Underground Coal Mining Methods and Their Impact on Safety

Abstract

In underground coal mining, the working environment is completely enclosed by the geologic medium, which consists of the coal seam and overlying and underlying strata. Depillaring is one of the most dangerous phases of underground coal mining. Even after centuries of experience in underground coal mining, there is no well-established depillaring method w.r.t existing geo-mining method. As far as fatal accidents are concerned, most of the fatal accidents occur mostly during depillaring operations. Method of extraction is one of the keys and underrated parameters, which plays an eminent role in successful depillaring in underground coal mining. This chapter highlights the underground coal mining methods and their impact on safety.

1. Introduction

Coal plays a critical role in the global economy and day-to-day development by contributing to the electricity sector and the steel and cement industries. Globally, around 37% of electricity and nearly 70% of steel are produced only using coal [1]. Coal is the backbone of both the steel and cement industries. As coal is not only the affordable option for energy in many markets, it will also remain the only feasible option for many critical industries. Coal also has a significant role in the construction of renewable energy infrastructure. It is significant to societal development, particularly in emerging economies. And also, coal is the single largest source of global carbon emissions. Advanced clean coal technologies have been developed to clean the coal by upgrading existing coal plants to the best available technology, which can decrease 2 gigatons of $C{O}_2$ emissions. The coal industry has seen consistent improvements against environmental and economic challenges by adopting current and emerging technologies [2].

It can also be observed that coal consumption, primarily in Asian countries, has increased many folds over the past decades due to the growing population and economy. In order to meet the increasing demand and consumption of coal requires a high production of coal through mining activities. The main point to be noted along with the production of coal is safety. Globally, coal mining is a highly regulated industry due to many inherent, operational, and occupational hazards. The work environment in mines is highly dynamic in nature and may change abruptly. The risks to the safety and health of employees due to the unknown and unforeseen hazards pose challenges for occupational safety and health professionals. Further, it requires comprehensive surveillance at workplaces and newer strategies to prevent accidents in mines.

At the same time, the mining sector is critical to a country, such as India, for its economic and social well-being. The mining and quarrying sector contributes around 1.63% of India’s gross domestic product (GDP). Indian economy is on the aspirational path of becoming a $5 trillion GDP economy by 2024−25 [3]. The mining industry will contribute to India’s envisaged GDP and wealth creation. As a developing economy, India depends on the energy sector’s growth to achieve the desired economic growth. Though there is significant global pressure on the immediate reduction of fossil fuels as an energy source, coal is and will remain one of India’s primary energy sources in the years to come. The past and present economic reforms have propelled the Indian mining industry into a new and competitive environment. The sector-wise GDP in India is shown in Figure 1.

Figure 1.

Sector-wise GDP in India.

1.1 Role of coal in India

Through a sustained program of investment and greater thrust on the application of modern technologies, it has been possible to raise the all-India production of coal to 716.08 million tonnes in 2020−21 (provisional). The all-India production of coal during 2021−22 was 778.19 MT (provisional), with a positive growth of 8.67%.

Coal India Limited (CIL) and its subsidiaries accounted for 596.221 million tonnes during 2020−21 as compared to the production of 602.129 million tonnes in 2019−20, showing a negative growth of 0.98%. Coal production of CIL, during 2021−22, was 622.634 MT (provisional), with a positive growth of 4.43%.

Singareni Collieries Company Limited (SCCL) is the main source of the supply of coal to the southern region of India. The company produced 50.580 million tonnes of coal during 2020−21 as against 64.044 million tonnes during the corresponding period last year. SCCL production of coal during 2021−22 was 65.022 MT(Provisional), with a positive growth of 28.55%. Small quantities of coal are also produced by TISCO, IISCO, DVC, and others [4]. In India, the total geological reserves of coal are around 3,44,021 MT, as per reports of the Geological Survey of India (GSI) as of 01.04.2020. The coal reserves of coking and non-coking coal are around 35,004 MT and 3,09,017 MT, respectively. From 1950 to 2020−21, the total coal extracted was around 1,72,96,897 TT [5, 6].

In India, open cast and underground mining are the two major coal mining methods. Opencast mining contributes about 95.74% of the total production. In contrast, underground mining contributes to the rest of the production by 4.26% during 2021−22 (provisional). The major contributors of coal in India are CIL, SCCL, SAIL, etc., in the public sector, and Reliance Power Limited, TISCO, etc., in the private sector. The overall coal production vs. open cast coal production vs. underground coal production is plotted in Figure 2.

Figure 2.

Opencast and underground coal production from 1996 to 2021−22.

1.2 Need to increase underground coal production

In India, coal mining is primarily carried out by opencast mining and underground mining. According to the Indian Bureau of Mines, there were 455 operating coal mines in India, out of which 219 were opencast, and 213 were underground. The remaining 23 were mixed collieries [7]. Even though the number of underground coal mines is proportionately equal to opencast coal mines, their production rates are nowhere on a comparable scale. The share of opencast and underground coal production in overall coal production is nearly 95.64% and 4.36%, respectively [8]. However, the thrust on increasing underground coal production is being felt by the coal companies because of many critical issues, such as difficulty in land acquisition for opencast mining under the current sociopolitical environment, the severe threat of irreparable environmental damage due to opencast mining vis-à-vis the growing concern for mitigating the environmental impact due to opencast mining and its associated cost, depletion of shallower deposits amenable to opencast mining, and at the same time, increasing demand for quality of coal production to maintain economic sustainability of the country [9, 10, 11, 12]. In order to increase coal production to meet the country’s coal demand, there is no other future option but to go for the exploitation of deeper deposits by underground mining methods.

1.3 Conventional mining

The majority of Indian underground coal mines are still working with conventional mining. This process involves drilling and blasting technology for the extraction of coal and LHD/SDL for conveyance/transport of blasted coal, followed by the installation of supports, which slows the rate of extraction and also becomes a source for adverse strata conditions in case of delay in the extraction of exposed pillar during the depillaring [13, 14]. A rib-and-slice method is commonly practiced in conventional depillaring [15, 16]. A rib/snook is left against the goaf along with breaker-line support in openings at the goaf edge [12, 17]. Left-out rib/snook is judiciously reduced at the time of retreat to facilitate the caving of the roof strata. This depillaring technique used to dominate in the past due to different techno-economic reasons, but the industry found it incapable of further incrementing coal production. The chronic problem of significantly less productivity leads to a considerable loss per ton of coal production and the lack of capability of the mass output by such technologies. Conventional mining methods and adopted low-level technologies result in a low production rate from underground coal mining [18].

It is high time to go for mass production from underground coal mining by adopting suitable technologies, such as longwall mining, shortwall/short longwall mining for developed coal pillars, room and pillar mining, and wongawilli or rib pillar extraction by continuous miner. Earlier attempts to develop mechanized underground mines were not very successful due to improper planning, lack of advanced geotechnical studies and R&D facilities, issues related to maintenance of the mechanized machinery implemented, technical lapses, high cost due to absence of indigenous equipment supplier, lack of understanding the nature of overlying strata concerning the depillaring method adopted and machinery implemented.

2. Mechanized underground coal mining

2.1 Longwall system

In India, the first mechanized longwall mining was implemented in Moonidh in 1978. Longwall mining is a new era in underground coal mining across the globe. This method can be adopted even in weak roof strata conditions with the help of an armoured face conveyor (AFC). Gassy seams are also easier to work on as there are few roadways, and ventilation presents a little problem. Longwall mining can be operated in two ways: longwall advancing and longwall retreating.

Proper scientific exploration is the critical key to the success of longwall mining. It should preferably be implemented in coal seams that are free from geological disturbances and dirt bands. The unforeseen encounter of such dirt band or geological disturbance hampers the underground production and, results in the idle time of machinery and, increases the chances of spontaneous combustion, may even result in substantial human and economic losses.

In a longwall advancing system, a rigid cycle of operations is to be followed. Faults or other geological disturbances cannot be proved in advance. For some distances behind the working face, the roadways are on moving ground. In such conditions, the uninterrupted working of roadway belt conveyors is more challenging to ensure.

When the boundary is reached, the salvage of all support materials and the removal of all machinery must be expedited if serious losses are to be avoided. The gate roads are maintained in solids and, therefore, they are well supported. In advancing the gate roads, as the boundary is reached, the longest length of conveyors, haulage tracks, etc., are required, but in retreating, these lengths go on decreasing, thus reducing the transport cost accordingly.

Packing goaf may be avoided completely, But if there are contiguous seams, packing may be required. The labor required for unproductive work of packing can be reduced. Hence, the number of productive shifts can be increased. There is no leakage of air, so ventilation is less affected [19, 20, 21, 22]. The schematic diagram of longwall mining is shown in Figure 3.

Figure 3.

Schematic diagram of longwall mining.

2.2 Longwall top coal caving

The longwall top coal caving (LTCC) method was first developed in the 1950s and 1960s in the former Soviet Union and France to increase coal resource extraction and improve productivity. During the 1970s, it was also adopted in Yugoslavia, Hungary, Romania, and former Czechoslovakia (now the Czech Republic) but generally did not receive much success at the time. The technology was formally introduced in China under extensive research and development. It has become progressively popular in China and rapidly became a significant means of extracting thick coal seams with great success in productivity, cost, and safety [23].

A thick seam is commonly considered to be one that cannot be mined underground safely and economically in single-pass cutting using the current technology, even though the exact thickness defined varies in different countries. Underground thick seam mining technologies have been developed and practiced worldwide for decades. Some methods used for extracting thick seams, including hydraulic mining and room and pillar methods, are less popular.

The multi-slice longwall mining practiced mainly in China is gradually being replaced by the LTCC method because of its lower development and operating costs, given thick seams have favorable caveability. The high-reach single-pass longwall used in Australia has been successful, but the cutting height is limited to less than 4.8 m with current equipment. To increase the recovery of Australian thick seam reserves, the LTCC mining technology is providing an alternative solution to economic underground thick seam mining in Australia [24, 25, 26, 27]. The conceptual model of longwall top coal caving (LTCC) is shown in Figure 4.

Figure 4.

Conceptual model of longwall top coal caving (after [23]).

2.3 Wongawilli mining

Australia has a history of successful pillar extraction over 90 years. In 1952, the first continuous miner was introduced at Wongawilli colliery and Huntley colliery. In 1955, they were also introduced in South Clifton colliery, Old Bulli colliery, Nebo colliery, and Kemira colliery, and later on, many continuous miners came into existence. The Wongawilli method was first adopted in Wongawilli colliery in 1958. Since then, this method has been gradually modified to suit both the mining conditions and the development of mining machinery [28].

This method provides a single working place and extraction of coal in stress relieving area. This system can be easily understood and followed by employees. With the Wongawilli system, an overall percentage of extraction of 80−90% can be expected, with improved worker safety, minimal development workings required, and high production rates achieved. The machinery used for panel development consists of two shuttle cars and a continuous miner. This system is straightforward, repetitive, and easily understood by the employees. The difficulties usually occur in lifting the final stooks at the end of each fender. This method maintains a straight line of extraction, which is helpful in mechanized depillaring in caving. The success of this method was working within distressed areas and can be adopted successfully to a depth of up to 600 meters. This method has been successfully practiced in Australia for over 90 years [29, 30]. The basic layout of the Wongawilli system of extraction is shown in Figure 5.

Figure 5.

Basic layout of Wongawilli system of extraction.

2.4 Room and pillar

In India, underground mining is highly dependent on bord and pillar/room and pillar mining due to their adaptability, flexibility, and accessibility. “room and pillar” mining involves a sequence of activities that are performed to first enter and develop the mine and then progressively extract the coal. The continuous miner extracts the coal as it moves forward, loading it onto an attached loader/shuttle car’, which, in turn, transfers the coal to the conveyor system. Bord and pillar method contributes 3−5% of total coal production and 65−75% of total underground production, whereas room & pillar mining, using continuous miners, contributes less than 1−2% of the total output and 20−25% of underground coal production [31].

The depth at which the pressure of the superincumbent strata reaches nearly the crushing strength of coal may be considered the limiting depth for room and pillar mining. Generally, at depths greater than 2000 ft, longwall mining is usually adopted.

Longwall is not usually suitable in faulted areas, and a room and pillar system is adopted. As it is difficult to dispose of the dirt bands in the room and pillar mining, clean seams free from dirt bands are suitable for the adoption of the room and pillar method. In room and pillar systems, roadways are supported by solid pillars. Thus, the roads must be maintained in safe conditions without· using many artificial supports. The relatively smaller size of the group of workers encourages team spirit. Supervision and maintenance of discipline are facilitated, and work progress can be readily checked shift by shift [31, 32, 33, 34]. In room and pillar work, the area is proved in advance by driving headings. In longwall, on meeting a fault, the whole output is lost. In room and pillar, on knowing the direction of the fault, the rest of the headings may be driven accordingly.

The deeper the seams, the greater the difficulty of working by room and pillar due to higher strata-pressure. In this method, the advance rate is sometimes slow because development takes more significant time. There is a danger of crushing during depillaring due to higher stresses of the superincumbent strata acting on the pillars.

This makes supervision difficult. If the mine is extensive, hauling lengths are greatly increased, involving increased haulage costs. In this system, the need for constant flitting of the machinery from· place to place is excellent. So, the number of idle hours of machinery in room and pillar will be greater than longwall. There is difficulty in the extraction of contiguous seams, especially during depillaring operations (without packing). However, this difficulty is reduced by working the seams in descending order. The roof control is comparatively difficult during the extraction of ·pillars when no packing is adopted. If pillars are weak, additional stresses may be thrown on the pillars in the depillaring area, which may cause the overriding of pillars or premature collapse.

If some pillars are left in the goaf, the goaf will not settle quickly, and this will produce an undulating subsided surface. In highly mechanized mines, a high standard of planning and organization and a larger staff of skilled technicians are needed to achieve maximum efficiency in mining operations.

2.4.1 Split and fender

Split and fender include pocket and wing, split and lift, and pocket and fender. Split and fender is the most practiced method in India because it can be implemented in shallow and deeper mines. In this method, initially, roadways are prepared with appropriate size for smooth movement of machinery employed, that is, continuous miner and shuttle car.

In this method, two pillars work simultaneously, one pillar under splitting and another under fender extraction. Unless the first pillar is completely extracted, CM is not allowed to start working on third pillar. A pillar is divided into two or three fenders by driving a split gallery in dip–rising direction into the pillar, depending on the pillar size and local strata conditions. Now the driven split gallery is supported by roof bolting with resin-grouted rock bolts and the installation of breaker lines; meanwhile, CM will be splitting the second pillar. The fender extraction is started by leaving a small portion of coal from the corner of the pillar; thus, slices are driven at an angle of 60 degrees with a suitable cut width depending on the fender size [12, 13, 35, 36].

A rib of 3 m coal is left against the pillar after driving the slices for stability purposes. The rib size may be 2 m or 3 m, and it is completely dependent on the pillar size and local strata conditions. After slices are extracted, a small portion of coal will be left against the goaf edge known as snook. Similarly, fender-B is also extracted.

If the caving in the goaf was delayed, induced blasting of the level junction was done. Before going for induced blasting over the junction of splits and level galleries after extraction of each fender, the snooks/ribs were further reduced or knocked down to facilitate the roof caving in the extracted area. The schematic manner of extraction in the split and fender depillaring method is shown in Figure 6.

Figure 6.

Split and fender method.

2.4.2 Christmas tree method

Christmas tree is also known as the left-right method, fish and tail method, or twinning method. This method has been practiced in the Pinoura mine in the Johilla area (Umaria coalfields), South Eastern Coal Fields Ltd. In this method, coal is extracted with the help of continuous miner by cutting slices into the pillar from both level galleries and then extracting pushouts [37]. A straight line of extraction is being maintained and the sequence so followed is to facilitate the caving of the roof in a dip direction. The number of slices driven from both level galleries completely depends on the pillar size and the local strata conditions. The first slice is made most likely with a width of 3.50 m and a suitable length depending on pillar size by leaving a small portion of coal toward the goaf edge.

Similarly, two more slices are extracted in the same manner. The fourth slice will be extracted with a width less than the first. A similar manner of extraction is followed to extract other slices on the other side of the pillar, simultaneously driving the slices into the second pillar. Now, pushouts will be driven from the dip gallery, and the number of pushouts to be lifted is completed dependent on the pillar size and the local strata conditions [36].

In India, it is being practiced mainly at shallower depths due to its design, that is, extraction sequence, but it can be successfully implemented even at a depth of around 600 m. This method was successfully adopted at a depth of more than 600 m in the United States. It cannot be practiced for large pillars since the process involves driving slices where we provide supports only at rib pillars, which results in a considerable loss in extraction in case of large pillars and stability reasons. The schematic manner of extraction in the christmas tree depillaring method is shown in Figure 7.

Figure 7.

Christmas tree method.

2.4.3 Nevid method

Neels Joubert and David Posthma developed this extraction method, so it is named NEVID after them. In this method, the snooks are predesigned in such a way as to ensure that the roof to let down in a controlled and safe manner, that is, adequate strength (width to height ratio) is maintained. For every three rows, a stooper pillar was left in addition to snooks formed. It was found that sometimes the snooks created failed unexpectedly due to inadequate strength. The primary purpose of the stopper pillar is to take an additional load off the pillars, which are extracted while they are spaced far enough apart to crush as required, mainly when the goaf hangs [38, 39, 40].

The main reason for the immediate successful results of this method is that compared to the previous mining methods in South Africa, the cut’s length was reduced and maintained appropriately to pillar size. All the lifts were driven into the pillar at an angle of 45 degrees. Another aspect of the method’s success was all the lifts in the extraction sequence were marked with the help of survey pegs and direction lines. In 2001, this method adopted panel zoning, which is a process of identifying the hazards based on geological information with the help of geological mapping to zone a panel during the development phase into low-to-high-risk zones [38]. The schematic manner of extraction to be followed in the Nevid method is shown in Figure 8.

Figure 8.

Nevid method (a) pillar extraction sequence, and (b) pillar extraction sequence layout for a complete panel [38].

2.4.4 Yield pillar method

The yield pillar method is one of the oldest extraction methods employed in underground coal mining. The main principle of the yield pillar method is to design the size of remnant pillars in such a way that they will fail in a slow and controlled manner. Thus, the method involves leaving pillars in the back area in a systematic way so that they yield over a period of time. The remnant pillars, which are allowed to yield, are designed in such a way that their width-to-height ratios are greater than five. Slices are made into pillars in a systematic and sequenced way. Along with a remnant pillar for yielding, this method allows leaving snook. The extraction of snook is highly dependent on local strata behavior, and experiences gained [41, 42, 43]. The schematic manner of extraction in the yield pillar technique is shown in Figure 9.

Figure 9.

Yield pillar method.

Although yield pillar behavior is conceptually simple, in-mine implementation is often difficult. These difficulties arise, in part, from-

• A lack of one universally accepted yield pillar technique;

• Implementation of a successful design at operation(s) having different geo-mining conditions;

• Implementation of the yield pillar design under possibly inappropriate conditions, for example, under weak roof conditions or at insufficient depth to induce pillar yielding.

Advantages, disadvantages, and present status of various mining methods are shown in Table 1.

Parameters	Longwall	Longwall top coal caving	Wongawilli	Room and Pillar
Advantages	Low operating cost	Face relocation frequency is less	Development work is minimum to maximize the work in retreating.	Moderate operating cost
	High production rate	High production rate	High production rate	Rapid development rate/ Moderate production rate
	Low dilution	Less equipment is required	The number of active working faces will be minimum, reducing the interface tramming.	Excellent ventilation
	Applicable to thick seam mining	Adopted at various thicknesses, moderately steep thick coal seams (4−20 m)	Extraction sequences were simple, repetitive, and easily understood by the employees	Not suitable for thick seam mining
	High mechanization/continuous method	Successful application in steep dipping thick seams	Workings in destressed zone	Continuous production
Disadvantages	High capital investment	Compatibility of support and other mining equipment	Difficulties in removing stooks	Moderate capital costs
	High development	The risk of spontaneous combustion is high	Difficulties in ventilating rib pillar panels when the roof caves, thus filling voids in the goaf area and choking the ventilation flow	The risk of spontaneous combustion is low/limitation on the depth
	Low selectivity and flexibility	The amount of dust is high	Moderate selectivity and flexibility	Moderate selectivity and flexibility
	High subsidence	High subsidence	Moderate subsidence	Moderate subsidence
Present status	Highly successful in China, the US, UK, Australia, and semi-successful in India due to a lack of machinery	Successful in countries, such as China, the UK, France, the US, and Russia, and yet to be implemented in India.	Most successful in Australia and New Zealand and yet to be implemented in India	The most common and flexible method in all countries, but liable to less production rate

Table 1.

Advantages, disadvantages, and present status of various mining methods.

3. Advantages, disadvantages, and present status of various mining methods

4. Role of continuous miner in Indian coal mechanization

The new era of mechanized underground coal mining in India started with the implementation of continuous miner in 2002 [44]. Since then, there has been a steady increase in continuous miners implemented in Indian coal mines with increasing mechanization and safety in underground mines. But most of the 213 underground coal mines are still highly dependent on conventional mining. This is high time to adopt complete mechanized mining in all underground coal mines with continuous miner technology.

Even though continuous miner is a major change in mechanization and safety in Indian coal mines, but still are not able to increase underground coal production due to adverse strata conditions, lack of implementing the suitable mining method prior to existing geo-mining conditions, lack of understanding of the caving nature of overlying strata before starting depillaring, which eventually results in a loss in production by leaving the number of pillars untouched in safety point of view. Undoubtedly, the future of Indian underground coal mining lies in successful pillar extraction with continuous miners. This is high time to study the stability parameters influencing the continuous miner panel workings in underground coal mines. Evolution of continuous Miner [45, 46, 47, 48] is shown in Table 2.

1870	English channel machine
1877	Harrison compressed air puncher
1877	Jeffrey (Lechmer) breast machines
1897	Sullivan air operated shortwall cutter
1898	Sullivan electrically operated shortwall cutter
1907	Goodman shortwall machine
1911	Jeffrey track cutter
1920	Jeffrey entry driver
1945	Joy 10-RU cutter
1948	First jeffrey colmol
1948	Joy 3-JCM continuous miner
1950	Lee Norse koal master
1950	Caterpillar introduces its first self-propelled wheel tractor-scraper
1951	Crawler mounted Lee Norse miner
1968	Lee Norse CM-26H miner
1954	Joy 8-CM oscillating continuous miner
1967	Jeffrey heli miner
1968	Joy ripper chain miner
1970	National mine service marietta 3060 miner
1970	Lee Norse 265 continuous miner
1972	Lee Norse 245 continuous miner
1972	Joy high-seam (12CM series) low-seam (14CM series) continuous miners
1974	Remote controlled continuous miner
1974	Joy 14-CM miner with remote control & dust collector (Jewell Ridge Coal Co.)
2000	Mid-seam high-voltage continuous miner (model 14CM27)
2012	Cat CM235 continuous miner
2020s Currently operating CMs	Joy 12 HM series (12HM21-AAA, 12HM31-B, 12HM37-A, 12HM37-B, 12HM37-C)
	Joy 14 HM series (14HM15, 14HM27)
	Joy 12 series (12CM30, 12CM27, 12CM15, 12CM12)
	Joy 14 series (14CM27, 14CM15, 14CM10, 14CM9)
	Caterpillar CM200 series (CM210, CM220, CM230, CM235, CM240)
	Caterpillar CM300 series (CM330, CM340, CM345)
	Caterpillar CM400 series (CM440, CM445)
	Caterpillar miner bolter (CM845)
	Sandvik MC series (MC470, MC430, MC390, MC350, MC250)
	Wildcat JAE42 continuous miner

Table 2.

Evolution of continuous miner.

Since the implementation of continuous miner, there has been a steady increase in the deployment of continuous miners in India. Here, some of the operating mines using continuous miners in India is listed below [49, 50].

Some of the lists of mines in Eastern Coalfields Limited (ECL) are Jhanjra mine, Shyamsundarpur UG, Kumardih-B CM UG, and Khottadih CM UG. List of mines in South Eastern Coalfields Limited (SECL) is Anjan Hill, Vindhya, Haldhibadi, Khairaha, Bangwar, Pinoura mine, NCPH Old mine, Rani Atari, Sheetaldhara-Kurja mine, Churcha R.O mine, and Vijaya west mine. The list of Mines in Central Coalfields Limited (CCL) and Western Coalfields Limited (WCL) is Churi UG mine and Tandsi mine, respectively. The list of mines in Singareni Collieries Company Limited (SCCL) are VK-7, GDK-11, Kundapuram, PVK-5, Shantikhani, and Vakilpalli mine. The list of mines operating with Continuous miners in India is shown in Table 3.

Sl.no	Mine	Sl.no	Mine
1	Jhanjra mine (ECL)	12	Churcha R.O mine (SECL)
2	Shyamsundarpur UG (ECL)	13	Vijaya west (SECL)
3	Kumardih-B CM UG (ECL)	14	Churi UG mine (CCL)
4	Anjan Hill (SECL)	15	VK-7(SCCL)
5	Vindhya (SECL)	16	GDK-11 (SCCL)
6	Haldhibadi (SECL)	17	Kundapuram (SCCL)
7	Khairaha (SECL)	18	PVK-5 (SCCL)
8	Bangwar (SECL)	19	Shantikhani (SCCL)
9	Pinoura mine (SECL)	20	Vakilpalli (SCCL)
10	NCPH Old mine (SECL)	21	Khottadih CM UG (ECL)
11	Rani Atari (SECL)	22	Sheetaldhara-Kurja mine (SECL)

Table 3.

List of mines operating with continuous miners.

5. Summary

It is necessary to point out that the many concrete recommendations made, for example, values of factor of safety (FOS), are intended only as a general guide. It is always dangerous to generalize in mining, where geological and other conditions vary widely. The recommendations should always, therefore, be reassessed in terms of local experience. Although the safety factor of the pillars is important to ensure that the pillar strength is adequate to support the overlying strata, it is only the start of the design process. Other factors that play equally important roles are the mining method to be practiced, the equipment to be used, and the behavior of the local strata. In the past, the safety factor was too often the only parameter used to determine the size of the pillars, and other critical factors were ignored. The behavior of the overburden strata is possibly the most critical Factor as it also affects the mining method, the type of mining method that can be used, and the support required during the development and pillar extraction process.

The mining method should be predetermined, and the pillar should be designed to accommodate this method. The type and size of the mining equipment also has a major influence on the type of mining that can be practiced, and the pillar dimensions required for safe and effective extraction. The critical factors that influence the manner of pillar extraction are depth of cover, physicomechanical properties of overlying strata and seam, incubation period, dip and thickness of the seam, and technology available for extraction. In the future, continuous miner is going to be the future of coal mining. It is highly recommended to understand the importance of mining methods and their impact on safety.

Acknowledgments

The authors express their gratitude to IIT(ISM), Dhanbad, for providing all the necessary facilities for carrying out research work at IIT (ISM).

Funding acknowledgement

This research has no funding from any source.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.