Exploitation of mica deposits at Nellore mica belt, Andhra Pradesh, India

India is the leading producer of sheet mica and a major part of this is exported. Nellore mica belt is the largest mica-producing area covering part of Nellore district in Andhra Pradesh, India. As most of the mines are old and privately operated, they are developed and operated purely based on local experiences. In this article, we highlight the problems associated with the present mica-mining practices in the Nellore mica belt, and scientific approaches that have been adopted for fixing different parameters associated with mica extraction. Based on detailed field study, geo-mechanical data and tested rock properties, extensive numerical modelling is done to suggest the best possible method of mining for safe and sustainable mica extraction from the area

Role of geological discontinuities during application of continuous miner technology in underground coal mines

A major portion of ground control problems encountered in underground coal mines can be attributed to geological discontinuities in the coal seam and the strata surrounding the seam. The role of geological discontinuities becomes more vital during application of mass production technology like continuous miner in maintaining safety, production and efficiency. Careful and detailed geological mapping to pick up the trend of joints, faults/slips, cleat etc., and documentation of roof instability problems encountered during development is essential in anticipating and controlling instability in the mine roof before planning of pillar extraction. Mapping helps in planning the orientation of extraction line to induce caving during depillaring and improving stability at the face significantly. Loss or damage of machines including decrease in production and productivity due to trapping of continuous miner may also be avoided. This technique estimates the orientation of major principal stress and avoids heavy expenditure on conducting in-situ stress measurements. It enhances in understanding the influence of drivage direction on roadway conditions and re-orientation of the roadways to attain stability in the roof. In the present paper, authors have dealt two cases, VK-7 incline and GDK-II incline mines of Singareni Collieries Company Limited (SCCL), where geological discontinuities played a crucial role during application of continuous miner technology in underground coal mines

Investigation of Failure Mechanism of Inclined Coal Pillars: Numerical Modelling and Tensorial Statistical Analysis with Field Validations

Analysis of the failure mechanism of inclined coal pillars is one of the complicated issues. The wide variability of dip angles of inclined coal pillars makes it more complex. The asymmetric stress distribution and the tendency of shearing along the bedding planes make the inclined coal pillars to behave differently from the flat coal pillars. There is a need for in-depth investigation of the failure mechanism for addressing the instability problems of the inclined coal pillars. Most of the literature quantifies only the magnitudes of the mean principal stresses by classical statistics. As the stress is a second-order tensor having six independent components, the classical statistics is not appropriate to calculate the mean and variability of the principal stresses at the onset of failure of the pillars. In this paper, a comprehensive analysis is done to understand the complex failure mechanism of the inclined coal pillar using numerical modelling as well as tensorial statistics and validated the results with field measurement data of failure cases. The failure mechanism is analysed by quantification of the characteristics of the inclined coal pillars by the principal-stress magnitude and its orientation, induced at the time of failure. Since the spatial variability of the magnitudes and orientations of the induced principal stresses exist within the inclined coal pillars, the mean induced principal stresses (σ1¯¯¯¯¯,σ2¯¯¯¯¯ and σ3¯¯¯¯¯) are used to quantify the stress states within it. The failure stress states within the coal pillars having different dip angles are generated by the calibrated elasto-plastic numerical modelling with the ubiquitous joint model. Several statistical parameters are calculated to quantify the stress-tensor variability and the correlation among the stress-tensor components. It is found that the correlation coefficients among the shear components increase significantly with the increase of the coal pillar dip angle. Therefore, the inclined coal pillars are highly susceptible to shear failure. The magnitudes, as well as orientations of the mean induced principal stresses within the coal pillars obtained through numerical modelling, are quantified by the tensorial as well as classical statistics. It is found that the magnitude of the mean major induced principal stress (σ1¯¯¯¯¯) at the time of failure, i.e. the strength of the pillar decreases with the increase of the dip angles. The validation of the results with the actual stress measurement data shows that all the failed pillar cases are correctly predicted by the tensorial statistical approach whereas the classical statistical approach does not effectively predict the actual failed condition of the pillars. The study would help to characterise the behaviour of the inclined pillars and address the instability issues for safe and efficient mining of inclined coal seams

Assessment of roof convergence during driving roadways in underground coal mines by continuous miner

In India, a number of coal seams are being extracted by using the continuous miner (CM). There is a need to standardise some of the major parameters like cut-out distance (COD) and the convergence of the roof strata for safe operation of the CM. The production and productivity of this system mainly depend on the COD, i.e. the length of the stable drivage made in the coal seam by the CM at a time without installing any reinforcement/support. This factor is determined by assessing the convergence of the roof strata. Therefore, prior to driving of the roadways by the CM, the estimation of the convergence of the roof strata is necessary for safe and stable driving of the roadways. No suitable empirical relationship is found between the convergence and the COD. This paper deals with the assessment of the roof convergence during development of the coal seams by the CM. Variations of the convergence with the major influencing parameters, i.e. the COD, the rock mass rating and the width of the gallery, are obtained by the parametric study through elasto-plastic numerical modelling. The trend of the convergence data with respect to the input parameters is found non-linear. Thus, a non-linear multivariate model is framed by putting the constant and exponents in the input parameters. These constant and exponents are determined through regression analysis to develop a predictive model. The coefficient of determination of the model is around 0.98. The model is validated with the monitoring data of different mines. This model can be applied for the assessment of the convergence of the roof in underground coal mines for safe driving of the roadways by the CM. Alternatively, optimum COD of the CM can be designed by fixing the threshold limit of the convergence value for a particular geomining condition

Probabilistic stability analysis of failed and stable cases of coal pillars

The conventional design criterion of the safety factor for pillar stability is a deterministic approach characterised by non-random and exact values of its input parameters i.e. strength and stress. The integrity of pillars often jeopardized when some fluctuations or deviations occur in their strength and stress due to uncertainties in field conditions. To counter these unknown fluctuations or deviations, the coal pillars are designed with safety factors greater than unity and their exact values are selected based on local geomining factors. This practice does not guarantee stability against the geomining uncertainties and consequently lock-up the coal in the bigger size of the pillars. Therefore, to assess the variability in the input parameters in pillar design, this paper presents a probabilistic approach to analyse the stability of coal pillars considering the cases of stable and failed pillars in Indian coalfields. The databases of input design parameters are collected for stable and failed coal pillars from different Indian coal mines. The probabilistic distribution fittings of strengths, stresses and safety factors of stable flat, stable inclined, failed flat and failed inclined pillar cases are derived to know their means and standard deviations. The Monte Carlo Simulation and the First Order Reliability Method have been implemented to solve the limit state functions. The failure probabilities estimated for stable flat, stable inclined, failed flat and failed inclined cases are 0.29, 0.28, 0.56, and 0.99 respectively. The analysis recommends the safety factors of corresponding threshold failure probabilities of stable flat pillars as 1.61 and for stable inclined pillars as 1.41 in Indian mines with the ranges of design parameters considered in their databases. Therefore, the probabilistic stability analysis can provide an additional design criterion to select the optimum safety factor of pillars for improved recovery rates in coal mines

Underground extraction methodology of contiguous coal seams ensuring the safety of the parting and the surface structures

In India, the coal seams are called contiguous, when the thickness of the parting formation between two coal seams is within 9 m. When the extraction is carried out in one seam, it not only disturbs the stress regime of the current working and its adjoining mined out areas but also alters the stress regime of the overlying and underlying formations. The interactions among coal seams become more pronounced when the thicknesses of the partings are lesser. If the parting between two coal seams collapses during their extractions, there may be a chance of casualty. The extraction of the contiguous seams becomes more difficult if the surface structures are present above the seams. If the seams are situated at a shallow depth of cover, the effects of the contiguous seams working easily reach to the ground surface. Therefore, the contiguous coal seams require suitable extraction methodology to ensure safety. In this study, a methodology is designed for safe extraction of the contiguous coal seams under surface structures. The safety of the parting is assessed by the analytical models, the numerical simulations and field investigation. The support system is designed to ensure the safety of the parting during the extraction of the coal seams. To assess the impact of mining on the surface structures, subsidence parameters are evaluated which are found to be within the permissible limits. Thus, this extraction methodology and the safety control measures can be applied for safe extraction of the contiguous coal seams having similar geomining conditions