A Study of Respirable Silica in Underground Coal Mines: Sources

An ongoing resurgence of occupational lung disease among coal miners in the United States has been linked to respirable crystalline silica (RCS). To better protect miners, a deeper understanding of key exposure factors is needed. As part of a larger investigation of RCS in 15 coal mines, this paper describes analysis of silica mass content in two types of samples: (1) respirable coal mine dust (RCMD) collected in standardized locations in each mine; and (2) respirable dust generated in the laboratory from primary source materials, including coal and rock strata being mined at the production face, material obtained from the dust collection system on roof bolter machines, and rock dust products being applied by the mine. As expected, results indicate that rock strata drilled for roof bolting or being extracted along with the coal are a major source of RCS in many coal mines—although the coal seam itself can contain significant silica in some mines. While silica content of rock strata encountered in central Appalachian mines is not necessarily higher than in other regions, the sheer abundance of rock being extracted in thin-seam central Appalachian mines can explain the relatively higher silica content typically observed in RCMD from this region

A Study of Respirable Silica in Underground Coal Mines: Particle Characteristics

Respirable crystalline silica is now considered to be a major culprit of resurgent lung disease among US coal miners—especially in central Appalachia—though questions remain regarding the specific circumstances around exposure to it. As part of a larger investigation of dust in 15 US coal mines, a recent study examined the silica content in both the respirable mine dust samples and the samples of respirable dust generated in the laboratory from primary source materials (i.e., coal and rock strata and rock dusting products). It concluded the rock strata that is being drilled for roof bolting or is being cut along with the coal is the most significant source of respirable silica in many mines, which is consistent with the expectations based on other scattered datasets. However, little information is available on the characteristics of respirable silica particles which might be important for understanding the exposure risks better. In the current study, which represents another part of the aforementioned investigation in 15 mines, scanning electron microcopy with energy dispersive X-ray spectroscopy (SEM–EDX) were used to analyze the size and surface condition (i.e., degree of surface-associated clay) of 1685 silica particles identified in 58 respirable mine dust samples. The results indicated that silica is typically finer in locations nearby to drilling and cutting activities than it is in other locations within a mine, but the silica in the Central Appalachian mines is not necessarily finer than it is in the mines in other regions. An analysis of the particle surfaces revealed that respirable silica in coal mines often does not occur as “free”, high-purity particles. Rather, there can be a range of occurrences including silica particles having a thin “occlusion” layer of clay, silica within agglomerates that can also contain other particle types including clays, or even silica ingrained within other particles such as coal

Effects of geometric factors on mode I fracture toughness for modified ring tests

Stress intensity factors of specimen models with various external diameters, inner hole diameters, and extents of flattened loading boundaries for the modified ring testing method for mode I fracture are computed by finite element modeling. Effects of boundary conditions and geometric factors on stress intensity factor computation are analyzed. Parametric expressions are proposed to estimate critical crack length position and corresponding maximum stress intensity factor. An expression in terms of inner hole and specimen diameter, and loading angle is developed for estimation of maximum stress intensity factor. Mode I fracture toughness tests with modified ring method are conducted on two different rock types; andesite and marble. Results of the modified ring tests with different external specimen diameters are compared to the results obtained by the cracked chevron notched Brazilian disk method. Geometric parameters of modified ring specimens yielding fracture toughness results close to the results of the suggested method are identified. Fracture toughness of large diameter specimens with small inner hole sizes is found to be higher than the results of the suggested method. A similar trend is observed with decreasing width of loaded boundaries. Effects of specimen and inner hole size, and loaded and free boundaries on fracture process zone and fracture toughness are discussed

Mode I fracture toughness determination with straight notched disk bending method

A new method called the straight notched disk bending method is developed for mode I fracture toughness determination using rock cores. Disk specimens of andesite and marble having a single straight edge notch were subjected to three-point bending loads. Dimensionless stress intensity factor estimations and fracture toughness tests were conducted for different notch lengths, span lengths, thicknesses and diameters of the cylindrical rock specimens. Stress intensity factors were computed by three-dimensional finite element modeling and the results were presented for a wide range of specimen geometrical parameters. Results of experiments were compared to the results of well-known mode I fracture toughness testing methods. For specimens having thickness equal to the radius, mode I fracture toughness was lower and close to the results obtained by semi-circular bending method. When thickness was increased and doubled, mode I fracture toughness increased and approached to the value found by the suggested cracked chevron notched Brazilian disk method. Advantages of the new method included easy specimen preparation and testing procedure, stiffer specimen geometry, smaller fracture process zone, and flexibility of the specimen geometry for the investigation of the size effect behavior

Investigation of proper specimen geometry for mode I fracture toughness testing with flattened Brazilian disc method

Investigation of geometrical parameters for flattened Brazilian disc method is important, since this is a simple and attractive method for mode I fracture toughness testing on rock cores. Evaluating numerical modeling results, a parametric equation in terms of principal stresses at the center of the disc and the loading angle of the flattened end was developed. An equation was proposed for maximum stress intensity factors at critical crack lengths around stable to unstable crack propagation. Comparing fracture toughness results of flattened Brazilian disc method to the results of the suggested cracked chevron notched Brazilian disc method, geometrical parameters for flattened Brazilian discs were investigated. Diameter, loading angle of flattened ends, and thickness of andesite rock core specimens were changed to obtain comparable results to the suggested method. The closest results to the suggested method were obtained by 54 mm diameter discs with loading angles larger than 32A degrees, and thicknesses between 19 and 34 mm. Results were confirmed by the flattened Brazilian disc tests on a marble rock. In flattened Brazilian disc tests with smaller loading angles and larger diameters, larger fracture toughness values than the results of the suggested cracked chevron notched were obtained. However, excluding tests with large loading angles over 27A degrees; specimen size was less effective on the results of these tests. Critical crack length parameters computed from modeling and experiments were close to each other for the flattened Brazilian disc specimens with smaller loading angles around 20A degrees and thickness/radius ratio equal or less than 1.1

A Study of Respirable Silica in Underground Coal Mines: Sources

An ongoing resurgence of occupational lung disease among coal miners in the United States has been linked to respirable crystalline silica (RCS). To better protect miners, a deeper understanding of key exposure factors is needed. As part of a larger investigation of RCS in 15 coal mines, this paper describes analysis of silica mass content in two types of samples: (1) respirable coal mine dust (RCMD) collected in standardized locations in each mine; and (2) respirable dust generated in the laboratory from primary source materials, including coal and rock strata being mined at the production face, material obtained from the dust collection system on roof bolter machines, and rock dust products being applied by the mine. As expected, results indicate that rock strata drilled for roof bolting or being extracted along with the coal are a major source of RCS in many coal mines—although the coal seam itself can contain significant silica in some mines. While silica content of rock strata encountered in central Appalachian mines is not necessarily higher than in other regions, the sheer abundance of rock being extracted in thin-seam central Appalachian mines can explain the relatively higher silica content typically observed in RCMD from this region

Risk aversion in Entrepreneurship Panels: Measurement Problems and Alternative Explanations

In this study, we investigate the pitfalls associated with measuring risk aversion within studies of entrepreneurial behavior. First, we raise substantial concerns as to whether standard questions employed can be used to infer risk aversion among nascent entrepreneurs. In our work we show that the US, Canadian and Swedish panel study datasets do not offer evidence that entrepreneurs are more risk averse than non‐entrepreneurs. In fact, we show that the measurements used for risk aversion in these studies are not compatible with classic expected utility theory. Furthermore, our analysis reveals that probability weighting may even counteract the respondent's risk attitude. Therefore, inferring the respondent's risk attitude from choices in the panel study datasets can be misleading in the presence of probability weighting. We therefore suggest that alternative theories of decision making under risk, like prospect theory, are relevant and should be taken into account in future studies on entrepreneurship

Exploring the Effect of Particle Loading Density on Respirable Dust Classification by SEM-EDX

Exposure to respirable coal mine dust (RCMD) still poses health risks to miners. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) is a powerful tool for RCMD characterization because it provides particle-level data, including elemental ratios (via the EDX signals) that can enable classification by inferred mineralogy. However, if the particle loading density (PLD) is high on the analyzed substrate (filter sample), interference between neighboring particles could cause misclassification. To investigate this possibility, a two-part study was conducted. First, the effect of PLD on RCMD classification was isolated by comparing dust particles recovered from the same parent filters under both low- and high-PLD conditions, and a set of modified classification criteria were established to correct for high PLD. Second, the modified criteria were applied to RCMD particles on pairs of filters, with each pair having one filter that was analyzed directly (frequently high PLD) and another filter from which particles were recovered and redeposited prior to analysis (frequently lower PLD). It was expected that application of the modified criteria would improve the agreement between mineralogy distributions for paired filters; however, relatively little change was observed for most pairs. These results suggest that factors other than PLD, including particle agglomeration, can have a substantial effect on the particle EDX data collected during direct-on-filter analysis