Report: APP CMHS Project 1

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3.4.7. Thick Seam Mining

Indian Coalfields are limited to the states in the eastern side of the country where large numbers of seams are closely held together with varying partings of as little as 3 m. Mining in multi seam scenarios have a variety of difficulties such as: stability of workings during mining in the immediate top and bottom seams, time delay between extraction of different seams, sequence of extraction, strata control problems, etc.

As per the Coal mines regulations (CMR 1957) no work in a higher seam or section can be done over an area in a lower seam or section which may collapse. Further as any two coal seams or two sections within the same coal seam which are within 9 m from each other are called as contiguous seams/sections. In such contiguous workings, the pillars in one seam or section are vertically above or below the pillars in the other seam or section, unless the strata is inclined at an angle of more than 30 degrees from the horizontal. The parting left between any two such seams or section is not less than three metres.

The working of contiguous seams is difficult in nature due to the strata control problems like collapse of parting and load transfer on to the pillars. In the normal method of working in contiguous seams the top seam/ section is developed and depillaring completed and the caved goaf area allowed to settle for some time before commencing depillaring in the bottom seam/section. There is no method to estimate the condition of the caved strata and hence the bottom seam or section extraction is generally delayed. Problems like excessive stresses over the pillars in the bottom seam/section due to the presence of left over stooks in the top seam/section, accumulations of water and/or gas. Chances of spon com in the upper seam/section are also very common in this type of working. The presence of thin partings adds to the risk of mining in such multi seam/section workings.

Several research studies were undertaken to deal with the problems of multi seam/section mining in India by CIMFR and other scientific agencies. Some of the findings are:

  • Contiguous coal seams can be safely and economically extracted by stabilizing the parting between the seams through underpinning. The practice was followed at Chirimiri area mine of the state owned south eastern Coal fields. Use of grouted steel ropes for support of the roof coal band improved the safety of the overhanging strata span and successfully applied for the depillaring of the 12.5 thick Zero seam. Installation of full column-grouted 6–6.5 m long rope dowels at a grid pattern of 1.2×1.2 m in the floor of the top section in the development gallery, split and slices consolidated the composite parting of nearly 6.2 m thickness. Underpinning is shown in Figure 54.

Figure 54 Simultaneous depillaring with underpinning and roof bolts

  • In one case at RK-8 mine, The Singareni Collieries Company Ltd, a state owned mining company, simultaneous extraction of pillars was done in three contiguous seams with a parting of 7-9 m and with seam thickness of about 2.5 m with the technical assistance of a government scientific agency, National Institute of Rock mechanics, Kolar. The results showed no significant influence of the abutment loading even at the time of goaf settlement due to the de-stressed zone created by the seam of the above panel.

Extraction of thick seam (10.5 m) in a single lift using Blasting gallery method in a previously developed multi section; posed problems of pillar collapse at GDK -8 incline of The Singareni Collieries. The 10.5 m thick seam was developed along the top section at a height of 3.0 m. In order to extract the entire seam using the blasting gallery method the bottom section is required to be developed. Special permission from the Mining inspectorate allowed the bottom section to be developed in a staggered manner without vertical superimposition of the pillars. The results show that the staggered method of development of bottom section for under winning of roof coal is effective for a thick coal seam already developed along the roof horizon.

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