ICGCM Papers:
Pillar Design and Global Stability
 
 
Optimal Extraction of Coal From Developed Pillars Locked-Up under Different Surface/Sub-surface Structures
35th International Conference on Ground Control in Mining
Optimal Extraction of Coal From Developed Pillars Locked-Up under Different Surface/Sub-surface Structures
by
Prabhat Kumar MandalArka Jyoti Das, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, India
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Pillar Design and Global Stability
[Author] Prabhat Kumar MandalArka Jyoti Das, CSIR-Central Institute of Mining and Fuel Research, Dhanbad, India
[Abstract] 
Key Conclusions:
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Simulation studies through numerical modelling show that the extraction of locked-up coal is possible by the two methods i.e. extraction by erecting artificial pillars and extraction by strengthening rib/remnant pillars as discussed above. The optimum size of the artificial pillar for a particular geo-mining condition will be found out by the methodology discussed in this study. Partial extraction of locked-up coal is proposed by strengthening the rib/remnant pillars by FRP. As the laboratory experiment shows that there is significant increase of strength of coal sample due to the confinement provided by FRP, simulation study is carried out to know the increase of percentage of extraction of locked-up coal if the same is extracted by strengthening the rib pillars with FRP with encouraging results.
Key Findings:
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By considering the importance of extraction of locked-up coal, two types of extraction methodologies are conceptualized and studied for different mining conditions. In the first type of extraction methodology where the recovery of coal is supposed to be maximum (around 80 to 100%), the locked-up coal is proposed to be extracted by erecting artificial pillars.The idea of erecting artificial pillars after extraction of coal pillars may be tried to support the strata to control strata movement for protecting surface/sub-surface features. The artificial pillars and its extraction methodology should be designed in such a way that it should meet the requirements of protection of surface/sub-surface features as well as facilitate economic and safe extraction of coal from standing pillars. In this research study, numerical modelling is carried out to obtain the suitable properties of the material for which strength of the artificial pillar will be optimal. With obtained material properties, the methodology is designed to optimize the size of the artificial pillar. Although, a number of extraction methodologies are designed and analyzed but a suitable extraction methodology as shown in Fig. 1a is presented here. The methodology is studied through numerical modelling by assessing the stability of surrounding rock mass (Fig. 1b) and artificial pillars as erected (Fig. 1c). The second methodology as proposed is basically a partial extraction method to improve the extraction percentage of locked-up coal by strengthening the rib/remnant pillars with Fibre Reinforced Polymer (FRP). Due to the confining effect of FRP, the rib/remnant pillars can take comparatively more overburden load. Laboratory experiment is carried out to know the confining effect of FRP on the compressive strength of coal samples. A Uniaxial Compressive Strength (UCS) testing of coal sample wrapped with double layers of Carbon Fibre Reinforced Polymer (CFRP) is shown in Fig. 2a. It is found that the compressive strength of NX size coal sample increases three times due to the confinement provided by FRP. Confining effect is further studied through the numerical modelling (Fig. 2b) to get detailed insight into the coal FRP interface. The detailed comparative study is carried out to know the increase of extraction percentage of locked-up coal by strengthening the rib pillars with FRP. Extraction methodology (Fig. 2c) is designed and studied through numerical modelling for its stability analysis to evaluate its suitability of application in underground.
Objective of the Paper:
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Design of methodologies for optimal extraction of locked-up coal in pillars under different surface/subsurface constraints.
Problem Statement:
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Coal mining in India has strategic importance for energy security of the country. Presently, the majority of coal production is coming from opencast mining under shallow depth of cover range which is fast depleting. The future coal production is mainly depended on underground mining only. But, the percentage of extraction from underground mines is very low in compared to opencast mining. Even, in most of the coalfields in India, there are many mines where the seams are only developed by “bord and pillar” system by extracting 15-20% of coal only but the pillars could not be extracted due to presence of different surface or sub-surface structures like villages, forest land, aquifers, surface water bodies, etc. and also non-availability of suitable stowing materials which lead to huge amount of coal (more than 3000 million tonnes) being locked-up in underground. Even, after stowing with sand or similar material, due to compaction there may be chances of strata movement or subsidence which may damage delicate surface/sub-surface structures. There is a pressing need for the mining industry to extract these locked-up coal pillars to meet the growing demand of coal in the country. But, extraction of the same is a serious techno-economical challenge for the industry due to the absence of suitable methodology. Spontaneous heating and fire, accumulation of poisonous gases, severe stability problems leading to unsafe working conditions and environmental hazards are the major problems associated with these locked-up coal. It would be difficult to extract these locked-up coal pillars in future if an early action is not taken for development of a suitable technology for their extraction.