Rock Mechanics challenges of depillaring at deep cover

Underground coal mining at deeper cover encounters difficult underground environment due to increase in gas content and rise in temperature transition of a mining practice from shallow to high depth cover encounters a big change in the rock mass characteristics and the stress condition becomes more complex. At higher depth of cover, the excavation starts encountering stress control regime rather than structural control behavior of the rock mass. Mechanisation and automation of underground mining activities is a solution to improve the performance of deeper mines but the approach should match with the rock mass and stress conditions of the site. Since inception, CIMFR (formerly, CMRI) is continuously working to understand behavior of the rock mass through laboratory testing, field investigations and study on simulated models. Obtained experiences during these investigations are observed to be of strategic importance during application of a modern technology to improve practical mining conditions. This paper reviews rock mechanics aspects of different mechanized pillar extraction approaches during mining of a deep seated coal seam and, also attempts to present an appraisal of some of the recent technical developments to overcome the challenges of a deep underground coal mining

Developments Made for Mechanised Extraction of Locked-Up Coal Pillars in Indian Geomining Conditions

Bord and Pillar method of underground mining has been used extensively to develop Indian coal seams into pillars and galleries. This results in only 20–30% recovery of coal and rest coal remain locked up in developed pillars. Indian coalfields are famous in the world for its uniqueness and complexity of the geomining conditions which makes the extraction of the locked-up coal pillars a difficult and hazardous activity using different underground mining methods. Indian mining industry has introduced mechanisation since last 10 years to deal with the various underground rock mechanics issues in order to improve the efficiency and safety during recovery of locked-up coal pillars. But mere introduction of mechanisation did not solve all the rock mechanics problems due to requirement of indigenous design of different involved geotechnical elements for Indian geomining conditions. CSIR-CIMFR is a national research organisation engaged in improving conditions of underground coal mines. It has developed rock mechanics advances, namely, design of irregular shaped heightened rib/snook, roof bolt-based breaker-line support, warning limit of roof sagging, and cut-out distance for continuous miner-based mechanised depillaring. This chapter presents the developments made and highlights challenges to pursue future research studies for mechanised depillaring-based mass coal production from Indian underground mines

Rib/snook design in mechanised depillaring of rectangular/square pillars

A field study at different mechanized depillaring( MD) operations in Indian coalfields (with depth ran- ging from 60–377 m and caveability Index variation from 2300–10500) found mixed performances of adopted sizes of the ribs/snooks. Formation of an irregular shaped rib/snook during MD of the existing square/rectangular pillars by a continuous miner and uniqueness of the existing geo-mining conditions also limit scope of application of the conventional rib/snook design approaches. Taking guidance from the field studies, a parametric investigation is conducted in laboratory on the calibrated simulated models using FLAC3D. An analysis of stress redistribution for different stages of the MD in simulated models provided a different characteristic of an irregular shaped ribs/snooks failure. Presence of mod- erate roof strata is found to be, relatively, more significant for the rib/snook design. Based on the si- mulation results, an attempt is made to provide a model for the rib/snook design in MD

Underground extraction of contiguous coal seams/sections consisting thin parting: a case study

Underground depillaring of contiguous coal seams/sections is a difficult problem, which becomes even more complex if the interburden/parting between them is thin. Conventionally, the depillaring with caving of contiguous seams/sections is done in descending order. However, if the parting is thin, the influence of the top sections/seams becomes extremely important for the safety and optimal exploitation of coal. Field investigations were carried out at Nowrozabad East mine where superimposed pillars of the Johilla top section were depillared above the developed pillars of the Johilla bottom section with a 3 m thick parting between the two sections. The competency of the 3 m thick parting (which consists of alternate layers of shale, sandstone and shaly sandstone) was improved by reinforcement (underpinning). The strata control investigations did not show any considerable instability problem of the 3 m thick reinforced parting and pillars of the bottom section due to top section depillaring. On the bases of this field investigation and a laboratory study on simulated models, simultaneous extraction of both the sections, along with reinforcement of the parting, was suggested for the geomining conditions of the Johilla top and bottom sections. Adopting the suggested simultaneous depillaring process, two sub panels of johilla top and bottom sections have successfully been extracted without any strata control problem. This paper presents this case study along with the results of the strata monitoring conducted to evaluate performance of the underground structures during the field trial

Performance of a coal Pillar at deeper cover: Field and simulation studies

Exhaustion of coal reserve, extractable by conventional longwall method of working at Czech mines, provided an opportunity to try modified Room and Pillar (R&P) method to extract high grade coal locked-up in larger shaft protection pillar at around 900 m depth. The shaft pillar is divided into various R&P panels and the development process in a panel followed a constant gallery width of 5.2 m and 3.5 m height. This development in the shaft pillar by bolter miner resulted oblique and inclined pillars of 860–1225 m2 area. Stability of these pillars is realised to be vital for optimum recovery of high-grade coal from these panels. No empirical formulation is found suitable for the design of pillars at such a depth. Therefore, an attempt is made to select a better representing one through comparative study of the available formulation. CMRI formula is found to be an alternative solution for designing pillars at this depth as it contains depth of cover as a design parameter. This parameter in the formulation is included to take care of the in situ stress conditions and compactness of the material at higher depth of cover. Further, a number of geo-technical instruments are installed in two pillars of Panel V for an analysis of their stability during the development. An attempt is made to develop a strainsoftening based numerical modelling approach, calibrated by field studies and CMRI empirical formulation, to assess pillar performance at deeper cover. This development is found to be helpful in pillar stability analysis, when the installed instruments ceased to work. Further, an apprehension of pillar failure by extrapolation of field results is not confirmed on simulated models. This paper briefly presents a discussion of difficulties involved in pillar design at higher depth followed by results of field and simulation conducted for the deep Czech mine

Influence of Overlying Roof Strata on Rib Design in Mechanised Depillaring

Depillaring of the existing pillars (square or rectangular in shape) by continuous miner provides irregular shaped ribs. An assessment of strength of such a rib becomes a difficult task. Literature review finds a number of approaches for the rib design but the scope of straight forward application of these approaches is limited due to the uniqueness of geo-mining conditions of the Indian coalfields. This review also finds that a wide spectrum of moderate roof conditions is yet to be properly addressed. Field studies at some depillaring operations in Indian coalfields indicated the influence of roof strata over the stability of a rib is considerably high. A systematic study of the roof strata influence on simulated models provided interesting results. A model for rib design is conceived on the basis of the simulation results. Discussing couple of field experiences of rib stability during mechanised depillaring under two different types of roof strata of Indian coalfields, this paper presents some results of the simulation along with the conceived model of a rib design

Performance of a coal Pillar at deeper cover: Field and simulation studies

Exhaustion of coal reserve, extractable by conventional longwall method of working at Czech mines, provided an opportunity to try modified Room and Pillar (R&P) method to extract high grade coal locked-up in larger shaft protection pillar at around 900 m depth. The shaft pillar is divided into various R&P panels and the development process in a panel followed a constant gallery width of 5.2 m and 3.5 m height. This development in the shaft pillar by bolter miner resulted oblique and inclined pillars of 860–1225 m2 area. Stability of these pillars is realised to be vital for optimum recovery of high-grade coal from these panels. No empirical formulation is found suitable for the design of pillars at such a depth. Therefore, an attempt is made to select a better representing one through comparative study of the available formulation. CMRI formula is found to be an alternative solution for designing pillars at this depth as it contains depth of cover as a design parameter. This parameter in the formulation is included to take care of the in situ stress conditions and compactness of the material at higher depth of cover. Further, a number of geo-technical instruments are installed in two pillars of Panel V for an analysis of their stability during the development. An attempt is made to develop a strainsoftening based numerical modelling approach, calibrated by field studies and CMRI empirical formulation, to assess pillar performance at deeper cover. This development is found to be helpful in pillar stability analysis, when the installed instruments ceased to work. Further, an apprehension of pillar failure by extrapolation of field results is not confirmed on simulated models. This paper briefly presents a discussion of difficulties involved in pillar design at higher depth followed by results of field and simulation conducted for the deep Czech mine

Field and numerical modelling studies for an efficient placement of roof bolts as breaker line support

Depending upon the site conditions, field studies of roof bolt-based breaker line support (RBBLS) found that its positions need to vary from 0.5 to 2 m out-bye side from the goaf edge for a better performance. This variation is done to adjust the extent of spalling/loosening of sides of the surrounding natural supports. As per the studied site conditions of different mechanised depillaring operations, a detailed parametric investigation is conducted on the simulated models to estimate rock load height (RLH) for a given site conditions. Results of the static (elastic) simulation study are used for an estimation length of a bolt in the RBBLS. But the field measurements found that the bolts were subjected to dynamic loading during caving of the roof strata. Accordingly, the support resistance of the RBBLS is estimated considering the dynamic effect of caving, derived from the field measurements. On the basis of these investigations, empirical formulations are attempted among the relevant geo-mining parameters for the design of a RBBLS