About the Department

Apart from the important contributions by the department in the S&T and prestigious industry sponsored projects, the most significant contributions of the department are:

Based on simple rock mechanics principles, the idea of using a grouted steel rope under tension for supporting a high roof as well as overlying coal band, and to improve safe span of overhanging strata near goaf edge for semi-mechanized depillaring of a thick coal seam standing on pillars was first applied at NCPH mine of the Chirimiri area of SECL. Taking advantage of the massiveness of the coal seam and pattern of stress redistribution around a depillaring face, field application of this technique made it possible to extract the full thickness (6.0 to 8.0m) of the No. 3 seam of the NCPH colliery in a single lift, which is developed along the floor with 2.5m (avg.) gallery height with improved the strata condition, safety and achievement in respect of conservation, production and productivity. The success of the field trial triggered a number of applications of this technique in many other mines in India due to its technical superiority over the conventional method for underground extraction of thick and developed coal seams. Feasibility study of the method for its application in different coal mines has been done.

Underpinning based method of mining for depillaring of thick contiguous seams/sections under fragile parting

Extraction of clean coal from thick and contiguous seams/sections is a major problem for mining engineers. Coal mining industry of the country needs a suitable technology for extraction of the thick and contiguous coal seams under thin, weak and laminated parting. An idea of underpinning[1] was conceived[2] to facilitate safe and optimal exploitation of coal from contiguous seams/sections under thin, weak and laminated parting. The underpinning provided solution for a three fold problem like (i) reinforcement of the laminated parting to enhance the interface cohesion and thereby enhancement in the effective strength of the roof rock mass, (ii) safely leaving a thick and stable band of coal along the roof of the bottom section, if required to enhance safety and (iii) systematic support of the roof rock mass of the bottom section, consisting coal band and laminated parting, to ensure coal cum parting as a single composite beam so that the roof coal band of necessary thickness be maintained during depillaring. Underpinning consolidated parting stability through reinforcement and provided additional thickness to the critical parting as the roof coal band of the thick bottom section is stitched together with the parting. The technology was first successfully experimented at Zero seam of Chirimiri colliery with lots of appreciation from the industry. This method could find another quick successful application at another mine.

b) Wide stall mining (eco-friendly mining method) for partial extraction of coal from locked-up pillars under various surface and sub-surface features

Mining of minerals, especially, opencast mining is facing rough weather due to growing concern over its environmental impact. Though underground mining is itself eco-friendly and impact on overlying surface environment for deeper cover is comparatively less but extraction of reserve lying at shallow depth and especially, in thick seams experiences direct impact of mining on ecology by damaging surface and sub-surface features due to large scale strata movement including subsidence. The methods adopted in Indian coal mining industry for winning of thick seams were invariably in conjunction with stowing in ascending order even that also imposed serious constraints of sand and sand stowing. But even with complete stowing some strata movement is likely to be there for most of the mining methods in vogue. Moreover, due to non-availability of stowing material, the methods adopted for the exploitation of thick seam under fragile ecology were invariably very partial in form of pillar development as natural support with low recovery below 30-35 percent by way of pillar mining in sections. In view of these problems and for the region where stowing material is not available for packing of underground voids, an eco-friendly mining method i.e., Wide stall method of mining was developed to avoid surface subsidence with optimal recovery of good quality coal, improvement in production, productivity and safety of the workers and workings. The method was first successfully experimented in No. III seam underneath Bartunga hill at Chirimiri colliery, where mining posed serious problems due to protected forestland, the presence of the seam No. II in the close vicinity and inaccessible terrain. Another experimental trial was done for optimal extraction of coal locked in pillars under aquifer and fragile ecology at Umaria colliery of Johilla Area of SECL. This method of partial extraction had a great success at Umaria colliery near Umaria district town also. This colliery still extracting coal by application of this method. The competency of immediate roof and the size of pillars were matched to optimise the width and height of the developed galleries for higher production and adequate safety. Investigations into stress and deformation during wide stall formation in field and the study for stability of pillars and stalls helped in optimising the recovery of coal from locked-up pillars under surface/subsurface features.

Staggered development based method of mining for final extraction of a critically thick coal seam standing on pillars and the development made along the roof horizon

As per Coal Mines Regulations 1957, superimposed development is a statutory requirement for contiguous sections. While, on the other hand, minimum 3m thickness of the parting is must between the workings of two close/contiguous sections/seams. Critically thick seams are those, which do not provide the required minimum 3m thickness between the two contiguous sections to be developed for optimal exploitation of the seam. A complex situation arises if a critically thick coal seam is developed along the roof horizon. One simple option is to depillar developed top section first and then bottom section can be developed and depillared after settlement of the top section goaf. Stress concentration over the stooks, left inside the goaf of the top section, poses serious threat to the safety of the thin and incompetent parting (less than 3m) during bottom section working besides problems of gas, heating etc. Idea of cross development based controlled sub-level caving for extraction of total thickness in one lift, has been developed on the basis of numerical modeling and the Singareni Collieries Company Limited (SCCL) has requested us to conduct the field trial at their mine under the formulated S&T project.

Performance evaluation of country's first Continuous Miner face at Anjan Hill Mine, SECL

Underground extraction of coal seam is struggling to compete with the high productivity of opencast mines. Further, the changing policies of the government are making the competition global. Application of Continuous Miner and Shuttle car along with roof bolting technology for mass production at Anjan Hill Mine probably showed the right direction to tackle the competition. At Anjan Hill mine of M/s South Eastern Coalfields Ltd. (SECL), Zero seam of average 5.33 m thickness was developed along the floor leaving 0.6 to 1.0 m thick coal band along the roof. Average size of the developed pillars was 33 x 33 m (centre to centre) and average gallery width was 4.8 m. .In technical collaboration with Joy Mining Machinery South Africa, and Rock Mechanics Technology (RMT) U.K., depillaring of the seam was undertaken utilizing continuous miner and shuttle car combination with roof bolter. India’s first Continuous Miner depillaring panel (panel C) at Anjan Hill Mine, consisting 70 pillars was successfully extracted with outstanding techno-economical performance. This face achieved productivity over 12 tonnes per manshift and remained, almost, free from any strata control problem.

Roof bolts were used for systematic supports in and around working areas as well as breaker lines at the goaf edges during depillaring of the panel. Statistically, the application of roof bolts as breaker line was observed to be a great success for the geo-mining conditions of the depillaring panel of the Anjan Hill Mine.

The depillaring panel was equipped with a number of instruments to monitor the mining induced stress (vertical), load distribution over roof bolts, bed separation and closure between roof and floor along with goaf settlement. Some of these instruments were installed by RMT (UK) while others were installed and monitored by CMRI. In fact, CMRI studied stress redistribution and roof to floor convergence by vibrating wire stress meters and remote convergence indicators respectively using datalogger, a microprocessor based automatic monitoring system. These instruments were installed after a complete analysis of the geo-mining condition of the panel and aimed to generate an effective data set for the evaluation of performance of different underground structures during the mining.

CMRI study demonstrated that the value of mining induced stress (vertical) and convergence were maximum near the goaf edge and gradually reduced to zero at a distance of one and half pillar ahead of the goaf edge. Maximum stresses of 6.24 and 97.87 kg/cm² were observed due to splitting and slicing of instrumented pillar respectively. Similarly, maximum 8.1 and 128.5 mm of convergence was observed due to splitting and slicing of instrumented pillar respectively before fall.

Regular caving of roof strata was observed during extraction in the panel. Application of the pretensioned and stiff active support of high capacity (25 tonnes) effectively arrested the strata movement due increase in the extraction ratio during final extraction. The first Indian experience of depillaring using only resin grouted roof bolts to control goaf edge encroachment proved to be highly successful. In general, the caving of roof was limited up to the breaker line supports. On the basis of a detailed statistical analysis of the observed strata control parameters, limit for the rate of convergence and mining induced stress and roof to floor convergence rate before fall is tabulated in the report.

[1] Application of full column grouted flexible bolt (mainly steel rope) of suitable length in floor of the top section working.

[2] On the basis of extensive study on different simulated models (numerical and physical) and field testing.