveys of tailings dam failures (e.g., ICOLD, 2001; Martin and Davies, 2000; USCOLD, 1994; Vick, 2000~. Because the range of experience in these reviews is much greater than that in coal waste embankments alone, they provide a record of potential failure modes, many of which are directly relevant to failure modes that could befall coal waste embankments. The U.S. Committee on Large Dams (USCOLD, 1994) defines incidents as dam breaks or loss of impoundments leading to a release of tailings and impoundment fluids; accidents that stressed the dam in some form without release; and groundwater contamination. A primary factor differentiating incident cause was the type of dam construction. The distinction of causes in terms of their relative proportion differs between upstream-type and ._
LIMITING POTENTIAL FAILURES 113 downstream-type dams (S. Vick, consultant, personal communication, 2001). The preponderance of failures occurred during active operation of the impoundment with surface water on the tailings deposit. By contrast, only a few failures occurred for inactive impoundments subsequent to water removal upon abandonment. These were caused mostly by overtopping attributable to inadequate post-closure spillways (S. Vick, consultant, personal communication, 2001~. Slope instability and earthquake effects dominate failure causes for upstream-type `dams. However, seepage, overtopping, and foundation instability are other important considerations. In recognition of the need to improve the design and construction of tailings dams, a number of guides to good practice have been prepared (e.g., ICOLD, 1989b, 1994, 1995a, b, 1996~. Frequency of failure has declined during the past decade, which has been attributed to more rigorous engineering and regulations (Cambridge, 2001~. The failure at Buffalo Creek (Sidebar 1.3) was a pivotal experience in U.S. practice with regard to the need to improve design and regulation of coal waste embankments. Since MSHA was established, there have been rho incidents of embankment instability, other than overtopping of starter dams early in construction. Nevertheless, it is prudent not to be complacent. Worldwide experience with upstream-constructed tailings dams indicates that many, particularly those with wide subaerial beaches, have performed well in significant seismic events, when subjected to intense rainfall, and sometimes in spite of questionable operating practices. Davies and Martin (2000) outline the requirements for construction of a safe upstream tailings dam. Unless alternative processing and disposal methods are adopted on a large scale (Chapter 7), embankments in the future will likely be higher than in the past. Given this challenge, and given the fact that some modern dams in other extractive industries have failed, the committee concludes that it is essential that MSHA and OSM stay current by ensuring that design criteria reflect the latest experience from all segments of the mining industry. Although the committee has not identified any deficiencies, it is a matter of due diligence that MSHA, OSM, and industry employ the best available current technology. The committee recommends that MSHA and OSM continue to adopt and promote the best available technology and practices with regard to the site evaluation, design, construction, and operation of impoundments. For example, MSHA and OSM should commission periodic reviews of existing technical procedures and practices, with particular attention to the basin. Results of the reviews should be disseminated to industry. Based on the outcome, MSHA and OSM may have to revise guidelines to establish minimum expectations and levels =
114 COAL WASTEIMPOUNDMENTS of investigation for site characterization, design, construction, operation, and closure of coal refuse impoundments. Structural Stability Structural stability is based on the same general principles as the stability of water-storage dams, and structural failure generally occurs by the same processes: slope instability, liquefaction (commonly due to seismic activity), and foundation failure. Evaluation of slope stability relates the resisting forces of the embankment and its materials to the driving forces of the impoundment. Evaluation of the seismic stability of the embankment is based on the seismicity of the site and the potential for the embankment material to liquefy or lose strength during shaking. The committee has identified the following factors that merit special attention in stability assessment, in particular for upstream-constructed embankments: . . . - Long-term dfurability of the coarse refuse. Since the coarse refuse is a component of the embankment structure, its resistance to weathering or durability should be assessed with respect to future settlement and shear strength. As discussed in Johnson (1999) and Linsey et al. (1982), the long-te~n performance of rock depends on the rock type as well as the site climate and setting for which the rock is used. Differing rock types vary in durability, with certain sedimentary rocks more susceptible to weathering due to the presence of clay minerals, which expand with moisture. Where coarse refuse is used for critical portions of the embankment, its durability should be assessed for the presence of these weathering minerals. Shear strength and higher embankments. For rockfill embankments, the shear strength characteristics of the granular materials in the embankment may change with increasing stresses as individual particles crush or break (tops, 1970; Marsal, 1973; Wilson and Marsal, 1979~. The shear strength and performance of embankment materials for a smaller embankment, for example, may not be the same as those for a high embankment because of particle crushing. The performance of the coarse refuse should be confirmed by shear strength testing under anticipated loading conditions. Increasing clay content in fine refuse. Fine refuse impoundments contain increasing amounts of clay, because more coal seams are
LIMITING POTENTIAL FAILURES 115 being mined that contain increasing amounts of clay partings. Since the clay particles typically slow the rate of consolidation and reduce shear strength properties of mill tailings and fine refuse (Vick, 1990), the consolidation and shear strength characteristics of the fine refuse should include the anticipated amount of clay that would be present. Seismic evaluation. Coal refuse impoundments are designed for stability under anticipated static and seismic conditions during the critical phases of construction and operation. For closed refuse impoundments, these stability conditions still apply, but the period for acceptable performance is indefinite. This means that for evaluation of seismic conditions (as well as precipitation events), events representing long-term recurrence intervals are required (such as SOO-year to maximum credible earthquake loading conditions). These long-term seismic loading conditions, coupled with whether the refuse impoundment remains saturated, should be included in the evaluation of the post-closure performance of slurry impoundments. Seepage and Piping Seepage through embankments can lead to failure by internal erosion, the process commonly known as piping. In design and construction of water- storage embankments, the possibility of seepage-caused piping is commonly prevented by installation of filters or drains. Although the use of internal filters or drains in embankments is not common practice in Appalachia, drainage through the embankment is an important consideration in the design and construction of the structure. The coarse coal refuse filters the fines while allowing clear seepage to flow. The drains that are included provide a secondary line of defense for the control of seepage. The lack of an internal filter is acceptable, but it places an extra burden on high-quality compaction control and the use of subaerial beaching at various locations around the perimeter of the impoundment (the beach area created near the discharge point of the slurry transfer pipe) as an additional line of defense. Beaching should also be considered as a line of defense around the basin.
116 COAL WASTEIMPOUNDMENTS Overtopping A coal refuse impoundment must be designed so that inflow does not exceed storage and outflow capacity. If inflow exceeds storage plus designed outflow (including that handled by spillways, decant facilities, diversion structures, and evaporation), the dam will be overtopped. Overtopping can cause substantial erosion of the embankment crest, which, if left uncontrolled, will usually work progressively downward, releasing water and coal refuse downstream. Inflow includes: direct precipitation onto the impoundment area; runoff from the contributing drainage basin due to precipitation or snowmelt; groundwater inflows to the basin; outflow from other ponds in the basin; mine-water disposal; and preparation-plant slurry. Especially important are natural floods produced by major storms. The designed precipitation event used for a coal refuse impoundment will vary depending on the hazard classification of the facility, but in nearly all cases in the Appalachian region will be the probable maximum precipitation event. This criterion is con- servative in that designers are obliged to provide sufficient storage in the facility to contain direct precipitation from the probable maximum precipi- tation event plus all other influent fluids from processing and runoff and still maintain 3 feet of freeboard. In determining the probable maximum precipi- tation event, designers rely on precipitation records and storm recurrence intervals to predict severe storm events such as hurricanes, as well as the effect of a reasonable foreseeable rain or snow runoff event. As discussed in Chapter 3, diligent slurry and water management is critically important to an effective coal waste impoundment system. BASIN FAILURE MODES Basin integrity is a routine consideration in all impoundment design. However, mining near a basin introduces special problems. In the evaluation of the basin of an impounding structure, the hydrogeological parameters are key in the determination of potential failure. The control and understanding of the leakage pathways such as subsidence, excessive seepage, or internal erosionare essential to determine the stability of the impoundment basin and the effect on the water balance and to comply with regulatory issues of these discharges. These are common considerations in all impoundment designs. It is essential that attention be paid to the identification, evaluation, and mitigation of potential failure modes in the basin. =
LIMITING POTENTIAL FAILURES 117 Subsidence Subsidence disturbs the strata above and adjoining the mined area and is an important failure mode (see Chapter 3~. It implies opening of tensile cracks on the surface, displacement along faults and joints, separation along bedding planes, and some distortion of the strata around the workings. The immediate roof tends to cave into the workings, and the floor may heave. Subsidence movements may combine to create leakage pathways from the bottom of the impoundment to the outside environment. Site characteriza- tion, design, operation, maintenance, and monitoring require additional considerations in areas with subsidence potential. Chapters 4 and 5 review ways to detect such problems in the basin area. Mine Openings As noted in Chapter 3, the position of underground mine workings relative to an impoundment is a factor in the evaluation of the potential for breakthrough. The relative elevations of local drainage and slurry height are also critical. If unidentified or not properly sealed, mine openings act as conduits for the flow of slurry or "black water," which may contaminate waterways outside the mine. Even if the workings do not allow water to flow out, there may be connections to local aquifers or other permeable strata. This may lead to seepage far from the impoundment. Therefore, identifying mine openings in or adjacent to the basin is of paramount importance. Basin failure above active or abandoned mine workings may involve one or more of the following modes: Leakage along naturally occurring joints and fractures. Joints and fractures may fill with ultra-fine material but do not necessarily develop resistance to the flow of fluids. Cleats in seams make the coal more permeable and are included in this category. The August 1996 failure at Lone Mountain was apparently caused by a breach in the mountainside (K. Mohn, Lone Mountain Processing, personal communication, 2001~. · Subsidence-induced tension or shear cracks and fractures at the bottom of the impoundment pool. Slurry tends to lubricate the joint and fracture surfaces and may promote movement, rather than inhibit flow. Entry of water between strata along bedding planes could accentuate subsidence effects and result in failure. The
118 COAL WASTE IMPOUNDMENTS October 1996 incident at Lone Mountain appears to have been induced by subsidence (K. Mohn, Lone Mountain Processing, personal communication, 2001~. Sinkhole or pit subsidence. If the interburden between the pool floor and the room-and-pillar workings is shallow (100 feet or [ess), sinkhole or pit subsidence may occur. This could cause a sudden inflow of the impoundment material into the mine openings. The November 1996 breakthrough into abandoned mine workings at Buchanan Mine was of such origin (B. Thacker, Geo/Environmental Associates, personal communication, 2001~. Catastrophic events. Heavy rainfall, debris flows, sudden snowmelt, breakage of seals, or blow-outs in mines above the slurry level would impose a severe load on the basin floor and could cause collapse of the pillars in workings below, creating a connection. The failure in Martin County in October 2000 occurred after heavy rain fell in a short period. · Piping Piping due to erosion may occur once seepage or leakage has been established. . If the width of the outcrop coal barrier or the overburden above the mined area are of insufficient thickness or the surrounding strata are too weak, impounded water could break into a mine and lead to a basin failure (see Chapter 3~. Conversely, an inundated mine under higher hydraulic head could introduce a large volume of water into the impoundment. Should a blow-out occur elsewhere in the watershed above pool level, it could cause an influx of substantial amounts of water into the impoundment. This, in turn, may overtop the embankment, damage the spillways, or induce a breakthrough in the pool bottom as a result of the additional load. Synergy between geologic and hydrogeologic conditions can compound the instabilities created by any failure mode (Figure 6.1~. Currently, no federal regulations address the width of outcrop barrier that should be left during underground coal mining. Kentucky and Virginia currently have standards for outcrop barriers, but allow variances where conditions are appropriate. In some cases it is prudent to allow openings for drainage. OSM has studied the problem of outcrop barriers but has yet to release any conclusions to date. The committee recommends that MSHA and OSM jointly pursue the issue of outcrop coal barrier width and overburden thickness and its competence and develop minimum standards for them. Mine workings below an impoundment should be avoided unless they can be confirmed to be deep enough and to contain an aquitard layer
LIMITING POTENTIAL FAILURES Slurry fines \ Low-cover area beneath valley 119 Slurry pool At,' Coarse blanket ,, \411\d ., l \~B MOB D 11 A/' not to scale L; Overlap of underground mine and pool FIGURE 6.1 Cross section of a hypothetical waste impoundment and geologic features that control site stability. (A) Deeply penetrating tectonic fracture sets. (B) Shallow, near- surface or stress-relief fracture zone. (C) Rock cut from contour to supply a coarse blanket for bark stabilization arid support. (D) Mined out areas of an underground coal seam. (E) Roof collapse above mine void, creating a zone of structural wealmess. Modified from S. Greb, J. Dinger, and D. Cumbie, Kentucky Geological Survey, personal communication, 2001. adequate to prevent uncontrolled entry of water into the openings. The strata in the aquitard must be identified and their thickness determined. This must be done with site-specific investigations and rock-properties data. If specific data cannot be obtained where longwall or other hill extraction pillar recovery has occurred, the surface fractured zone may be considered to be from 50 to 200 feet thick (Kipp and Dinger, 1987; Singh and Kendorski, 1981~. The permeable zone immediately above the openings may be 60
120 COAL WASTEIMPOUNDMENTS times the thickness of the extracted seam, if sufficient width of opening with respect to seam depth has been excavated. It should be recognized, however, that these values may have to be modified for known site conditions. MITIGATIVE MEASURES The ability to intervene in the event of undesirable performance and to introduce mitigation measures is an integral aspect of eliminating basin failure. The ease of doing so depends upon whether slurry elevation has already exceeded the level of the above-drainage coal seams or associated workings. If the basin is new or if the slurry elevation has not yet exceeded the level of above-drainage coal seams, the quality of the existing outcrop barrier between the basin and the coal seam should be evaluated. If the outcrop barrier is inadequate, it should be enhanced. A number of enhancement measures are available, including construction of additional barriers, installation of drains, promotion of seals such as what might occur with the consolidation of the slurry itself, and reinforcement of the existing barrier by grouting or other means. The solids deposited at the bottom of the impoundment can, under some circumstances, consolidate enough to create an impervious blanket of cohesive material. This reliance on the impervious zone can be evaluated using well-established procedures in geotechnical engmeer~ng. In the case where the slurry elevation has not exceeded the level of the above-drainage mine workings, the assessment of the outcrop barrier is generally feasible. However, if the condition of the outcrop barrier cannot be adequately assessed, the risk associated with developing a new facility or continuing with a preexisting one increases. If the costs of mitigation or the risks associated with the proposed operations are excessive, an alternative disposal strategy, which might even entail an alternate site, becomes necessary. When slurry elevation exceeds the level of the above-drainage workings, it becomes more difficult to assess the quality of the existing barrier. This may result in increased cost of investigation or greater uncertainty with the results or both. For example, as discussed in Chapter 5, the use of geophysical and remote sensing techniques in and adjacent to the basin area may reveal anomalies related to voids or geologic features such as fractures. These anomalies can provide clues about the potential mechanical and hydrogeological conditions in the proposed impoundment area. In the absence of geophysical, remote sensing, or hydrogeological anomalies,
LIMITING POTENTIAL FAILURES 121 clearing of the basin area may be warranted to uncover geologic anomalies that might otherwise go undetected. While the steps to be taken in the selection of mitigative measures are the same as noted above, many of the measures become more difficult to implement (Sidebar 6.1~. Hence reliability associated with assessment and intervention in this case is reduced. This can be overcome, to some degree, by increased reliance on monitonng. The assessment of below-drainage workings constitutes a subset of the second case above. Here the integrity of the cover must be evaluated by some combination of drilling, geophysics, fracture analysis, and subsidence assessment. A distinction should be made between active and inactive workings, because the safety of miners is at stake in active workings. Here, if basin sealing cannot be relied upon, and alternative methods of slurry management cannot be practiced, as may be in steep topography, then the size of the impoundment may have to be restricted. Coal seams above the maximum permitted slurry elevation may also be susceptible to blow-out. In this case, the assessment requires an understanding of the hydrogeological circumstances and whether they are favorable for blow-out. If so, mitigation is appropriate and some combination of drainage, sealing, and bamer construction, together with monitoring, is needed. The committee concludes that selecting the appropriate mitigative measures relies strongly on reliable basin characterization. The committee recommends that MSHA and OSM develop and promulgate guidelines for the site evaluation, design, construction, and operation of basins. They should be comparable in scope to the guidelines used in embankment design. If slurry from an impoundment leaks into active or abandoned mine workings, or may do so, bulkheads or seals may be constructed to preclude the water from escaping into the outside environment (Chekan, 1985~. As discussed above, many mitigative measures can be designed using established procedures; bulkheads designed to support high hydrostatic pressure present a different kind of problem. The committee recommends that MSHA review its current practice and develop guidelines for the design of bulkheads intended to withstand hydraulic heads associated with slurry impoundments. The bulkhead should be constructed of material that can withstand water action without deterioration in the presence of the various chemicals in the impoundment water. Further, the bulkhead should be suitably anchored in competent, unfractured strata. If such an area is not available, pressure grouting may be needed. Deterioration of the anchoring strata can be a major structural problem where the bulkhead is keyed into
122 COAL WASTEIMPOUNDMENTS SIDEBAR 6.1 Mitigation Measures at Lone Mountain and Buchanan, Virginia In August 19969 at the Lone Mountain, Virginia, slurry breached the impoundment and went into abandoned mine workings that were much closer to the pool than was shown on mine maps. The area was excavated to expose underlying bedrock. The exposed workings were sealed by backfilling with competent rock. This was covered with a geo-textile and further backfilled with compacted earthen materials. Backfill was placed over the seal to an additional depth of 20 feet and compacted. To protect the area further, along the western wall of the impoundment pool, a barrier of compacted earthen materials was constructed. In October 1996, two large sinkholes developed at a different location. Excavation revealed a fracture in the roof of old workings that had allowed slurry to enter the mine. The loose rock was removed, and the void was sealed with polyurethane grout. The exposed rock face was covered with a geo-textile and backfilled with compacted earthen materials (K. Mohn, Lone Mountain Processing? Inc., personal communication, 2001~. In November 1996, the impoundment at Buchanan, Virginia, failed and slurry entered old workings created by a different mining company. The area was excavated, exposing filled auger holes. Mine workings were backfilled; a filter fabric was placed along the entire perimeter of the coal seam; cohesive soil fill was compacted in lifts to create a barrier, and coarse refuse was then used to backfill the remaining excavation; and French drains were installed around the entire perimeter of the facility to drain the coarse refuse perimeter embankment {B. Thacker, Geo/Environmental Associates, personal com- munication, 2001 ). water-sensitive, clay-bearing strata. The size, integrity, and strength of the surrounding coal pillars, roof, and floor are critical to successful sealing. Generally, seals constructed for ventilation cannot withstand the anticipated water pressures. IMPOUNDMENT MANAGEMENT Risk reduction cannot be achieved by design and regulation alone, but requires adopting the best available construction and operating practices. The annual inspection review provides one check in this system, but it may not identify all construction and operation problems. Experience with tailings dam failures suggests that all concerned web safe impoundment management must pay additional attention. The International Committee on Large Dams (ICOLD, 2001) recently summarized lessons learned from tailings dam
LIMITING POTENTIAL FAILURES 123 failures. Its recommendations include: detailed site investigation; state-of- the-art procedures for design, construction, and operation; routine monitoring; safety audits; and occasional specialist reviews. Experience elsewhere indicates the desirability of corporations developing an impoundment or tailings management system that addresses policy, commitment, planning, implementation, checking, corrective action, and management review. The guidelines developed by the Mining Association of Canada (MAC, 1998) are en example. Coal operators are integral stake- holders in risk management and reduction. The committee suggests that coal operators develop an industry-wide procedure for evaluating impoundment management systems that could be adapted to specific properties and corporations. RISK ASSESSMENT AND MANAGEMENT In response to the basin failure at Inez, Kentucky (Sidebar 1.12), MSHA and a number of state agencies have initiated surveys to assess the risk associated with current impoundments. The committee agrees that identification of those impoundments within the existing inventory that have the greatest risk of failure has significant value. However, the two classification systems MSHA currently uses (see Chapter 1) are not consistent with accepted definitions of risk. The risk associated with a failure is defined as the product of hazard (the potential for a failure to occur) and the consequences of that failure (loss of life, costs of repairing damage to structures or facilities, environmental impacts). An impoundment could have relatively low risk if consequences of a failure were low even though the probability of a failure was moderate. On the other hand, an impoundment with a low probability of failure could be assigned high risk if the consequences of failure, in terms of danger to human life, damage to valuable structures or the environment were large. The pair of MSHA classification systems currently use the term hazard in a way that is not consistent between the two classifications system. In the first classification scheme, which deals with potential impacts of embankment failures, the term hazard refers to the consequences of failure. That classification system makes no attempt to assess the probability of a failure event for individual embankments. The second classification scheme, which deals with basin failures, comes closer to the standard definition of a risk assessment. That system includes an assessment of the proximity of underground workings and the potential for a failure that would lead to a release of water or slurry from the basin area into underground mine workings. In addition, the
124 COAL WASTEIMPOUNDMENTS potential impacts of such a failure are considered. Thus, the second classification scheme is a version of a risk assessment, according to the standard definition. The committee concludes that using different hazard classification methodologies for embankments and basins is inappropriate. Therefore, the committee recommends that: (1) MSHA and OSM review activities related to risk assessment for existing impoundments (including both embankments and basins) to ensure that they are consistent and that they distinguish appropriately between hazard and consequence assessment in the methodologies adopted; and (2) MSHA and OSM establish a single, consistent system, which should be used to assign both embankments and basins to risk categories. The ranking should be based on the combination of hazards and consequences, such as loss of life, cost, and environmental impact. Proposed new impoundments should also be assigned to risk categories, based on a combination of hazards and consequences, as was suggested for existing impoundments. The committee believes that this can be accomplished using qualitative risk assessment techniques. A consistent risk assessment system would allow decisions on impound- ments to be based on their relative risks. The committee also recommends that MSHA and OSM oversee a thorough assessment of potential mitigation measures for those impoundments that fall in the highest risk category and should determine which mitigation measures should be applied to reduce this risk to an acceptable level. Geotechnical engineering is used in the design and construction of waste impoundments. Given the inevitable uncertainties in site characterization, knowledge of material properties, and the need for use of idealized models to describe both physical and human behavior, risk is inherent. Therefore, managing risk is an essential consideration (Morgenstern, 1995~. Fortunately, powerful methods of risk management have evolved. The observational method is the first line of defense in managing risk in the face of identified uncertainties. This method involves the use of observation to review performance and refine subsequent design, construction, or operation. Peck (1969) identified the elements of the observational method as follows: Site exploration to establish (at a minimum) the general nature, pattern, and properties of subsurface materials; Assessment of the most probable conditions and most unfavorable deviations from these conditions; Establishment of the design based on a working hypothesis of anticipated behavior under the most probable conditions;
LIMITING POTENTIAL FAILURES 125 Selection of quantities to be observed as construction (or operations) proceeds, and calculation of anticipated values of these quantities on the basis of the working hypothesis; Calculation of values of the same quantities under the most unfavorable conditions compatible with available subsurface conditions; Selection in advance of a course of action or modification of design of every significant deviation from that predicted for the working hypothesis; Measurement of the quantities to be observed and evaluation of actual conditions; Modification of design to suit actual conditions. Because geotechnical uncertainty is intrinsic, designers, owners, and regulators should make conscious use of the observational method and accept the need to declare performance indicators, response procedures, and observational procedures, including advances in monitoring technology. The observational method of risk management is applicable to both existing and future impoundment systems. The value of the observational method is well recognized in geotech- nical practice, and MSHA regulations and current practice promote elements of it. However, the method has some limitations, such as difficulties in application to seismic or other rapid events. In particular, it requires anticipation of all eventualities as well as preparation for courses of action to meet whatever situation develops. MSHA and coal industry designers should be aware of the need to employ the observational method to the degree practical. Another method of risk management is the use of third party reviews. The committee recognizes the value of third-party reviews that have been used in projects such as the design and construction of large water dams. Such reviews often examine whether the project is being designed to appropriate standards, and the construction is being managed appropriately, and the committee suggests that coal companies consider whether similar reviews would add value and help manage risk in the design, construction, and operation of coal waste impoundments. To maximize the potential for risk reduction, the committee recom- mends that all impoundment designs be accompanied by a risk analysis utilizing qualitative methods. Examples of such methods include Potential Problem Analysis and Failure Modes and Effects Analysis. The performance requirements needed to correct failure modes, including the instrumentation
126 COAL WASTEIMPOUNDMENTS to be installed to evaluate performance must be identified, and the plan to be executed in the event that performance is not met should be spelled out. The committee believes there is a limit to risk tolerance, for both existing and new impoundments. When risk is high, and when mitigation, either through more reliable characterization or barrier constructing is im- possible, of limited precedent, or so expensive that it is infeasible, then a substantial change in operation of the impoundment is warranted. This may range from minimizing slurry fluidity to ceasing operations. If an impound- ment fails risk-assessment criteria and if risk cannot be mitigated it should be phased out or alternatives considered. MONITORING Monitoring is an integral part of the observational method used in geotechnical engineering. In the design and construction of coal waste impoundments, monitoring is critical since the construction process continues for the life of the facility. During the life of the impoundment, which may span decades or more, conditions may change. For example, the nature and characteristics of the refuse may differ because of the areal variations in the geology of the coal seam, mining of different coal seams, alterations in mining or preparation methods, or rate of waste generation. A well-planned monitoring program can help to detect when major changes are occurring so that design modifications can be implemented in a timely manner. The savings that may be realized by not designing for the most conservative scenario can justify the scale of the monitoring program. Monitoring can help ascertain when repairs, improvements, or other upkeep is needed. Monitoring procedures and instrumentation are well documented (ASCE, 1999; Dunnicliff, 1988) and need not be repeated here. Monitoring instrumentation requires that appropriate target sites be identified and accessible. Monitoring of potential failure modes of embankments typically measures pore pressures, surface and internal deformations, hydraulic parameters, and vibrations, especially if blasting is being conducted nearby. Occasionally, temperature and rock stress or soil pressure, especially if high horizontal stresses exist in the area, should be measured. Hydrogeologic monitoring and downstream flow and quality measurements may give evidence that would provide warning of impending basin failures. It cannot be overemphasized that instrumentation should be used as a complement to visual observation and not as a substitute. Instruments cannot adequately establish the extent of vegetation and undergrowth or its removal,
LIMITING POTENTIAL FAILURES 127 the origination of new seeps, initiation of erosion, and many other parameters. As discussed in Chapter 2, OSM and its state delegate programs require mine operators to monitor both surface and groundwater at coal mining sites. The vast majority of the work published on slurry impoundment composition and chemistry was conducted in the mid 1970s and early 1980s. However, considering the advances in coal cleaning techniques and in storage and abandonment of the refuse, and the likely impact on impoundment chemistry resulting from the implementation of these technologies, this data may not be representative of current conditions (Darrell Taulbee, Center for Applied Energy Research, Lexington, KY, personal communication, 2001~. In addition, a theme mentioned repeatedly in town meetings with coalfield citizens was their concern and desire for information concerning the chemical constituents in the coal waste, and how it affects Weir ground and surface water. As a result, the committee recommends that research be performed to identify the chemical constituents contained in the liquid and solid fractions of coal waste, and to characterize the hydrogeologic conditions around impoundments. An additional benefit to this research is that the characterization of the wastewater will aid in monitoring schemes that could aid in leak detection, which could foreshadow impoundment failure (Sidebar 6.2~. These monitoring systems focus on forensic petrologic and geochemical investigations that may aid in the early detection of fugitive solutions from a refuse impound- ment system. While these investigations and monitoring programs are not directly germane to limiting the potential for refuse impoundment breakthroughs and failures, they may act as an early warning system for mine operators to change their management of the impoundment system. Conjunctive research should also be conducted to demonstrate the use of continuous data loggers and real-time monitoring devices that can monitor and warn of changes in hydrologic, hydrogeologic, and geotechnical conditions around the impoundments. It must be clearly understood that for monitoring to be successful it should be applied to all modes of potential failure. Monitoring expenses can only be justified if data and analysis results are received in a timely manner. Acquisition and analysis of data should be sufficiently rapid to enable coal operators to make meaningful decisions. If data indicating movement or water pressure increases are not analyzed prior to a black-water release or failure, they are of limited value. Computerized processing can immediately transform the raw data into a format the engineer can use. If some crucial criteria are exceeded, visual or audible alarms can be triggered.
128 COAL WASTEIMPOUNDMENTS - 1 SIDEBAR 6.2 Forensic Hydrology Systematic changes in water chemistry are often used to detect plumes of contaminants around leaking landfills, underground storage tanks, and mine waste. Similarly, changes in the chemical character of water samples from ambient levels may indicate leakage from impoundments into the surrounding groundwater system. Hydrochemical facies are defined as areas of aquifers where the chemistry of the groundwater is predictable within defined limits. A hydrochemical facies model for the Appalachian coal field (Pennsylvania Department of Environ- mental Protection, 1999; Wunsch, 1993) can be used to detect groundwater contamination from coal waste. The water fraction of coal slurry will chemically reflect the concentrated waste material in coal and will also be the most mobile fraction to leak into the underlying or adjacent bedrock, coal seams, and fractures. For example, site-specific data show that coal waste effluent can be enriched in iron, aluminum, magnesium, and sulfate (D. Taulbee, University of Kentucky, personal communication, 2001~. Moreover, organic chemicals used in the beneficiation of coal waste may also be used as a groundwater tracer to identify leakage from impoundments. A more comprehensive suite of analyses could be used to monitor and detect leakage from an impoundment by employing geo- chemical modeling, which can uniquely identify and characterize water samples. Thus, monitoring the chemical composition of water adjacent to impoundments could be used to detect whether water is leaking from the coal slurry and, with the other site-specific information, to determine sensible mitigative programs. Several water quality parameters normally associated with mining impacts can be easily determined in-situ. For example, specific electrical conductance, which is a measurement of capacity of a fluid to transmit an electrical current, is directly proportional to increased solids or salt content in the water. Water from waste impoundments or mine waste usually contains increased dissolved salts, such that significant changes in electric current can be a predictive tool for leakage. The pH of mine waste-impacted water is often notably acidic, thus changes in pH can be indicative of contaminated ground or surface water leakage. Digital water quality monitoring equipment (i.e., data loggers) can simul- taneously record changes in several parameters that indicate water impacted by mining or mining wastes. These tools can also be used to obtain real-time water- quality data that can be accessed remotely over the Internet or by radio or satellite transmission, and can be used to monitor for leakage that may presage an impending failure. These instruments are relatively inexpensive (approximately U.S. $5,000) and can be used for predictive monitoring of hydrologic and chemical parameters that would warn of impoundment failure. In conjunction with chemical data, water levels could be monitored to establish the hydrostatic head conditions around the impoundment. Digital data collectors or data loggers can be used to monitor remotely anomalous changes in hydraulic head in the adjacent strata or mine workings. This too may indicate changing head conditions between the slurry impoundment and surrounding aquifers or mines. Digital data loggers to monitor these changes can be equipped with alarms that give real-time warnings that head levels have surpassed a pre- determined threshold. The instruments are proven in the field, relatively inexpensive, and widely available.
LIMITING POTENTIAL FAILURES 129 Continuous monitoring could provide timely warning in case of impending failure of an embankment or basin. Its use with weirs, for example, is well established, and it can be used with other types of instrumentation. The committee recommends that MSHA and OSM consider requiring additional continuous monitoring in specific instances and evaluate automation of monitoring instrumentation. EMERGENCY PLANNING AND RISK COMMUNICATION All coal companies that operate a coal refuse impoundment system are required to develop emergency response and evacuation plans that describe what may happen and what should be done to limit the damage of any reasonably foreseeable event. Because Appalachian refuse impoundment systems are usually positioned at the head of a stream, communities are often situated immediately downstream. A failure or breakthrough from refuse impoundment systems has the potential for significant adverse consequences for downstream communities, infrastructure, and the environment. Dam break analyses and evaluation of potential flooding resulting from impoundment failure are commonly undertaken in emergency preparedness planning. Public concern regarding emergency response and evacuation plans was a recurring theme in public comments made to the committee. Some residents were unaware of emergency evacuation plans; others had seen evacuation plans but disagreed with the logic behind the evacuation routes and would not have used the plan in the event of an emergency. Conversely, coal industry and regulatory agency representatives confirmed that these plans are being developed and shared with the public through the various community contacts (e.g., local fire departments, police, health care providers). The lack of realistic communication constitutes a fundamental barrier to the industry's ability to make stakeholders aware of the risk associated with coal refuse impoundment construction, operation, and closure and of steps taken to mitigate that risk. Based on these stakeholders' input, the committee concludes that communication concerning coal refuse impoundment system risk and emergency response between the industry and the local communities could be improved substantially. The committee suggests that the industry take steps through the appropriate emergency response agencies to address these problems.
130 COAL WASTE IMPOUNDMENTS method to aid in determining the extent of coal outcrop barriers and coal voids in mines adjacent to coal waste impoundments. SUMMARY One of the critical tasks in site characterization is ruling out the presence of voids. Evasive drilling programs can provide the necessary information. However, they may compromise the hydrological integrity above the mine, and their cost is often significant, both economically and environmentally. Well-planned and appropriate use of geophysical techniques can often help to minimize the amount of drilling required to detect mine voids. However, no single geophysical technique will work at all depths in all types of geology. From a practical standpoint, steep topography compounds the difficulty in collecting, processing, and interpreting geophysical data when surface methods are used, but these effects are minimized when borehole, cross-hole, and in-seam methods are used. ~ addition, trees and cultural features such as fences can impede geophysical data collection, processing, and interpretation. Multiple geophysical techniques may be necessary to reduce the probability for error to an acceptable level; drilling is required for confirmation. The committee conclucles that geophysical techniques are useful in some cases in coal mine void detection, especially the use of seismic surface waves, seismic reflection, ground-penetrating radar, and electrical resistivity methods. The committee also concludes that geophysical techniques have been underutilizedt in the coal-mining industry and could benefit from additional research. The committee recommends that demonstration projects using modern geophysical techniques be funded, and that the results be widely conveyed to the mining industry and to government regulatory personnel through workshops and continuing education. Continuing education could include the opportunity to attend short courses and seminars that present the latest technology along with case histories to support its use. The committee notes that much more work has been done using geophysical techniques on coal field problems than is indicated in the literature. Since a large amount of the work is proprietary or involved in Prepub~ication Version - Subject to Further Editorial Correction
