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FIELD AND LABORATORY CHARACTERIZATIONS OF IN SITU COAL SLURRY
ABSTRACT
Over 200 coal tailings impoundments are classified as having high hazard potential by the
Federal Emergency Management Agency’s hazard rating system. The coal tailings impoundment failures can be immensely destructive. The catastrophic tailings impoundment failures that happened in the United States such as the Buffalo Creek disaster in 1972, the Big Branch impoundment failure in 2000, and the Kingston Fossil Plant’s slurry spill in 2008, have resulted in huge social, economic, and environmental losses. Despite the scale of the failures, there are not enough data that are readily available for the stability analysis of the impoundments. Due to the scarcity of the current database, there is a need to conduct a comprehensive testing program to determine the necessary geotechnical parameters in order to realistically assess the stability of coal tailings impoundments upon various field impacts.
The testing programs in this study included the field testing such as the standard penetration test (SPT), the conventional laboratory testing, the seismic survey using the field non-destructive testing such as seismic refraction survey and multichannel analysis of surface waves (MASW) method, and the seismic monitoring using seismographs to determine the particle velocity of coal tailings upon dynamic loadings. The basic index properties, consolidation properties, hydraulic conductivity, shear strength, shear modulus, and damping ratio of coal tailings were determined from the laboratory testing program and can be used as input parameters to any numerical models. The liquefaction susceptibility of coal tailings was also evaluated based on the laboratory testing results. Shear wave and primary wave velocities of coal tailings were determined based on the seismic survey field data. The elastic moduli that are critical in understanding the stress-strain relationship of coal tailings were estimated. The in situ dynamic properties of coal tailings deposited in both active and inactive impoundments were compared. The liquefaction potential of coal tailings upon the blast loadings was assessed based on data derived from the seismic survey and the seismic monitoring programs. All results from several testing programs indicated that the coal tailings may potentially liquefy upon strong ground motions.
Other than the geotechnical properties, the mineralogical properties of the coal tailings were also characterized to evaluate the potential of coal tailings to be reutilized for fuel generation and extraction. The reutilization of coal tailings can potentially reduce, if not eliminate, the hazards posed by the coal tailings impoundments. In most studies, coal tailings were treated as having the same properties regardless of their origins (i.e. the coal ranks and basins from which the tailings are derived). Recognizing the differences in the properties of tailings resulting from the productions of coals with different ranks would help effectively select tailings that suit the requirements of various reprocessing and combustion technologies. In this study, the physical and mineralogical properties of two types of tailings produced from anthracite and bituminous coals were studied and compared. The following characteristics that have direct applications to the future coal tailings reprocessing and combustion were determined: absolute density, particle size, and elemental and mineral compositions. Based on the mineralogical properties, analyses show that coal fines are easier to be extracted or separated from anthracite tailings. The average particle size of both coal tailings, however, may need to be modified accordingly to meet the specifications of
available combustion techniques.
TABLE OF CONTENTS
LIST OF FIGURES ………………………………………………………………………………………………….. VIII
LIST OF TABLES …………………………………………………………………………………………………….. XII
ACKNOWLEDGEMENTS ………………………………………………………………………………………. XIII
CHAPTER 1. INTRODUCTION…………………………………………………………………………………… 1
1.1 Background……………………………………………………………………………………………………………. 1
1.1.1 Construction Methods of Coal Tailings Impoundments…………………………………………. 2
1.1.2 Tailings Impoundment Failures………………………………………………………………………….. 4
1.1.3 Coal Tailings Reutilization for Fuel Generation…………………………………………………… 4
1.2 Research Motivation and Objectives…………………………………………………………………………. 5
1.3 Thesis Outline………………………………………………………………………………………………………… 7
CHAPTER 2. LITERATURE REVIEW………………………………………………………………………… 9
2.1 Failure Modes of Tailings Impoundments………………………………………………………………….. 9
2.2 Previous Geotechnical Investigations on Coal Tailings……………………………………………… 10
2.6 Coal Fines Recovery and Reprocessing Techniques………………………………………………….. 17
CHAPTER 3. MINERALOGICAL CHARACTERIZATION……………………………………….. 19
3.1. Methodology of Mineralogical Characterization………………………………………………………. 19
3.1.1 Sample Selection and Collection………………………………………………………………………. 19
3.1.2 Methods of Analysis……………………………………………………………………………………….. 20
3.2 Results and Discussion…………………………………………………………………………………………… 22
3.2.1 Absolute Density and Particle Size Distribution…………………………………………………. 22
3.2.2 X-ray Photoelectron Spectroscopy……………………………………………………………………. 24
3.2.3 Scanning Electron Microscopy with Energy Dispersive Spectrometry………………….. 29
3.2.4 X-ray Diffraction……………………………………………………………………………………………. 36
3.3. Mineralogical Testing Conclusions………………………………………………………………………… 38
CHAPTER 4. FIELD INVESTIGATION……………………………………………………………………… 40
4.1 Site History………………………………………………………………………………………………………….. 40
4.2 Site Reconnaissance………………………………………………………………………………………………. 43
4.3 Field Investigation Approach………………………………………………………………………………….. 45
4.4 Sample Retrieval and Sample Storage……………………………………………………………………… 50
4.5 Field Testing Results……………………………………………………………………………………………… 51
CHAPTER 5. LABORATORY TESTING……………………………………………………………………. 53
5.1 Laboratory Testing Methodology……………………………………………………………………………. 53
5.2 Laboratory Testing Results…………………………………………………………………………………….. 55
5.2.1 Results of Index Properties of Fine Coal Refuse…………………………………………………. 55
5.2.2 Results of Dynamic Properties of Fine Coal Refuse……………………………………………. 61
5.3 Laboratory Testing Analysis and Discussion…………………………………………………………….. 64
5.3.1 Statistical Analysis of Laboratory Testing Results………………………………………………. 64
5.3.2 Liquefaction Susceptibility of Fine Coal Refuse…………………………………………………. 67
CHAPTER 6. SEISMIC SURVEY AND SEISMIC MONITORING……………………………… 69
6.1 Methodology of Seismic Survey……………………………………………………………………………… 71
6.2 Methodology of Seismic Monitoring……………………………………………………………………….. 74
6.2.1 Measurement of Ground Velocity…………………………………………………………………….. 77
6.2.2 Measurement of Dynamic Pore Water Pressure………………………………………………….. 79
6.3 Results and Discussion of Seismic Survey………………………………………………………………. 82
6.4 Results and Discussion of Seismic Monitoring…………………………………………………………. 95
6.4.1 Results and Discussion of the Peak Ground Velocity………………………………………….. 95
6.5 Liquefaction Potential of Fine Coal Refuse…………………………………………………………….. 101
6.5.1 Estimation of CSR from Earthquake Loading…………………………………………………… 104
6.5.2 Estimation of CSR from Blast Loading……………………………………………………………. 104
6.5.3 Estimation of CRR of Coal Tailings………………………………………………………………… 105
6.5.4 Liquefaction Potential of Coal Tailings upon Dynamic Loading…………………………. 106
6.5.5 Interpretation of Coal Tailings Susceptibility to Liquefaction…………………………….. 106
CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS…………………………………… 109
7.1 Conclusions………………………………………………………………………………………………………… 109
7.2 Recommendations for Future Research………………………………………………………………….. 112
REFERENCES…………………………………………………………………………………………………………… 113
APPENDIX A: MULTICHANNEL ANALYSIS OF SURFACE WAVES DATA………….. 117
APPENDIX B: SEISMIC REFRACTION DATA………………………………………………………… 123
APPENDIX C: CALCULATION OF LIQUEFACTION POTENTIAL……………………….. 134
CHAPTER 1. INTRODUCTION
1.1 Background
In the United States, over 200 coal waste tailings impoundments are classified as having high hazard potential by the Federal Emergency Management Agency’s hazard rating system. Although the coal tailings impoundment usually was perceived as stable under its self-weight, its stability upon field impacts, such as blasting events and earthquakes, remains unknown due to the limited geotechnical studies that had been conducted in the past.
Coal tailings impoundments are the most common and critical facility for coal refuse disposal. They are typically designed using coarse refuse and soil or rock fill materials for construction with placement of fine coal refuse slurry in the associated impoundment. Coal refuse, also known as tailings, are the materials left over after the process of separating the valuable fraction of coal mines. Coal refuse can have a wide range of size distributions from rock fragments such as friable shale materials to clay-size particles, and it typically includes varying amounts of coal that can influence material behavior. Fine refuse is a separate waste stream resulting from the wet processing of coal. It may be: (1) disposed as a slurry (fine coal refuse slurry) separate from the coarse refuse, (2) dewatered and disposed with the coarse refuse as combined refuse, or (3) dewatered and disposed separately from the coarse refuse (dewatered fine coal refuse or filter cake)
(D’Appolonia Engineering, Inc. 2009).
1.1.1 Construction Methods of Coal Tailings Impoundments
The configuration of coal refuse impoundments generally has four categories: (1) crossvalley impounding embankment, (2) incised impoundment, (3) side-hill impounding embankment, and (4) diked impounding embankment. Planning and design of an impounding embankment generally involve distinct development and construction stages. The direction of construction normally falls into two categories: upstream and downstream. Upstream construction, as shown in Figure 1-1 (a), involves initial construction and placement of coarse refuse in downstream areas to form the impoundment with sequential placement during subsequent stages in upstream locations, typically at higher elevations. Downstream construction, as shown in Figure 1-1 (b), involves initial construction and placement of coarse refuse in upstream areas with placement during subsequent stages in downstream locations. It is common to have both upstream and downstream construction stages as part of a disposal facility design. An intermediate development condition is centerline construction (which is essentially the same as alternating upstream and downstream construction), where refuse stages are constructed both upstream and downstream of the previous stage, with the crest of the two stages generally in alignment but separated by the elevation increment of the stage (Figure 1-1 (c)). Upstream construction, and to a lesser degree centerline construction, with placement of coarse refuse embankments on settled fine coal refuse, introduce stability concerns due to the potentially low strength of the fine coal refuse during initial covering and the potential for seismically-induced strength degradation (D’Appolonia Engineering, Inc.
2009).
- Upstream staging method
- Downstream staging method
- Center staging method
Figure 1-1. Schematic cross sections of coal refuse impoundment construction methods
(figures are from D’Appolonia Engineering, Inc. 2009)
1.1.2 Tailings Impoundment Failures
An extensive study by the International Commission of Large Dams (ICOLD) and the United Nations Environment Programme found that, on average, one major tailings dam incident occurs each year, although that figure doubled between 1995 to 2001 (ICOLD 2001). According to the statement from the National Inventory of Dams, there are more than 700 coal waste tailings dams in the United States, and 241 of them are classified as having high hazard potential (facilities whose failure could reasonably be expected to cause loss of human life, severe damage to houses, industrial and commercial buildings, important utilities, highways, and railroads) by the FEMA hazard rating system. Failures of coal refuse impoundments include breaching of the tailings dams such as the Kingston Fossil Plant coal fly ash slurry spill in Tennessee in 2008 and the breakthrough of impounded coal refuse slurry into underground mines such as the incident in Kentucky in 2000 (Michael et al. 2010). Failures of the coal refuse impoundments can be triggered by various conditions, such as the loading change due to the upstream dam construction on settled fine coal refuse, added overburden stress due to final soil cover during coal refuse impoundment reclamation, and seismic excitations due to earthquake or blasting.
1.1.3 Coal Tailings Reutilization for Fuel Generation
Before being sold to the market, coal ores mixed with debris need to be washed using water so that the valuable coals can be extracted. This washing process, also known as coal preparation process, produces coal tailings that are composed of water, non-valuable minerals, and coal fractions (MSHA 2009). The National Research Council reported that, in the United States alone, 70 to 90 million tons of coal tailings out of 1.1 billion tons of coal mined were discharged annually into impoundments (National 2002). There are approximately 713 active tailings impoundments in the United States that are being monitored by the Mine Safety and Health Administration
(MSHA). The deposition life span of a tailings impoundment is usually more than 20 years (National 2002). Thus, the costs associated with managing the safety and operation of the impoundments usually cause significant economical disadvantages to the mining companies and local governments. Even after impoundments have been totally filled, the low-strength waste materials stored in the impoundments still pose substantial risks to environment and public safety (ICOLD 2001; Kossoff et al. 2014). Due to the safety issues and high operating costs associated with tailings impoundments and the increasing demand for power generation, coal tailings have been considered as a potential material for structural fill (Indraratna 1994) and fuel generation and extraction (Lewitt 2011; Salih et al. 2015; Collins and Ciesielski 1994; Fehrenbach et al. 2003).
1.2 Research Motivation and Objectives
The catastrophic tailings impoundment failures happened in the past 50 years have resulted in huge social, economic, and environmental losses. The tailings dam failures that happened in the
United States include the Buffalo Creek disaster in 1972, the Big Branch impoundment failure in
2000, and the Kingston Fossil Plant’s slurry spill in 2008. These case histories show that the coal
tailings dam failures can be immensely destructive.
Despite this tremendous tragedy, the geotechnical studies conducted on the properties of coal tailings were very limited. There are many reasons behind the scarcity of database of the geotechnical properties of coal tailings. Other than the limited research funding, coal slurry impoundments are also in general hardly accessible. To access the impoundments and to retrieve coal tailings samples, the following factors need to be compromised: the agreement with the coal company to enter the impoundment for research purposes, the safety purposes for research personnel to stay in the impoundments, and the practicability of passing through the narrow entrances of the impoundments with large equipment. Even when these difficulties are successfully resolved, the uncertainty associated with the disturbed samples may still arise. In fact, the geotechnical studies on coal tailings that have been carried out in recent years mostly used reconstituted samples or relatively undisturbed samples; very limited field investigations have been conducted on coal tailings. To realistically assess the stability of coal tailings impoundments upon field impacts, there is a need to conduct a comprehensive geotechnical investigation, which includes laboratory testing, conventional field testing such as standard penetration test, and nondestructive field testing such as seismic refraction survey and multichannel analysis of surface waves method.
Due to the long life-span and the high maintenance cost of coal tailings impoundments, another approach that has been explored by past researchers is to reutilize coal tailings for future fuel generation and extraction. It is believed that this approach can potentially reduce the amount of tailings, which in turn lower the number of unstable impoundments, and therefore the number of impoundment failures.
Fuel generation and extraction techniques using coals have been developed for decades and there are many available options to choose from. To effectively select tailings that suit the requirements of various reprocessing and combustion technologies, it is important to recognize the differences in the properties of tailings resulting from the productions of coals with different ranks.
However, coal tailings were treated as having the same properties regardless of their origins (i.e. the coal ranks and basins from which the tailings are derived) in most studies. It is still an unknown whether the tailings produced from different types of coal could have different properties. Hence, there is a need to study and compare the physical and mineralogical properties, especially the characteristics that have direct applications to the future coal tailings reprocessing and combustion, of two types of tailings produced from anthracite and bituminous coals.
The main objectives of this thesis are to determine and compare the geotechnical properties of coal tailings from the laboratory testing, the conventional field testing, and the non-destructive field testing. Conclusions are made regarding the liquefaction susceptibility of coal tailings based on their geotechnical properties. The geotechnical properties of coal tailings from the active and inactive impoundments are compared. At the same time, in order to explore the practicability of reusing coal tailings for future fuel generation, this study also aims to compare the mineralogical properties of tailings produced from two major types of coals (i.e., bituminous coal and anthracite) so that the tailings can be effectively selected to suit the requirements of various reprocessing and combustion technologies.
1.3 Thesis Outline
This thesis consists of seven chapters. Chapter 1, where this thesis outline is located, elaborates on the case histories of tailings impoundment failures, construction methods of tailings impoundments, and the research motivation and objectives of this study.
Chapter 2 provides the literature review of the geotechnical properties of coal tailings. The geotechnical investigation, non-destructive field investigation, and mineralogical characterization of coal tailings that had been carried out in other studies were also thoroughly reviewed and are presented in this chapter.
Chapter 3 presents the mineralogical characterization of coal tailings. The physical and mineralogical properties of tailings produced from two major types of coals, bituminous coal and anthracite, are characterized and compared.
Chapter 4 presents the field investigation of coal tailings. The logistics of field sampling and testing are included in detail.
Chapter 5 presents the laboratory testing that was conducted to evaluate the basic and dynamic geotechnical properties of coal tailings. The statistical analyses of the coal tailings properties were performed.
Chapter 6 presents the seismic survey and seismic monitoring of coal tailings impoundments. The liquefaction potential of coal tailings is evaluated.
Chapter 7 summarizes the significant findings and conclusions of this study.
Recommendations for future work are also provided.
FIELD AND LABORATORY CHARACTERIZATIONS OF IN SITU COAL SLURRY
