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--> Appendix A Specific Comments on the Draft Report of the DOE Working Group Chapter 2 of this review presents general comments on the March 1997 draft DOE Working Group report, Reducing the Ambiguity and Visibility of Seismic Signals from Mining Activities. The NRC committee authoring this review also presents specific comments on many of the technical aspects of the DOE report within this appendix. The intent is to provide these comments for consideration by the DOE Working Group in any revision to their report. These specific comments have been placed in an appendix to avoid distracting readers from the principal focus of the NRC study, that is, voluntary efforts that could reduce the ambiguity associated with seismic signals originating from mining activities. Blasting Problem Statement The DOE Working Group identified important issues for the mining industry with respect to the possible impact of the CTBT, as well as specific problems relating to the visibility and ambiguity of mining blasts. However, statements relating to the magnitude and extent of these problems are undocumented and may be misleading. For example, the report asserts that one in 20 shots contains sympathetic detonations, represents misuse of explosives, or noncompliance with safety regulations. This assertion implies that, of the estimated 150,000 to 200,000 blasts that are conducted annually in the U.S. mining industry, 7,500 to 10,000 represent blasts in which mishaps occur and may produce anomalous signals. The NRC committee is not aware of data that could justify this assertion and none are presented in the DOE report. Although it is generally recognized that many blasts do not perform exactly as designed, such incidents usually do not detract significantly from the overall performance of the blasts.
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--> Representativeness of the Study Site The principal site of the research effort that forms the basis for defining the blasting issues of concern and for developing the report's recommendations comprises a single coal mining operation (Black Thunder, Wyoming). The site, a large coal mine with casting operations, may not be representative of most large blasting operations in the United States. Because of the narrow scope of the investigation, it is risky to extrapolate the scientific information acquired for one mining operation that may be representative only of blasting operations of that type (casting). Hence, the conclusions and recommendations lack the foundation that is needed if they are to be applied to operations throughout the United States. The draft report clearly points out that there is no difference in the signals from single chemical explosions and from nuclear explosions when energy is normalized to TNT equivalents and the same source geology is involved. Other aspects of the source such as coupling (defined as the efficiency of the transfer of explosive energy into seismic waves) further affects the amplitude and hence, visibility. This information, as well as resulting complexities in the seismic wave forms, has been recognized for many years by explosives engineers and mining engineers working in the vibration fields. With respect to the research leading to the conclusion that there is a difference between casting and fragmentation shots, the report does not distinguish between the two types of blasting with recommendations specific to each practice. The coal shots used to exemplify fragmentation shots do not necessarily represent fragmentation shots in hard rock mining (i.e., mining in which the mineralized ore deposit is blasted prior to comminution). The latter have very specific fragmentation requirements that are met using carefully placed time delays. Because the source geology and delay sequence used in these shots differ from those of coal mine blasting, the wave form frequencies also show quite different characteristics. Hence, the data forming the basis for recommendations that apply broadly to the mining industry as a whole have not included the subset of hard rock mining. Blasting Recommendations. Recommendations are discussed in Chapter 4 of the DOE Working Group report. Links between the research findings and the recommendations often are insufficiently established. One notable recommendation concerned the “slice mining” method, which was not referenced or explained in the report and is largely an untested method. Based on an oral explanation of the slice mining method to the NRC committee by members of the DOE Working Group, the committee found the method may create a highwall safety exposure risk, require operating under unsafe and congested conditions, and may not lend itself to the use of large-scale mining equipment. Each of the other blasting-related recommendations are considered below.
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--> Reduction of Charge Weights The recommendation about charge weight reduction can be implemented in a number of ways summarized in the DOE Working Group report. This includes reduction of borehole size, reduction of blasting bench height, the use of decked and decoupled charges within the hole, more stemming (implying shorter charge lengths), and less subgrade drilling. Unfortunately, the DOE Working Group report does not consider the economic impacts of these recommendations. The NRC committee believes that the economic impacts may be substantial because of the costs for additional drilling. Amount of Explosive per Delay The assertion that a “20 to 50 percent reduction of the explosive would be required before measurable reductions in vibration levels can be realized” (draft report's executive summary, see Appendix B, p. 49) is not substantiated and no references are cited. The report does not adequately discuss the feasibility of reducing the amount of explosive per delay while achieving fragmentation and haulage objectives, as well as overall financial goals. The NRC committee agrees that reducing the quantity of explosives detonated on one time delay will reduce ground motion amplitudes. However, the specific correlation between decreased explosives and the reduction of the seismic amplitudes is unresolved. Smaller Borehole Diameters and Smaller Bench Heights Employing shorter blastholes of smaller diameters generally increases the drilling costs. The economic impact of this can be assessed for a hypothetical example. In considering production blast design modifications (versus wall control blasts), the powder factor must remain constant, as it is set for a particular geology, fragmentation size, and ore displacement requirement. Given a typical bench height of 50 feet and a blasthole diameter of 10 inches (blasthole diameters are determined by the designed bench heights), to achieve a 50 percent reduction in explosive weight, a reduction in hole diameter and length or bench height is necessary. For operating convenience, it would be most appropriate to break the 50 feet bench into two 25 feet benches. Reducing the bench height to 25 feet would reduce the blasthole diameter to 8 inches. This would result in a reduction of the explosive loading density from 34.1 to 21.8 lb/ft. (assuming an average explosive density of 1 g/cm3 for mixtures of ammonium nitrate/fuel oil and emulsion-based products). As an illustrative example, the impact of this change on the costs of drilling can be roughly assessed (see Table 1, Chapter 2 of this report). The examples given are acceptable for good blast design and meet most production purposes. The explosive loading per hole is reduced by the greater than the 50 percent recommended by the DOE Working Group report.
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--> The doubling of annual drill footages almost certainly would require the acquisition of an additional drill rig (acquisition cost on the order of $2 million). The increase in blasting costs, although not as severe as the increase in drilling costs, also can be assessed. The cost of the explosives would remain the same for a constant powder factor. However, quadrupling the number of holes requires four times the number of boosters and detonators, as well as labor time of the blasting crew. Since these costs typically represent about 2.5 percent of the total blasting costs, an increase of about 10 percent in boosters and detonators would further add to the extra costs resulting from decreasing the charge per blasthole, as given in the hypothetical example. Decoupled and Decked Charges The working group's report claimed that sizable reductions in the explosive weight per delay can be achieved by decoupling the explosive charge from the borehole wall. The use of decoupled charges usually is not acceptable to most mining practices, as it represents a possible waste of explosive energy and dollars spent. However, for wall control, decoupled charges have been used for over 30 years by the mining industry to minimize damage to intermediate and final open pit walls in order to improve wall stability. In recent years, dozer ripping/trimming has been practiced to develop certain ultimate slope walls to eliminate blast damage altogether. It is well recognized that good blasting practices must achieve a balance between maximizing fragmentation and minimizing wall damage. The decking of charges, however, does represent a valid modification that will reduce the charge per delay that mines may be able to live with, but only to the extent that the stratigraphy within the borehole allows. More Stemming and Less Subgrade Drilling. The DOE Working Group report also recommended increasing the stemming and using less subgrade drilling as a way to decrease seismic visibility. These modifications usually are not acceptable, as these regions of the borehole (the top near the collar and the base below the pit floor) are critical in blast design. If the boreholes are not properly loaded with explosives, operating efficiency may be directly affected. Using less explosives at the hole collar produces oversized fragments that may be difficult to remove by excavation equipment. Less subdrilling in hard rock mines will produce irregular pit floors that are not cut to grade and on which equipment may be difficult to operate. Delay Period Specific delay periods are not recommended in the DOE Working Group report. However, delay periods of up to and over 100 milliseconds is noted as being desirable to increase the probability of mine blast discrimination and to lower
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--> seismic amplitudes. Although scientifically sound, this recommendation is made without considering the accompanying impact on mine operations. The 100 millisecond interval is, unfortunately, one of the worst intervals for near-mine site ground motion disturbance in most geologies. Such intervals generate frequencies near the natural frequencies of structures (near 10 Hz) and promote shaking to nearby residential dwellings. Moreover, blasts designed to achieve fine fragmentation must use small delay intervals on the order of 9 to 25 milliseconds. High delay intervals are not effective in such fragmentation design (versus casting) practices. The DOE Working Group report recommends signature-hole tests to establish optimum delay intervals, in order to shift near-field frequencies away from the natural frequencies of structures. However, these tests may not be as reliable as suggested. Variations in geology and explosive detonation properties from test to test yield varying results. The results of signature tests, when applied to full-scale production blasting, may not alleviate ambiguity. Use of Programmable Detonators The DOE Working Group stated that “precise, programmable electronic detonators hold considerable promise in the control of blast induced ground vibrations by eliminating the inherent scatter in the detonator firing times and providing unlimited choice of delay intervals.” However, programmable detonators are not widely available at this time and their projected costs (3 to 10 times the costs of currently available electric and nonelectric detonators) cannot be justified based only on the reliabilities measured under ideal (non-field) conditions. There is limited testing being conducted in operating situations and manufacturers are reluctant to specify both projected sale prices, as well as statistics for actual initiation timing for production situations. Therefore, the recommendation for wide-spread use of programmable detonators is unrealistic at this time. Current delay times for initiation systems are considered reliable, cost-effective, and within the acceptable ranges required by the mining industry. Most nonelectric detonation systems have a standard deviation from ±2 to 8 percent and have proven to be reliable, depending on the range of delay times—from a few tens of milliseconds to a few hundreds of milliseconds. The variation in actual detonation time is a function of lot number, storage conditions, and other factors and is usually taken into account by the blaster when designing blasts. It is uncertain that improvements to current detonator accuracies will be detected regionally in seismic signals. On the positive side, improvements in fragmentation and muckpile placement will accompany improvements in accuracy. Other factors that contribute to variations in initiation timing include explosive performance and the variability of explosive types that may be used within one mine blast. The variability in explosive quality within a blasthole and between blastholes in a shot for a 100 foot blasthole can cause variations in total explosive detonation times on the order of a few milliseconds, and possibly up to 10 milliseconds in the worst case.
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--> Close-in Seismic Warning Stations The DOE Working Group report suggests that mine operators establish permanent near-site seismograph warning systems at distances of 5, 10, or 15 miles from the mine site to allow a prewarning of signals that could trigger an international monitoring station. (Note that the description in the report of the near-site system given in Section 4.C does not match that of the on-site system recommended in section 4.A.1.3.) Currently, many mines use seismographs that are located 1.0 ± 0.5 mile from the mine site for vibration structure damage control. The NRC committee believes that the industry as a whole currently possesses the expertise and analytical capabilities to assess “desirable and undesirable” blast designs. The recommendation to install distant (more than 5 or 10 miles from the mine) monitoring stations would not likely benefit the mine operators and could be regarded as an excessive and unjustified burden. The main differences between the seismometers typically used in the mining industry and those used by seismologists is the ability for the latter to detect and measure low amplitude and low frequency responses. The equipment used in the mining industry has 8- to 12-bit resolution and frequency responses that cut off below 1 Hz. As a result, these seismometers do not detect many of the wave phases and long period amplitudes of interest to seismologists. The seismometers are used chiefly to determine if mine blasts generate amplitudes and frequencies that might lead to damage at near-by structures. If mine seismometers are to serve as a warning system for regional seismic systems, then the two recording systems should have similar sensitivities to detect the necessary wave characteristics of interest to both the mining industry and seismological community. Further, the on-site mine monitoring system that is proposed is relatively expensive and requires expertise that is not routinely available at the mine. A more effective means to obtain accurate blast times and locations may be the use of GPS (global positioning satellite) techniques. Ground Failures Underground failures that may produce large seismic signals are categorized in the DOE Working Group report as planned and unplanned failures. Planned failures include (1) controlled failures (e.g., planned pillar blasting and initiation of a block cave) and (2) uncontrolled failures (e.g., strata caving). Unplanned failures include catastrophic events such as pillar collapses, coal bumps, and rock bursts. Large-scale surface mine and waste dump failures that could generate distinguishing seismic signatures are not mentioned in the report. In terms of controlled mine failures, the case studies presented in the report do not indicate any significant visibility issues. The report uses the term “cascading pillar failure” (CPF) to describe unplanned, catastrophic events of pillar collapses. The occurrence of CPF is extensively discussed in the report, although such events cannot be considered as common occurrences. Three possible CPF cases since 1986 are reported in the study, most notably the 1995 collapse at the Solvay mine
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--> in Wyoming that resulted in seismic signals of a 5.2 (ML) magnitude. In typical mining operations in the United States, cascading pillar failures are rare, unlike coal bumps, and improved mine design techniques are likely to further reduce such pillar collapse events. Pillars usually fail due to secondary extraction in room-and-pillar operations. Also, yield pillars (e.g., in the headgate and tailgate entries of longwall panels) are designed to yield, to accommodate and accept variable stress loading. Such pillar design techniques are now mature, well-proven practices in domestic mining operations.9 The DOE Working Group report presents simplistic concepts of mine planning measures to avoid cascading pillar failures (i.e., containment, prevention, and full extraction) that may be applicable only to some coal mine systems. These are basically prescriptive suggestions, but cannot account for individual mine design goals and objectives unless there is thorough consideration of production impacts and financial implications. A considerable percentage of the coal bump events referenced in the report (Table 3A.1) is from a single mine (Lynch 37) in Kentucky, which is currently closed. Also, it is noted that multiple discrete events are listed for a single day, which is typical of the stress redistribution process. Bump activity10 and rock burst episodes are probably related to regional tectonic conditions. Contemporary seismic monitoring techniques, including analytical methods for data analysis, are now available that can integrate information from seismic activity at a mine property into a warning system for safety considerations. A prototype of such a warning system for the occurrence of rock bursts is operated at the Lucky Friday mine in Mullen, Idaho, which was installed in cooperation with the former U.S. Bureau of Mines. Calibration Of Mining Signals. The DOE Working Group report recommends calibration and fingerprinting of active mines, in order to minimize the impact of the CTBT verification system by minimizing false alarms. The working group proposes obtaining this information by deploying portable instruments at “visible” mines to record large blasts. These recommendations are scientifically valid and practical; the NRC committee amplifies them in its proposed solution in Chapter 4 of this review. Although the 9 It should be noted, however, that modern mining techniques and operating pressures to achieve high production goals may inevitably lead to larger mining panels or sections (i.e., size increases in longwall panel dimensions). That trend may also increase the seismicity potential (i.e., during caving operations) of reaching “visible” levels in the future. As a result, seismic signals from engineered mine collapses are expected to become more visible in the future as mine production increases and caving operations include stronger more massive rocks. 10 It may be reasonable to assume that bump severity might increase in the future, as coal mining operations progress to deeper cover depths.
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--> measures themselves would not reduce seismic visibility, they could result in reduced ambiguity by improving identification of blast event locations and providing a fingerprint of mines with large blast events. However, most mines do not maintain the technical expertise or equipment necessary to undertake such a calibration effort and would have to rely on outside expertise to carry this out. The DOE Working Group has not indicated who should perform this task nor to whom the data, once calibrated, should be provided. On the other hand, the NRC committee believes that a more immediate benefit would accrue from the simple collection and reporting of accurate mine blast locations and times utilizing less sophisticated and less costly equipment and techniques than those proposed in the report. These procedures are described in Chapter 4 of this review. They do not replace the on-site calibration and fingerprinting system proposed by the DOE Working Group, but could be used to improve the characterization of mine blasts in a broad, more cost-effective, and timely way. Presentation Of Data Although the DOE Working Group report includes valuable technical information, the presentation could benefit from improved organization and writing. By improving the presentation, useful information could be synthesized in a more logical manner and the significance of data could be appreciated more readily. For example, terminology used to describe details of seismic signals is complex and is discipline-specific to the seismologic community. A high level of knowledge and experience in seismology is needed to comprehend a large portion of the report. If the intent of the working group report is to serve as a reference document for a community that includes the mining industry, the report should clearly convey the issues defining the problem in terms familiar to that industry. Presentation of data or supporting documentation of several assertions would improve the credibility of the report. For example, in the executive summary of the working group's draft report (see Appendix B) the statement is made that “…the U.S. government and its National Laboratories have engaged in an extensive partnership and joint research with the U.S. mining industry, to identify mining practices that provide intrinsic benefit to mining companies…” A list of companies and types of cooperation and research would be useful, as would be the results of such activities and resulting benefits. In the draft report's presentation of technical data, equivalent data types are not displayed in the same context and in many cases, graphs and plots of data are missing labels and/or units for axes (e.g., Figure 2.B.7 and 2.B.8). Greater attention to such details would help improve the presentation, particularly for understanding points made in the text and for the evaluation of the data sets. One figure (Figure 2.A.10) has a portion labeled “safe damage zone,” whereas in the referenced Office of Surface Mining chart, that portion is labeled the “safe blasting zone.” The level below which seismic signals from mining operations should be of no concern, in terms of visibility, is not adequately defined or discussed. For example, are events only with seismic signals above ML = 3.0 relevant to CTBT monitoring or
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--> might visibility questions arise from events in the ML = 2.0 to 3.0 range? Seismic magnitudes (M) used in the report need to be defined and labeled consistently. Frequently, it is not clear which of the commonly used earthquake magnitudes are employed. A goal of the IMS is to have uniform detection capability to some threshold magnitude (3.0 is often mentioned as a goal). This magnitude is a teleseismic measure, mb (body wave magnitude). Most studies of U.S. mining have made use of regional distance seismic recordings and related these to local magnitude (ML) or regional magnitude (e.g., mb, mbLg). In the case of earthquakes, mb and ML are well correlated in the magnitude range of 3.0–4.5. However, for mining seismicity, this correlation is poor. This is because delayed detonation procedures distribute the P wave over a time window lasting up to several seconds; the shorter period signals (like SV or Rg) may still be large. The net effect is that local and regional magnitudes may be larger than mb for most mining explosions.
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Representative terms from entire chapter: