The underground coal mines under the soft cover require a phenomenal understanding of strata control to maintain mine stability for overburden load transfer during progressive depillaring operations in prevailing Indian geo-mining conditions. This paper highlights the Indian underground mines under soft cover conditions, which have challenges leading to instability of the parting strata due to uncontrolled load transfer of the dead load at the time of commencement and continuance of the working operations. A numerical modelling study was conducted in this work to assess the strata and support behaviour during progressive extraction of the developed pillars under varying thicknesses of the parting strata (PS) and the soft cover (SC) at the cover depth of 150–350 m. The modelling results were analysed to develop an approach for estimating the safe thickness of PS for a controlled and sustainable load transfer. It considered the maximum allowable convergence at the goaf edge (MACGE), which was further confirmed by the trend of the peak settlement rate (PSR) and the location of failure of the parting strata during the progressive depillaring. The laboratory investigation and numerical modelling of the prevailing soft cover condition in Kuiya Colliery were performed, and the modelling results of the parametric study were validated using the results of Kuiya Colliery and other relevant published data. The optimum capacity of the goaf edge support has also been estimated for the safe PS and the MACGE to ensure safe extraction in the given geo-mining conditions.

Globally, design-based shortcomings caused many dump slope instability in the last two decades. Thus, the reliable design and a thorough understanding of the influencing parameters of an overburden dump structure is critical for its safe operation and maintenance. This paper reports a numerical modelling-based sensitivity analysis to develop an approach for designing an optimally safe dump structure in a limited space. The study was based on the large scale opencast workings of central India. The geometrical dimensions of dump structures were compiled from the major opencast workings, and geotechnical properties were based on the laboratory findings of geotechnical tests conducted on the dump material. The effect of total dump height, bench height, bench slope angle, bench width, cohesion and friction angle were evaluated considering a strong floor through the numerical simulation using FLAC 2D software. The friction angle possessed highest and total dump height had least influence on the dump slope structure. The FoS can be less than 1 even if a single bench fails instead of the entire dump structure. Hence, Displacement and shear strain parameters were incorporated in this study along with the safety factor to identify the instability prone zones. The statistical analysis was performed using SAS v7.15 software to determine the combined effect of considered stability governing parameters on the stability of dump structure by developing a mathematical model. The R-Square of the model was 0.988 with a standard error of 2.54%. The classification of output parameters is also done to deduce the state of stability.

A modelling approach consisting of best-fit relations to estimate the post-yield strength parameters is presented for simulating post-peak behavior beyond the point of residual strength of coal pillars having different w/h ratios. The model was developed based on back-analysis of the complete stress-strain behavior of specimens belonging to six different Indian coal seams with different w/h ratios of 0.5–13.5. It was found that the simultaneous degradation of the cohesion and friction angle of the Mohr-Coulomb rock material characterizes the post-peak strength behavior of the rock. The resulting expressions are simplistic as they require parameters that can be easily determined using uniaxial and triaxial compression results. Eventually, the developed model was validated by simulating the triaxial tests of coal specimens with different sizes under varying confining stresses and comparing its findings with the published test results. The study showed that its implementation in the numerical model could reproduce laboratory-observed mechanical response, deformation behavior, and failure mechanism very closely.

Protective water barrier pillars (PWBPs) are inter-mine barrier pillars. An adequate PWBP can protect active mine workings from the danger of inundation from adjoining inundated workings. This paper discusses a hydro-mechanical coupled numerical modeling approach for the design of PWBPs, considering different flow regimes. The coupled model considers the effect of seepage through the roof, pillar, and floor on the mechanical strength of the rock mass and vice-versa. A statistical model based on the extent of positive volumetric strain zones (ZoPVS,

A continual increase in the quantity of wastes produced make stabilization of waste dumps a formidable task both from economic and technological standpoints. Of late, bio-engineering approach has been emerging as one of the most, affordable, eco-friendly, efficient and widely used methods of stabilizing such waste dumps. This study enumerates various plant species that can be used to reinforce coal mine waste dumps, the factors that affect the choice, and the techniques used to measure their effectiveness as a bio-engineering tool. The outcome of this work can be used for the selection of suitable plant species that would ensure long-term maintenance of the ecosystem as well as mechanical stability of the dumps. Locally available native species that are likely to blend in with the surroundings and adapt quickly to the climate have been suggested for the vegetation of sterile dumps. The Miyawaki plantation approach is recommended as it promotes higher survival, aids in ecological restoration, and improves the mechanical stability of the dump slope.

The size of chain pillars in the Indian geo-mining conditions is primarily decided in accordance with the regulatory provisions. These provisions are of little help for deep longwall workings, exceeding the cover depth of 360 m. Furthermore, these provisions are primarily meant to support pillars in bord and pillar workings. They may not be suitable for the chain pillars owing to a significant difference in layouts and functional requirements. Hence, it is imperative to develop a method to evaluate the stability of the chain pillar considering the complex loading and geo-mining conditions. To this end, a machine learning-based model was developed in this study to assess the stability of the chain pillars under high depth of cover considering the field-representative conditions. The data for this work was generated by conducting a parametric study using a field-validated numerical model. The strength and deformability parameters of the caved, fractured, and continuous deformation zones were established by calibrating the numerical model outcomes against the site-specific field observations. The developed ML model was used to evaluate the influence of pillar width, pillar height, cover depth, abutment angle, face length, coal strength, moduli ratio of the roof and floor strata to the coal seam, and the elastic modulus and unit weight of the overburden on the chain pillar stability. An interesting finding of this study showed that the factor of safety of the chain pillar increased with the increasing abutment angle, which may be indicative of the confining effect of the abutment angle on the pillar. Eventually, the ML model was verified by comparing its outcomes with the calibrated numerical model for a typical longwall working from Indian geo-mining conditions.

Providing a suitable protective water barrier pillar (PWBP) is common to reduce inundation hazards in underground coal mines. The imperative factors influencing its performance include the water head acting on the pillar, cover depth, pillar width, strength properties, and permeability characteristics. The mechanical failure of such pillar is a stress-controlled phenomenon, whereas the hydraulic failure is a strain-based phenomenon. A finite-difference numerical modeling approach was developed to study the hydro-mechanical coupled behavior of protective water barrier pillars. The mechanical stability was evaluated in terms of the percentage of failed (ZoF) and intact zones. The influence of the strain-controlled weakening on the permeability of the flow medium was studied through the coupling of the mechanical and hydraulic effects. The coupled steady-state model was used to estimate the outflow rate and its hydraulic stability. The adequacy of the protective pillar was also investigated by assessing mechanical stability and capability to resist hydraulic pressure against the maximum expected water head. A seepage rate-based classification system has also been proposed to evaluate the seepage potential and assess the hydraulic stability. The model has been validated for two case studies at the cover depth of 136–189.5 m and the existing pillar width of 16–42 m against the water head of 25–141 m.

A large volume of overlying waste material is removed to access deep-seated mineral deposits and stored near mines or eventually dumped as backfill. Overburden is stored in stacked dumps due to space constraints and high stripping ratios. The height and slope of these overburden dumps are enormous. This study is a parametric evaluation of the impact of interface and blasting-induced seismic loading on the stability of dump structures having total heights varying between 60 and 120 m. The study reveals that for 20 degrees of internal friction of the interface, a factor of safety (FoS) of the slope structure increases with increasing cohesion (10-30 kPa). However, as the friction angle increases from 20 degrees to 25 degrees, the relative increase in FoS is reduced. Thus FoS remains unchanged with increasing cohesion for a friction angle of 29 degrees. The stability of the dump reduces when subjected to blasting-induced seismic loading. The damage is more due to the shock waves imposing seismic loading in the horizontal direction than in the vertical direction.

A reliable design of protective water barrier pillars is critically essential for safe operation in underground mine workings. Different approaches are applied for the design of such barrier pillars, but none of them have ever evolved as a standard. This paper presents the outcome of a literature review covering various aspects of protective water barrier pillar design, including its hydro-mechanical modeling. Width/height (w/h) ratio, cover depth, water head, permeability, and discontinuities have been identified as the critical parameters influencing the performance of such protective pillars. For a given water head and the depth of cover, a lower w/h ratio of the pillar can result in an increased seepage from the pillars and inundation hazard in the worst condition. The review shows that in-depth research is required for an improved understanding of the mechanism of water flow and the failure of such pillars. This will not only help in assessing the adequacy of existing pillars but also in forming guidelines for the design of new protective pillars based on an updated knowledge base and improved understanding of the subject.

Proper investigation of the face spalling mechanism with its associated rock mechanics indicators is crucial for effective ground control in deep longwall workings, particularly under massive strata conditions comprising high strength and large thickness. This paper deals with the three-dimensional elastoplastic modeling study to understand the damage and associated spalling characteristics in a retreating longwall working. The stress–strain redistribution was simulated with progressive face retreat till the occurrence of the first weighting. The quantification of the various rock mechanics indicators viz. major principal stress, axial strain, and lateral strain were presented in the three distinct face loading conditions, namely normal, peak-stress, and weighting. The excavation damage concept was implemented to quantify the extent of EDZ (excavation damaged zone) and HDZ (highly damaged zone) at the face. The role of the massive key roof in differential loading along the length of the face was also examined. Finally, a set of design criteria for the three-dimensional assessment of the extent of face spalling was proposed integrating the results of the numerical modeling and field observations. The study revealed that the central section of the face experienced a higher intensity of damage than the gate end section when the face was exposed to extreme stress conditions. The model results indicated that 52% of the face length extending over 130 m in the central section of the 250 m long face was affected by spalling. The spalling was initiated at the middle position of the 3.5 m high face and extended to its top position affecting the 0–3 m region ahead of the face. The study also showed that the extent of spalling during the first weighting varied between 24 to 40% of the zones within the affected region. The model-observed extent of face spalling was validated against the field observation. The modeling results also agreed well with the redistribution pattern of induced stresses, differential load transfer at the face, and the mechanism of damage in the overlying strata as well as the coal face generally observed in the field. The findings of this study present a unique perspective for the rational design of face length for effective roof control in deep longwall workings.

The depths of open-pit mines have been increasing in the last few decades. This transformation generates a vast amount of waste rock material per unit mining area imposing a significant economic, social and environmental liability on the mine operators. The scarcity of dumping land along with the rise in population, competing urbanization and associated environmental clearance problems require accommodating this ever-growing volume of waste rock material at restricted available space. On the contrary, the frequency of accidents due to the instability of the waste dump slope structures has also increased, resulting in significant fatalities, apart from the economic, social and environmental impacts of these disasters. Numerous scientific studies have been conducted to reduce the occurrence of such incidents. This paper conducts a critical review of the numerical modelling-based stability analysis of such waste dump slope structures. The popularly cited cases of dump slope instability have been analysed to synthesize pertinent findings regarding the approach of stability analysis, broader design criteria, and optimization. The critical parameters to numerical modelling-based design of a safe dump slope structure are discussed in detail. The significant output parameters, apart from the factor of safety, are also outlined for evaluating the state of stability.

Prediction of pillar stability is one of the most critical tasks in underground mining industries. This pillar stability analysis requires many input parameters and some of them are difficult to be determined. Various statistical based analysis is presented in literature for assessing pillar stability successfully. In the present work, the data from three mines had been to determine the factor of safety. A total of 63 pillar cases had been collected from the mines. Principal component analysis (PCA) and Stepwise selection and elimination (SSE) models were developed by using multi variate linear regression (MLR) on 45 data sets and subsequently the proposed models were validated on 18 different data sets. The value of coefficient of determination (R2) is 0.86 and 0.84 for PCA and SSE respectively. The root mean square error for PCA and SSE are found to be 0.112 and 0.123 respectively. On validation of the proposed model developed by PCA and SSE, the PCA model provided a better validation results. Hence, PCA is recommended for modelling pillar stability.

The knowledge of the compaction behavior of goaf is critical for the stability and rational design of depillaring workings in a given geomining condition, particularly in conditions where part of the overburden is distressed in the presence of softcover comprising of dump material. In this study, a Bord and Pillar working from Jharia coalfield has been considered for the study of progressive caving behavior and goaf compaction under a similar geomining condition. An algorithm was developed and implemented with the help of FISH subroutines for plane-strain simulation of progressive caving of strata, filling of goaf and its compaction in a depillaring working using finite difference software—FLAC. The compaction and stress recovery of different goaf filled zones have also been evaluated with the progressive depillaring till the extent of working becomes extensive enough to offset the effect of cover depth. The findings show that the magnitude of the stress accommodated at the bottom of the goaf material is governed by the span of fractured strata and the distance between the face and the point of maximum stress in the goaf. The caved goaf material recovers almost 15% of the in situ stress after the main fall, whereas 89% of the stress recovery is observed at 200 m of face advance, which is almost twice the depth of cover. The goaf material spanning up to 40 m from the goaf edge achieves 50% of the maximum stress recovery at this stage. The modeling approach developed in this paper with the help of various subroutines amply explains the mechanism of compaction and settlement of the goaf material filled cyclically following progressive caving of roof in the depillaring working.

The majority of the longwall panels in India have been worked underneath massive sandstone main roof with soft coal/shale immediate roof. As the mining goes deeper, ensuring stability and consistent production from such workings is a great challenge for their safety and sustainable performance. In this scenario, understanding the face instability mechanism and its rock mechanics precursors have evolved as a significant design challenge for obtaining an optimal longwall geometry in complex geo-mining conditions. Research to date mostly focused on the failure mechanism and, in some part, displacement characteristics of the coal wall. In contrast, minimal work has been performed to address the critical research gap from the face failure to spalling with the associated rock mechanics parameters and its three-dimensional quantification. This paper considered the longwall panel of 250 m face length and 3.5 m working height in the Godavari Valley Coalfield, which was worked underneath 23.5 m massive sandstone roof at a cover depth of 323 m. The finite difference software FLAC(3D) was employed to investigate stress redistribution, failure characteristics, strain dynamics, and horizontal displacement of the coal wall until the occurrence of the main weighting. Accordingly, a set of criteria, namely, Percentage of Critically Strained Zone (PCSZ) and Percentage of Spalling Zones (PSZ), were proposed based on the critical limits of the lateral strain and the horizontal displacement of the face. The formation of these critical zones around the face was further correlated with the failure and caving characteristics of the massive sandstone roof. The study revealed that within 0-3 m distance ahead in the central region of the face, the extent of PCSZ in the Peak stress period was 46, which further enhanced and contributed in PSZ of 78 as noted during the weighting period. It is indicated that the impact of spalling dominated in the middle and the middle to the top portions of the face, covering 53% of total face length within 0-3 m regions ahead of the face. The relative influence of the failure and caving characteristics of the key roof positively correlated with the PCSZ and PSZ at the face. The critical limits of lateral strain (0.58%) and horizontal displacement (-15.40 mm), as projected in this study for evaluation of face instability, provided closer agreement with field observed extent of face spalling. The findings of the study supplements the existing knowledge about the mechanism of face spalling and its quantification in the Indian geo-mining conditions. The approach developed in this paper can be employed to analyze the extent of face instability during weighting periods.

A field representative modeling of the gob behavior is critical for the understanding of the associated mechanics of strata behavior in longwall workings. The findings of theoretical models and laboratory tests on the simulated gob material have shown the strain hardening behavior of the gob material. The double-yield material model, which is a built-in constitutive model in FLAC3D, has significant potential to simulate the gob compaction process as it represents materials which undergo irreversible compaction in addition to shear yielding. Apart from this, several researchers have attempted using modified elastic model or applied nodal force in reaction to the strain in the gob area as per the well-known theoretical relations to meet this requirement. Although these models can simulate the mechanical response of the gob for subcritical and critical extraction widths, they do not apply to super-critical extraction width as the gob stress may exceed the in situ stress level in such cases. For overcoming these limitations, a standard approach has been developed and validated in this paper, which can be used for simulating the gob behavior in super-critical longwall workings as well. The complete study is divided into two stages wherein the laboratory scale compression test on the simulated gob material has been numerically replicated to develop preliminary confidence followed by a parametric study of the double-yield constitutive parameters for a rational estimation of their values in different conditions. The calibrated model has been used for verifying its results by plane strain modeling of two different longwall workings, using well-established empirical and analytical findings and field observations. The approach suggested in this study can be used for the efficient modeling of the gob behavior of longwall panels under varying geo-mining conditions without compromising the accuracy of the numerical model. The meticulously validated model can be helpful in rational estimation of stress redistributions around the gob area apart from load transfer on the barrier pillar.

This paper presents a critical review of the various approaches for the selection of the optimum length of longwall face considering ground control and cost parameters. Various factors affecting the efficacy of roof control and their impact on the productivity of a mechanized longwall face have been identified. The outcome of the study has been used to assess the knowledge gap in existing know-how and exploring the possibility of developing a hybrid methodology for the design of techno-economical optimum length of face combining the experiences of ground control and production economics of such operations. The critical ground control parameters identified for the purpose include massiveness of the strata formation and abutment loading that creates severe roof convergence and cavity formation in the face area apart from uncontrolled goaf settlement. Such roof control difficulties not only lead to frequent production delays and stoppages of the face but also contribute to the increased cost of production and reduced face productivity. Establishing a field representative relationship between the face length and severity of roof weighting and its integration with the existing cost economics is proposed for the optimum selection of face length to ensure smoother face operation with consistent face productivity.

This research focuses on the strata control issues in a Bord and Pillar depillaring working under the influence of dead load condition of voluminous and fragmented softcover of overburden dump material. It reports the modeling of a mine working through the FLAC 2D software under the Indian geo-mining conditions. The modeling results of an actual mine working under softcover were compared with that under the intact overburden condition to assess the influence of the soft overburden over the depillaring working. The modeling work involved progressive caving of the strata and the cyclic goaf filling following the model-simulated occurrence of main fall and periodic caving. Mohr-Coulomb Failure criteria along with a simplified strain-softening (MCSS) model was used to study the failure and caving mechanism. Salamon’s compaction model was used to simulate compaction and resultant stress recovery in the goaf material. Comparison of the model findings was done in terms of load on supports, abutment stress, and convergence, apart from the stress redistribution and failure mechanism. The load transfer mechanism of the soft overburden was also studied for an overall understanding of the strata behavior in such workings.

Numerical modelling-based design and stability analyses outcomes of longwall structures critically depend on a reliable simulation of the effect of the gob compaction process. The approaches available for modelling of the gob behaviour are either oversimplification of the actual phenomenon or too complicated. In this paper, a modified elastic model along with its algorithm was developed to simulate the strain hardening behaviour of the gob material. The proposed approach was implemented in the finite difference-based code—FLAC 3D through single-element model, and its applicability was validated by comparing the model response with the laboratory-validated theoretical gob compaction model. The proposed approach can be implemented in any numerical modelling code using only three input parameters: initial modulus, density and maximum strain. Plane strain models of two longwall workings: one supercritical and the other subcritical, were constructed for its field-scale implementation to compare its findings with theoretical and field observations of the maximum surface subsidence. The modelling results of the gob stress redistribution were also verified against the findings in the literature. The study shows that the load transfer distance critically depends on the geological conditions of the overlying strata in addition to the depth of cover. The model observed profiles of abutment stress decay and the peak abutment stress of the two cases were in line with the field observations in typical geo-mining conditions in India.

India has been seeking technology for mass production of coal below ground. Longwall technology was accepted as one of the options a few decades ago. Unfortunately, it failed to meet the expected benchmark for success. This shortfall was deciphered and attributed to the inadequate understanding of cavability, leading to a mismatch in support performances. Hence, the present study attempted to bridge the gap in understanding of the strata behavior in longwall workings for the Indian geological formations. Four longwall panels representative of the major Indian coalfields were selected for the investigation. An effort was made to supplement the design-based knowledge of longwall workings, under different geo-mining and strata conditions. The stress redistribution was observed with the progressive mining along with the model and field-observed mechanism of strata failure, caving, and support loading. The effect of mining activities on the failure and deformation characteristics of the strata was studied using Finite Difference software FLAC(3D). It was found that the geo-mechanical properties of the overlying strata and the depth of mining were the most influential factors in controlling the behavior of the strata. At shallow depth, mechanical strength and the thickness of the overlying strata were identified as the main controlling parameters for most of the ground control events. However, at greater depths, face instability appeared to be the major contributor to these events. It is inferred that the proposed numerical modeling approach could be effectively utilized for evaluation of the stress redistribution, mechanism of failure, quantification of caving span, face convergence, and support loading for longwall panels in Indian geo-mining conditions.

Most of the underground coal mines in the Indian Geo-mining conditions are developed and depillared by Bord and Pillar method. Caving behaviour and strata control are the major concerns to be considered while extraction of coal seams in such workings. This paper describes the results of a scientific strata control monitoring study during depillaring in a contiguous seam working. It includes instrumentation for determining the induced stresses in the pillars and roof-floor convergence in the galleries and junctions with distance from the goaf edges in the working. The mining induced stresses on the pillars were observed to be more in the top seam than that in the bottom seam working. Convergence monitoring was done with Telescopic Convergence Indicators (TCI) and Remote convergence recorders. In general, higher convergence was observed in the junctions as compared to the galleries. The junctions located at distant location suffered more convergence than the galleries is nearer to the goaf edge. Based on the field observation, convergence classification has also been done in terms of average distance of the goaf edge from the point of observation to assess the state of 'No-convergence', 'Initial convergence' and 'Maximum convergence'. The convergence observed in the bottom seam is more than that of the top seam.