Integrated LEM-FEM framework for evaluating the role of fly ash for enhancing overburden dump stability

Stability of mine overburden (OB) dump slope has been a major concern over the years and needs a proper design for safe mining operation and economy of project. Incorporation of fly ash along with the OB materials may provide an alternative mode of fly ash utilization and stability of OB dump. In the present study, fly ash was mixed at a different percentage with OB material to evaluate various geotechnical characteristics. Numerical modelling based on the finite difference method was carried out to assess the stability of fly ash mixed OB dump and to suggest the optimum dump height. The result indicates that addition of 10% fly ash by dry weight with OB material results in 21.38% and 16.31% higher California bearing ratio value than CBR value of native OB material under unsoaked and soaked conditions, respectively. The maximum unconfined compressive strength and cohesion values are observed for the OB material mixed with 20% fly ash. Based on the stability analysis, optimum dump height of 55 and 50 m is observed for 35° and 37° slope angle, respectively, with an addition of 10% fly ash with OB material. The decrease in shear strain rate and the vector velocity with the addition of fly ash makes it suitable to utilize with OB material. Mathematical expression presented in this paper provides a reliable prediction of factor of safety. Abbreviations: ASTM: American Society for Testing and Materials; CBR: California Bearing Ratio; EDX: Energy Dispersive X-ray; FLAC: Fast Lagrangian Analysis of Continua; FOS: Factor of Safety; MDD: Maximum Dry Density; OB: Overburden; OMC: Optimum Moisture Content; SEM: Scanning Electron Microscope; SSR: Shear Strain Rate; UCS: Unconfined Compressive Strength; V.V. : Vector Velocity

Effect of blast induced vibration on coal mine overburden dump slope through discrete element method

This paper is mainly focused over the possible utilization of fly ash along with OB dump to enhance the stability of OB dump and thus provide a sustainable approach for better waste management of both these materials simultaneously. Instability of coal mine overburden (OB) dumps is an important problem in most of the coal mines like Jharia coalfields in India. This is mainly occurring due to sliding nature of the rock material, lack of vegetation etc. Numbers of Environmental and health issues are associated with these unstable OB dumps. As it may easily flow with running water can contaminate the nearby water resource as well as carbonaceous content of the dump causes air pollution due to simultaneous combustion. On the other hand management of coal ash that is produced from thermal plants is also an important task. Dumping of fly ash in open may cause number of environmental problems.Various geotechnical and physical parameters such as particle size analysis, specific gravity, density, and friction angle/cohesion test have been performed to check the stability of OB dump and to analyze impacts of fly ash utilization to stabilize the OB dump.

Stability of the coal mine overburden (OB) dump slopes is many times compromised owing to the lesser availability of the in-situ characteristics of the OB dump materials and their spatial heterogeneity. In this study, multi-channel analysis of surface waves (MASW) investigation was carried out for the characterization of OB dump materials of Jambad opencast coal mine of India and the variation of heterogeneous in-situ characteristics has been reported in terms of statistical parameters. Next, the stability of a two-bench OB dump slope against seismic forces was investigated through finite element limit analyses coupled with random field theory utilizing Karhunen-Loeve expansion method for the spatial heterogeneity of the OB dump material. The effects of spatial heterogeneity of the OB dump material on the probability of failure and the critical pseudostatic seismic coefficient of the slope were investigated through Monte-Carlo simulations. The influence of spatial heterogeneity of OB dump materials were investigated with both isotropic and anisotropic correlation lengths. Finally, seismic slope stability analyses were also carried out using the discrete element method for the size heterogeneity of the OB dump materials. This study summarizes the effects of heterogeneity of material property and size on the seismic stability of OB dump slope.

The scarcity of conventional aggregates with tremendous growth in highway construction and the indiscriminate dumping of industrial waste materials in precious landfills has become a huge global concern. This study is aimed at utilizing wastes from various industries, including coalmine overburden (OB) dump, basic oxygen furnace (BOF) slag, and fly ash to produce suitable and sustainable cement-treated subbase/base course layers (CBSB/CTB) for flexible pavement construction. Response surface methodology was used to optimize the composition of the blended material considering unconfined compressive strength (UCS) and Poisson's ratio. Results demonstrated that 50% OB dump, 40% slag, 5% fly ash, and 5% cement achieved a 7-day UCS of 4.84MPa and Poisson's ratio of 0.25, in line with IRC:37-2018 guidelines. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) analysis confirmed the presence of ettringite crystals, calcium-silicate-hydrate (C-S-H), and calcium-aluminosilicate-hydrate (C-A-S-H) gels which are the source of strength development in the blend. Further, a soaked California bearing ratio (CBR) of 136.08% and flexural strength of 2.06MPa after 7days and 28days of curing, respectively, demonstrates the overall strength of the stabilized waste. Approximately 4% weight loss was observed after wet-dry durability tests, indicating exceptional performance of the optimal blend in inclement weather conditions. Furthermore, the environmental impact of the blended material was studied through a leaching study. Fly ash had a high zinc (Zn) level, while BOF slag showed a rich concentrationof chromium (Cr), manganese (Mn), and iron (Fe) in the acid digestion test. In spite of this, the toxicity characteristics leaching procedure (TCLP) test indicated that the levels of heavy metals that leached from the stabilized material stayed considerably below the permissible limits set forth in Indian Standard, IS:10500 (2012). Finally, cost analysis showed a 51.6% reduction in construction cost with cement-treated industrial wastes instead of granular base/subbase made with conventional aggregates. The study recommends the suitability of the stabilized waste material as an alternate construction material for large-scale field applications, which could encourage the construction of flexible pavements that are environmentally benign, economical, and sustainable.

Fly ash generation, its utilization and safe disposal is a major problem faced by thermal power plants (TPP) in India. Despite several efforts of the government, the utilization of fly ash is reached to only 55% of the total fly ash generated and remaining 45% fly ash is still being dumped into poorly designed and maintained ash ponds. At present, fly ash is mainly being utilized for making cement, bricks, concrete, roads and small quantity in mine void filling. The consumption of fly ash in construction activity has reached to almost saturation level and there is not much potential to consume more fly ash in these segments. Mine void filling is the only potential area where bulk quantity of fly ash can be utilized and 100% utilization target can be achieved. This paper investigates the suitability of fly ash to be disposed off by mixing it with overburden (OB) dumps in coal mines. Characterization of both the OB dump material and fly ash were carried out in the laboratory. The compaction and shear tests were performed on OB dump material and the same mixed with 25% fly ash by volume as per the guidelines issued by Ministry of Environment Forest and Climate Change, Govt. of India. The stability condition of both OB dump and fly ash mixed OB dump at varying slope angles were analyzed using FLAC 3D slope stability software and dump angle were optimized for safe disposal of the fly ash. Article DOI: https://dx.doi.org/10.20319/mijst.2017.32.105121 This work is licensed under the Creative Commons Attribution-Non-commercial 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/4.0/ or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.

Overburden (OB) dumps and associated haulage are the significant contributors to increased respirable particulate levels in mining areas. Earlier studies have only focused on reporting seasonal variation of size-segregated particle mass concentration, limiting the role of specific emission sources on sensitive receptors nearby. This study estimated the impact of OB dump expansion (between years 2016 and 2018) with associated haulage on spatial pattern of particulate concentration, associated health effects, and health cost. Furthermore, a model to identify critical health risk zones was also developed. Haulage of OB and its unloading contributed to a significant increase in particulate concentration on the windward side. Moreover, OB dumping resulted in a higher respiratory dose for workers and inhabitants nearby the OB dumpsite. The results indicated that coughing along with lower respiratory problems were the dominant health effects. Moreover, the cases of lower respiratory symptoms due to PM10 emissions from OB dumps increased in 2018. The risk potential model indicated a 4.9% increase in high risk category for the population exposed to PM10 emission from OB expansion within two years. An alternative management option was proposed to reduce health risk potential. The control resulted in 73% peak concentration curtailment and 84% reduction in the surface area exceeding prescribed PM10 (100 µg/m3) levels. The said study will be useful in demarcating risk zones and findings have particular significance for dispersion of particulates emanating from OB dumps.

With the rapid increase in the accumulation rate of coal mine overburden (OB) dumps, the geoenvironmental hazards due to their failures have taken a toll. A major segment of the standard Indian regulations laid down for the overall geometry of the OB dump is based on limit equilibrium method (LEM) without any information about the displacement of the failed slope, confining stresses, stress–strain behaviour of slope material, time factor and heterogeneity (shape and size). This paper highlights the various methodologies used in the stability analysis of coal mine OB dumps. The problem domain in continuum approaches is considered as a single continuous body, so they are unable to model the discrete particle failure in the OB dump mass, whereas in the discontinuum approaches (like discrete element method (DEM)) it is treated as an assembly of independent particles giving a better realistic representation. Using DEM, the studies on slope stability analysis of OB dumps considering the aspects of heterogeneity of particles, site and environmental factors are very limited. Though OB dump consists of heterogeneous material, the previous literature is silent on the aspect of reliability analysis. The work presented here sheds light on all the above aspects from past literature and is believed to serve as a significant contribution in knowing the shortcomings, challenges and moving towards the feasible solutions utilizing the most appropriate computational technique.KeywordsCoal mine overburdenContinuumDiscontinuumDiscrete element methodNumerical modellingSlope stability analysis

Quality of land once disturbed due to mining either by opencast or underground mines though it cannot be fully restored back, it can be reclaimed by phyto-management. However, its success depends on the selection of the plant species based on their remediation ability. Here, an attempt was made to select native plant species which are dominant in a mining area of Kuju and Charhi, Jharkhand, India and have good metal accumulating capacity from coal mine overburden (OB) dumps. First, vegetation community study was performed to identify the dominant herb and shrub species through quadrat method. Seven dominant plant species were selected from the OB dumps and analyzed for metal uptake. Bioconcentration factor (BCF) and translocation factor (TF) of Cd, Ni, As, Fe, Pb, Mn, Zn and Se were calculated to understand the pattern of bioaccumulation and translocation into the various plant parts, respectively. Four plant species viz. Cynodon dactylon, Eulaliopsis binata, Croton oblongifolius, Lantana indica were found to be abundant in the area and efficient accumulators of metals from the soil. The order of uptake for the various metals in mg/kg of dry weight by the abundant plant species were Fe: 1191 > Mn: 441 > Zn: 232 > As: 12.5 > Ni: 10.91 > Pb: 8.6 > Se: 0.7 > Cd: 0.15. These species can thus be further utilized for phytoremediation of the degraded OB dumps and similar studies can help in other OB dump reclamation in other geographical and climatic regimes.

Chapter 10 - A geospatial approach to analyze the stability of mine overburden dump over reclaimed land

Fly ash is one of the major waste line products in coal power sector and simultaneously one of the most desirable products in the construction industry, due to its pozzolanic nature. This has resulted in fly ash being used as a partial replacement of cement and in various composites, for civil and mining industries. Recent studies and applications have shown the viability of fly ash as an economical and eco-friendly stabiliser for dump slopes in opencast mines. Apart from the analysis of structural stability due to the pozzolanic binding nature of fly ash, it is also important to study the hydrological aspects of dump slopes in conjugation with the fly ash stabiliser. This study performs the numerical simulation for the dump slopes using finite element analysis (FEA) for studying the effect of fly ash stabilisation layer on water movement and associated hydrological investigation. The effect of water head build-up (due to fly ash stabilisation layer) on the overall stability of dump slope is analysed and compared to give an estimate for the increase in a factor of safety of slope and a viable dimensional increase of slope geometry. It is seen that fly ash layer stabilisation is effective even in the saturated state of overburden dumps, with the factor of safety increasing from 1.09 to 1.32. Proper drainage channels are found to be essential to prevent any excess stress build-up due to the low permeability of fly ash layers.

Inappropriate management of coal mine overburden (OB) dumps creates environmental hazards. In many countries, high concentration of Mn was observed in groundwater near the coal mine OB dump sites. Comparatively new coal mines in Barjora coalfield were chosen in this study to assess the impact of OB dumps in groundwater. Acid Digestion (AD), Toxicity Characteristics Leaching Procedure (TCLP) and Batch Leaching Test (BLT) were performed to estimate Mn concentration in OB dumps and its leaching behaviour. Shale present in OB dump has potential to leach high concentration of Mn. The aquifer in this region is mainly unconfined type and water table is very shallow. In situ soil is characterized by 44.3% sand, 37.5% silt and 18.2% clay with a saturated hydraulic conductivity of 5.58 cm/day. Column study and HYDRUS 1D numerical modelling were used to simulate the migration of Mn in vadose zone. Subsurface soil does not possess appreciable attenuating potential to retard the transport of Mn in vadose zone. HYDRUS 1D model explains that 1.6 m thick soil profile of the study area may release Mn in groundwater after 11th year and it can attain the complete breakthrough at 17th year. A groundwater contaminant transport model based on available hydrogeological data has been developed using VISUAL MODFLOW 2009.1 software. The movement of Mn in aquifer has been determined using MT3DMS code. It indicates ≃ 8 km2 area around the two Open Cast Coal Mines (OCCMs) namely OCCM 1 and OCCM 2 may suffer groundwater contamination of Mn by 2050.

In the last two decades, opencast coal mining has dominated the coal mining sector of India in comparison to the underground mining. This has caused an increase in large quantity of overburden (OB) waste material which is generally dumped in the form of slope commonly known as OB dump slope. The highly heterogeneous OB dump materials are generally dumped in the loose state implying that the OB dump slopes are highly vulnerable to failure. Therefore, proper functioning of the mining activities requires stable OB dump slopes. This paper briefly provides an overview on the formation of OB dump slopes and some catastrophic failures highlighting the importance of proper and adequate analyses. Next, the major factors affecting the OB dump stability are discussed elaborately with respect to the performance of the slopes under static and dynamic loading. Finally, various approaches available for the assessment of their stability are collated and discussed. The important aspect of consideration of heterogeneity on the performance of slope is highlighted and the need of detailed future research is envisaged.

Increasing stripping ratios in surface mines have led to the removal and movement of huge volumes of overburden that is generally stored in form of internal or external dump within the mine boundaries. For safe storage in limited areas, the overburden dumps are stabilized using several popular methods. One of the recent economic and environmentally safe techniques is the stabilization of dumps using fly ash composite material. This study analyzes the sensitivity of the overall stability of the dump slope on its geometrical and material properties, when stabilized using fly ash composite. Four different geometric parameters and six material properties were varied in simulated dump models to find the sensitivity of the factor of safety (FOS) using limit equilibrium (LEM) and finite element methods (FEM). The sensitivity analysis was also carried out for minimum thickness of fly ash composite layers required for better stabilization of dump slopes. From the study, it was observed that FOS of the dump slope was most sensitive to the material properties of overburden, apart from the slope angle, dump height, number of fly ash layers, and cohesion of fly ash composite (Sensitivity Index (SI) > 0.2), while it was less sensitive to other geometrical parameters and material properties. The critical composite layer thickness was found to be most sensitive to all the geometrical parameters, but very less sensitive to the material parameters (SI < 0.1). These sensitivities give a clear idea of the criticality of various parameters for a stable dump slope design.

The ever-increasing urbanization, globalization and population have led to construction of pavement in rural areas. It has been a cause of concern of depletion of natural resources at an alarming rate but also gradually becoming a challenge for sustainability. This paper presents the application of mine overburden (OB) dump material for replacing the natural aggregate in rural roads. OB dump also known as colliery spoil which is the solid residual material resulting from the mining of coal. It is likely to contain varying proportions of sandstone, shale, mudstone and coal fragments. These overburden can be used in various ways as a substitute for conventional material for rural roads. The coal producing countries (including India) accumulate large amounts of OB close to collieries. The existing coal mine overburden might be a good alternative for aggregate and provide a suitable base and sub base material in rural roads. Use of OB may provide an economical solution to reduce the transportation cost that has to be hauled from a long distance under such conditions also reduce the environmental influence caused by coal mining. In this present study the 1:3 scaled laboratory model tests have been carried out on an unpaved pavements with OB as base and sub-base layers. Monotonic as well as cyclic tests have been conducted using computer controlled pneumatic cyclic actuator. The tests were conducted with various soil conditions and loading patterns. A finite element analysis for monotonic loading has been done with different soil conditions. Based on the test results, it can be concluded that Dhanbad soil of subgrade CBR 3 gives better results than other subgrade of low CBR. The overburden aggregate shows similar behaviour to that of virgin aggregates. Therefore overburden dumps can be used as a replacement of base and sub-base layers for rural roads where the utility of low volume roads are high.