SUBSIDENCE RESEARCH IN INDIA. N.C.Saxena and B.Singh Central Mining Research Station Dhanbad, India. Abstract Due to lack of know-how of subsidence

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1 SUBSIDENCE RESEARCH IN INDIA. N.C.Saxena and B.Singh Central Mining Research Station Dhanbad, India. Abstract Due to lack of know-how of subsidence behaviour of Indian Coal Measures seams underneath surface properties have mostly remained unexploited. The research by the Central Mining Research Station, Dhanbad has made it possible to partially develop subsidence indices and also to extract more than 6-million tonne of coal underneath surface properties. Subsidence profiles have been continuous, discontinuous as well as assymmetrical with different shapes. Approaches have been suggested for anticipation of assymmetrical profiles of different shapes for sub-critical and critical widths. The subsidence movements were greatly influenced by the percentage sandstone in superincumbent strata. The non-effective width increased with the percentage of sandstone. The maximum subsidence over hydraulically sand stowed workings was less than 5 percent of extraction thickness. Hydro-pneumatic stowing system for bling back-filling of unapproachable old workings is being developed. The first field trial has shown encouraging results. Programmes for extensive investigations have been drawn. Introduction Although the importance of subsidence research in India was realised as long back as the beginning of the century, systematic investigations were started in the sixth decade by the Central Mining Research Station (CMRS), Dhanbad. The research gained impetus after nationalisation of coal mines in 1971 and 1973» The investigations have been completed over 31 workings and are in progress over ^5» Even today there is a lot to be done and programmes have been drawn for extensive investigations for next 15 years. The coal measures in India mainly belong to Gondwana and Tertiary periods and the total indicated reserves up to a depth of about 600 m are about 83-blllion tonne. Recent explorations have indicated some more reserves. The coalfields having multiple number of seams are spread over a large area in the eastern and central parts. Mining activities underground have been mainly confined in upper seams at comparatively shallow depths. There are only a few mines deeper than 500 m. The prédominent method of mining has so far been bord and pillar. The contribution of longwall system was just over one percent of the total in , which was approximately 133-milllon tonne. The share of underground mining being about 83-million. In almost all the coalfields problems are being faced in exploitation of seams due to presence of surface properties, e.g., roads, railways, rivers, ponds, buildings, etc. The subsidence research conducted so far has been helpful in extracting more than 6-million tonne of coal underneath 661

2 surface properties. A main railway line at Sudamdih in Jharia coalfield has been made to subside gradually by a maximum of kk8 mm, while three seams dipping at 1 in 2 were extracted with hydraulic sand stowing. The paste mining practices have left a legacy of unapproachable, unknown, waterlogged and dry old workings posing danger to surface properties due to collapse of pillar remnants. Many sudden collapses have taken place in such workings resulting in extensive damages to surface properties due to subsidence movements. Hydro-pneumatic system for blind backfilling is being developed for stabilising such workings. Indian Coal Measures The coal measures in Indian coalfields consist of mainly sandstones, shales, and sub-soils. In some areas the thickness of soils/clays is more than 250 m. The sandstones vary from coarse to very fine grained and the thickness of individual beds up to 80 m and more. The shales, finely bedded, interlace sandstone and coal beds and their percentage is up to maximum of about ko. The maximum thickness of a coal seam is about 150 m (jhingurda seam in Singrauli). The general thickness being 3 m to 9 m. Seams up to a thickness of 1.2 m are in general being considered unworkable. The thick seams have been extensively developed in one or more sections on bord and pillar pattern. Coal Winning The most prédominent method of underground winning of seams is bord and pillar. The longwall system has so far found very limited application. Mechanisation of mines has been on a very modest scale. The mining activities underground are confined to shallow depths in general. Extraction of thick seams developed in two or more sections has been of concern to most of the mining engineers in the country. Subsidence Profiles Visually both continuous and dis continuous subsidence profiles have been observed in Indian coalfields (Fig.1), Stepped subsidences have mainly taken place over thick seams extracted by STEPPED NUMBER OF SMALL SMOOTH bord and pillar system SUBSIDENCE STEPS IN SUBSIDENCE SUBSIDENCE PROFILE PROFILE PROFILE under shallow covers having sandstones predominently. The subsidence Fig. 1 - Nature of subsidence profiles observed were profiles. not symmetrical and their shape also varied. The norms and methods used in other countries were in general not suitable for anticipation of subsidence movements. On the basis of experience in field investigations the followd for anticipation ing equations have been suggeste of 662

3 subsidence movement profiles for sub-critical and critical widths, which can be used to obtain assymmetrical subsidence profiles of different shapes and they satisfy boundary conditions also. For sub-critical widths (Fig. 2), the boundary conditions are x = O, s a S, g = 0 and /p s» some value x = b 1 or b 2» s«0» g = 0 and Jj/p = 0 ( ii) and the equations suggested are = S m g a -s X- b - x 2 b x n 2 (b' 2^2 2x~b 2x2 x ) + V 2x2 <b* - *') o 3x * - 2nx o 2. b 2... (1)... (2)... (3) \ * * l > 4'v h 1 ' î w- V. t,/ X \ \ V, \ h V <.,! Fig. 2 - Subsidence, slope and curvature profil@ for sub-criileal widths. Fig. 3 - Subsidence, slope and curvature profiles for critical widths. For critical widths (Fig. 3)» the boundary conditions x = 0, s=ss, g = 0 and /p = 0 x = b 1 or b_, s = 0, g = 0 and /p = 0 and the equations suggested are h (ii) s» S e g = -«b - x h b k x 3 n (b* - x 4 ) 2 (M (5) 663

4 5x + 2b x - *mx b ) (6) In the above equations s, g and /pare the subsidence, slope and curvature at any point at a distance x from the origin, S is the maximum subsidenc e, b is length of each flank of subsidence profile, and n is a factor controlling the shape of the profiles. Assymmet rical profiles can be plotted by using two different values of b for the two flanks. The values of n for sub critical cases examined so far has been between 1,0 and 1 5, and in case of a critical width area having 2^-30 m quarry overburden debris on surface it was found to be 30. Non-effective Width The subsidence investigations in Indian coalfields have been confined to areas having single seam workings, in which a phenomenon of delayed subsidence was observed. The minimum width causing start of subsidence has been termed as 'noneffective width* and is expressed in terms of depth. It was found to be 0.3 to 1.0 times the depth in different coalfields and was influenced by the percentage of sandstone in the overlying strata as seen from Fig. J*. The percentage of, 2 (. sandstone tends to increase i the non-effective width. Fig % SANDSTONE k - Relationship between non-effective width and percentage of sandstone. Angle of Draw It was earlier believed that the angle of draw in Indian coal measures is negative, i.e. the area of subsidence on surface is less than that extracted underground. This view was formed mainly due to lack of information. The investigations conducted have shown that the area influenced on the surface was always more than that extracted underground, thereby the angle of draw is positive, The maximum angle of draw in case of flat seams has been between 20o and 25. It was generally more on the starting side than on the finishing side, probably due to the fact that the first break of superincumbent strata takes place in the condition of being supported on all the four sides and subsequent breaks take place in the state of having support on three sides. This phenomenon is also responsible for assymmetrical subsidence profiles. The percentage and the thickness of sub-soils and loose rocks in the superincumbent strata caused increase in the angle of draw. The maximum being about k0 in an area having about 150 m of clays. 664

5 In case of clipping seams the angle of draw was more on the dip side than on the rise side, and the subsidence movements were in confirmation with normal theorey. Maximum Subsidence The maximum subsidence observed over hydraullcally sand stowed longwall and bord and pillar workings has been about 5 percent of extraction thickness. Fig. 5 shows the maximum subsidence over stowed workings plotted against their widthdepth ratio and Fig. 6 shows a similar plot for caved workings. The figures do not indicate any relationship between these two parameters, In Fig. 7 percentage of sandstone has been incorporated with these para- ^ meters for caved workings, '""" " " which shows that the per-..' centage of sandstone also * influences the magnitude of * ', y.,j, t, t,:,,,,;,j '.b s u b s i d e n c e. Fig. 5 - Maximum subsidence Relationship between vs width-depth ratio Subsidence Parameters over stowed faces. In the following general relationships between maximum subsidence (s), slope (G), and strain (E) and I- i -! * 40 r ' M j % SANDSTONE WIDTH/ DEPTH Fig, 6 - Maximum subsidence vs width-depth ratio Fig, 7 - Relationship between maximum subsidence, per over caved faces. centage sandstone and width-depth ratio. average depth (h) the values of the constants were computed from the field observations, which are plotted against width-depth ratio in Fig. 8. It can be seen that k1 tends to decrease with the increase in the width-depth ratio, whereas kg and k.j tend to increase. G s k 1 S/h (7) E (- -) = k 2 G (8) E ( + ) = k 3 G (9) 665

6 Safe Limits of Subsidence Movements The experience in field investigations led to provisionallydefining the safe limits of subsidence movements for railway lines, buildinga and water bodies. Water bodies The maximum permissible tensile strain in the beds of water bodies is 3 mm/m. The limit has been defined considering the nature of superincumbent strata in Indian coal- fields and the nature of beds of water bodies, which are sandy in general. Fig. 8 - Relationship between ki, k2 and k3 and width-depth ratio. Railway Lines - For regularly jointed rail construction the maximum permissible strain is 3 mm/m. The limiting operating gradient is 1 in 100 or 10 mm/m. Buildinga - The maximum permissible compression or expansion in the buildings is 60 mm, which is expected to cause slight damage which can be easily repaired. Case Studies The know-how of subsidence behaviour of Indian coal measures developed after field investigations, described earlier, has been used in studying- 48 problems involving extraction of coal seams underneath surface properties and protection there-of» Six problems are under study. As a result of the studies it has been possible to help the industry to extract about 6-million tonne of coal underneath surface properties. A few important examples are given hereunder» At Sudamdih shaft and incline mines of Bharat Coking Coal Limited (BCCL) in Jharia coalfield three seams namely 7.5 m thick XI/XII seam at depth ranging between 35 m and 400 m, 3-6 m thickness in IX/X seam at depth ranging between 200 m and 300», and J-k.5 m thick VIIIA seam at depth ranging between 200 m and 400 m, dipping at 1 in 2 have been extracted underneath and in the vicinity of a main railway line, Damodar river, and other surface properties by longwall ascending slicing with hydraulic sand stowing. The total quantity extracted so far is about 2-million tonne. The railway line has been made to subside by a maximum of kk8 mm in a period of about 12 years without any disturbance in its normal operation. The maximum strain suffered by the line was 2.9 mm/m and its steepest long gradient after subsidence was 1 in 103. Another 4-5 million tonne of coal is to be extracted in future» At Surakachhar colliery of Western Coalfields Limited successful extraction has been done underneath water bodies and their highest flood level and an assisted railway siding in m thick G-III seam at depth ranging from 50 m to 140 m by longwall system with hydraulic sand stowing. The maximum subsidence observed over the stowed workings was less than 5 percent of extraction thickness. 666

7 Ten blocks in 1.7-1«8 in thick XVII-Top seam have been extracted successfully by longwall caving system at depth ranging from 220 m to 4^0 m mainly underneath built-up areas at Moonidih Project of BCCL. As a result of the extraction a number of buildings were slightly damaged, which were repaired at a nominal cost of Re 1/- ($0.1) per tonne of coal extracted. The non-effective width was found to be o,4-0.5 times the depth. The maximum subsidence observed was about 30 percent of extraction thickness for a width-depth ratio of 0.6. For the protection of surface properties in some areas the width of extraction underground was restricted to times the depth and times the depth wide barriers were left between the blocks. Successful extraction underneath and in the vicinity of surface properties has also been done at Loyabad, Pootki, Dhemomain, Ningah, Bhelatand-Sijua and other collieries. Hydro-pneumatic Stowing Laboratory investigations on a scale model led to development of hydro-pneumatic stowing system of blind backfilling of unapproachable and unknown underground workings. Schematic diagram of the system is shown in Fig. 9. The force of compressed air being released at about 300 mm below the end of the bore hole is used to keep on breaking the formation of cone below the bore hole and also to transport solids in the voids. Due to buoyancy of air the movement of solids was more towards rise side. The first field trial of the system for stabilisation of old workings in XIV seam at a depth of Fig. 9 - Schematic diagram of about k5 m underneath a hydro-pneumatic stowing jore bed at Jogta Fire system. Project of BCCL has shown encouraging results. More than 2,000 cubic metre of solids have been stowed from the first bore hole without being choked. Soon field experiments with the system would be made set a few more places. Qffinçlusio.n^ Although some know-how has been developed in respect of subsidence behaviour of coal measures in Indian coalfields more developments are necessary to fulfil the existing gaps. Programmes have been drawn for extensive research for the next 15 years for this purpose. Acknowledgements The authors are thankful to the Director, CMRS, Dhanbad for according permission to present this paper. The views espressed in the paper are those of the authors and not necessarily of the CMRS. 667

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