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

Deformation analysis in the surroundings of the roadway ahead of longwall mining, Staszic-Wujek mine, Poland

Accurate knowledge of the stress-strain state in rock mass is absolutely critical to optimise support design. Therefore, the rock mass stresses are often measured for reasons of safety and efficiency in underground mining. Investigation of the rock mass stress is usually carried out by interpretation of the rock mass deformation processes. These can be monitored and measured. The compact conical-ended borehole overcoring (CCBO) probe was used to measure the pre-mining full stress tensor and afterwards three compact conical-ended borehole monitoring (CCBM) probes were installed to continuously monitor stress changes in rock mass ahead of the advancing longwall II/501/C mining seam 501 in the Staszic-Wujek coal mine in Poland. The purpose of the monitoring was to quantify changes in stress tensor during the mining process. At the monitoring site the extracted seam had an average thickness of 3.4 m at a depth of about 890 m and lies within the Polish part of the Upper Silesian Coal Basin. The maximum measured stress of 30 MPa was oriented close to horizontal and amost parallel to the monitored roadway. The deformation analysis of roadway was carried out using a pulse 3D laser scanner at the geotechnical station. Due to favourable orientation of the lateral stress, the deformation changes in the roadway surroundings the monitoring station were minimal

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

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

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 simulation study for rock bolt loading characteristics under high stress conditions

Rhomboid shaped coal pillars (35 m x 30 m to 26 m x16 m) were formed by a modified Room and Pillar method below 850 m depth from surface at the CSM mine in the Czech Republic. The pillars were developed in a shaft protective pillar by driving roadways of 3.5-4.5 m in height and 5.2 m in width within Panel V of Seam No. 30. Development of pillars at such great depth is prone to spalling/fracturing (pillar rib dilation) due to redistribution of the high stress regime. The induced stress driven dilation was measured during partial extraction of the coal seam within the shaft protective pillar using rib extensometers. In order to stabilize the pillar ribs, four rows of rock bolts with 2.4 m length were installed into the pillar from all sides at different heights. The immediate roof was also supported by rock bolts at a 1 m grid pattern. Three-way intersections were made to control the deformation of developed pillars and other underground structures. Further, an attempt was made to understand the rock bolt loading characteristics at different stages of rib dilation using numerical modelling with the available properties of rock mass and reinforcement for the studied site. Elastic and Mohr Coulomb strain-softening constitutive models are considered in FLAC3D to evaluate the performance of the rock bolts. Results obtained on numerical models were found to be in good tune with the rock bolt loading characteristics monitored during the field study. This paper presents a discussion about the impact of rib bolting on pillar safety factor and induced load on rock bolt with respect to the dilation/spalling of pillar ribs at the studied site

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

Parametric study to design competent irregular-shaped remnants in mechanised depillaring

Mechanised depillaring (MD) has been proved to be a panacea for faster extraction of the developed pillars which is blocking access to deeper deposits and locking more than 3200 Mt of coal, developed by Bord and Pillar mining method in Indian coalfields. Depillaring of these square/rectangular-shaped developed pillars by continuous miner creates irregular-shapedribs/snooks. Stability assessment of such ribs/snooks becomes a challenging task due to various issues faced in estimation ofload acting upon it and their strength. Area-based approach for the design of rib/snook is found to be the most suitable criteriaduring MD. Field investigations supported for parametric investigation for a competent size of rib/snook with varying nature of roof and depth of cover. Taking help from previous studies to design such ribs/snooks on numerical models using FLAC3D,aparametric study is carried out using 3DEC. Strength of such ribs/snooks and load acting upon them is calculated with the help of calibrated numerical models in order to estimate their factor of safety. This paper presents a review of the previous researches and a novel numerical simulation technique for estimation of a competent size of rib/snook in a given geo-mining condition

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