International Journal of Mining Science and Technology xxx (2017) xxx–xxx Contents lists available at ScienceDirect International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Das Arka Jyoti ⇑, Mandal Prabhat Kumar, Ghosh Chandra Nath, Sinha Amalendu CSIR-Central Institute of Mining and Fuel Research, Dhanbad 826001, India a r t i c l e i n f o Article history: Received 25 May 2015 Received in revised form 27 August 2015 Accepted 12 November 2015 Available online xxxx Keywords: Locked-up coal Confined core Yield criterion Strengthened rib pillar Strain softening a b s t r a c t In some of the coalfields in India, coal seams are only developed but no extraction of pillars is possible due to the presence of surface or sub-surface structures and also non-availability of stowing materials which leads to huge amounts of coal being locked-up underground Spontaneous heating and fire, accumulation of poisonous gases, severe stability issues leading to unsafe workings and environmental hazards are the major problems associated with the developed coal pillars So, there is a pressing need for a technology for the mining industry to extract the huge amount of coal locked-up under different constraints In this study, the locked-up coal is proposed to be extracted by artificially strengthening the rib pillars The detailed comparative study is carried out to know the increase of extraction percentage of locked-up coal by strengthening the rib pillars with FRP Extraction methodology is designed and studied through numerical modelling for its stability analysis to evaluate its suitability of application in underground Ó 2017 Published by Elsevier B.V on behalf of China University of Mining & Technology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction Coal being a finite and non-renewable natural resource, its conservation is an important issue The aspect of conservation of coal is considered right from the planning stage and maximum recovery should be ensured during the implementation stage In order to play a leading role in the coal mining industry and to meet the primary energy requirement of the country, indigenous development of suitable underground mining methods is of strategic importance In some of the coalfields, coal seams are developed but no extraction of pillars is possible due to the presence of surface or sub-surface structures and also non-availability of stowing materials which leads to huge amounts of coal being locked-up underground [1,2] Spontaneous heating and fire, accumulation of poisonous gases, severe stability issues [3] leading to unsafe workings and environmental hazards are the major problems associated with the development of coal pillars So, there is a pressing need for a technology in the mining industry to extract the huge amount of coal locked-up under different constraints [4] It may not be possible to recover this coal in future if early action is not initiated for the development of a suitable technology Therefore, a study was undertaken to develop a methodology to extract the maximum percentage of locked-up coal from developed pillars under con⇑ Corresponding author E-mail address: arkajyoti19@gmail.com (A.J Das) straints by strengthening of reduced coal/rib pillars In case of non-availability of stowing material, locked-up coal may be extracted with the help of this methodology Strengthening of remnant pillars [5] is planned to be done by wrapping the reduced coal/rib pillars with suitable materials In general, a large size of remnant/rib pillar is required to be left to ensure the long-term stability in the conventional method of partial extraction where the percentage of extraction reduces drastically If the remnant/rib pillars are strengthened, it may be possible to reduce the size of the remnant/rib pillars which provide enhancement of recovery of coal Although stowing increases the strength of remnant/rib pillars by imposing lateral confinement [6], rib pillars may lose their strength gradually due to compaction of stowing material Therefore, it should be ensured that for extraction of coal under constraints by strengthening the rib pillars, the pillars should take the load of overlying strata without causing any failure In this paper, several methodologies of extraction of locked-up coal by strengthening the rib pillars are studied through numerical modelling to ensure the increase of percentage of recovery of coal Laboratory testing of artificially strengthening of coal samples For development of methodologies for extraction of locked-up coal by strengthening rib pillars, data related to laboratory testing of artificially strengthening of coal samples using different type of http://dx.doi.org/10.1016/j.ijmst.2017.01.024 2095-2686/Ó 2017 Published by Elsevier B.V on behalf of China University of Mining & Technology This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Nomenclature rcc rc rl k qcom Acom dc Ecom r1 r3 rcm rtm bm strength of the confined concrete (MPa) strength of the unconfined concrete (MPa) lateral stress produced by the confinement (MPa) confinement effectiveness coefficient volumetric ratio of FRP composite qcom ¼ 4Adcom c area of coal sample with FRP (m2) diameter of coal sample (m) modulus of elasticity of coal sample with FRP (GPa) major principle stress (MPa) minor principle stress (MPa) uniaixial rock mass compressive strength (MPa) rock mass tensile strength (MPa) exponent in failure criterion for rock mass wrapping materials is required The wrapping of a coal pillar by high strength-low weight fibre provides passive confinement, which increases both strength and ductility [7] It not only provides passive confinement and increases the concrete strength, but also provides significant strength against shear [8] Numbers of NX size coal samples are tested to understand the effect of strength with several types of wrapping material The ratio of height to diameter of samples is two First, uniaxial and tri-axial compressive tests are carried out for NX size coal samples Then, the same sample is wrapped by two types of materials i.e Glass Fibre Reinforced Polymer (GFRP), Carbon Fibre Reinforced Polymer (CFRP) [9,10] Fig shows a photograph of wrapping of a coal sample by GFRP These materials are attached to the surface of coal samples by using some additives, so that it does not lose grip during testing Fig shows the testing of NX size coal samples by wrapping with double layers of CFRP As the wrapping material provides lateral confinement to the sample, its strength increases as compared to the normal coal sample Numerical modelling is carried out to evaluate the increment of strength of the coal sample wrapped with CFRP (Fig 3a) The properties of CFRP used for numerical modelling are shown in Table Fig 3b shows the increment of strength of a coal sample using double layers of CFRP material rc rt r1i r3i ssm s0m /0m H h c Xb / uniaixial intact rock compressive strength (MPa) intact rock tensile strength (MPa) induced major principle stress (MPa) induced minor principle stress (MPa) rock mass shear strength (MPa) rock mass coefficient of internal friction (MPa) rock mass angle of internal friction (MPa) depth of working (m) height of working (m) unit weight of rock, (t/m3) width of yield zone (m) internal friction angle (°) Fig UCS testing of coal sample wrapped with double layers of CFRP behaviour in both strength and ductility than a pillar under uniaxial compression Since the FRP jacket acts to contain damaged sections of the pillar, the maximum usable strain level in the pillar is limited only by the ultimate strain in the FRP jacket and not by pillar crushing The following relation is found to be suitable to estimate strength of coal sample due to confinement for FRP [11] rcc ẳ rc ỵ krl Concept of confinement As a pillar is uniaxially compressed, Poisson’s effect induces lateral strains that result in radial expansion of the pillar which leads to volumetric expansion By confining the pillar using a continuous FRP jacket, the fibres resist the lateral expansion of the pillar The effect of confining pressure provided by FRP is to induce a triaxial state of stress in the coal pillar which thus exhibits superior GFRP-1 layer The above equation has been used by most researchers to estimate the ultimate strength of confined material, assuming that failure of the system occurs when the confined pressure reaches its maximum The value of k is assumed to be 4.1 [12] The lateral confining stress rl can be computed according to the equation given below: rl ¼ qcom Ecom ecom Fig Wrapping of coal samples by GFRP single layer and double layers ð2Þ From Fig 3b, it is found that the confinement provided by the CFRP is around MPa During numerical modelling, the Young’s modulus of a coal sample wrapped with CFRP is estimated by Eq (3) [11] Ecom ¼ 1:10 ðrc Þ1=3 GFRP-2 layer ð1Þ ð3Þ Wilson’s [13] confined core approach suggests that the strength of a coal pillar increases with confinement of the core Due to confinement, the width of the yield zone decreases and the elastic confined core increases As the rib pillars are wrapped with FRP, it provides the lateral confinement equivalent to the rl stated in Eq (2) which reduces the width of yield zone in the rib pillar The width of the yield zone is defined as: Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx 10-5 m CFRP wrapped sample lab test 60 Coal sample 50 UCS (MPa) 40 30 20 UCS of coal 10 Uz =0 CFRP Coal sample (a) Numerical modelling for uniaxial compressive strength test for coal sample wrapped with CFRP (b) Comparison of strength of coal samples with double layers of CFRP Fig Numerical modelling for uniaxial compressive strength test and comparison of strength of coal samples Failure criterion of rock mass Table Properties of CFRP Properties Value Young’s modulus (GPa) Poisson’s ratio Tensile strength (MPa) Ultimate tensile strain (%) Unit weight (t/m3) 82 0.3 834 0.85 1.5 / where K ẳ 1ỵsin ,Fẳ 1sin / k1 p k ỵ k12 k r1 ẳ rc ỵ r3 rt b 5ị This equation is changed for rock masses as h cH for rigid roof-floor w ¼ 2X b ¼ ln F rc þ rl " # 1À1 h cH k ¼ for soft roof-floor rc ỵ rl Sheorey [14] has adopted Balmer’s criterion for intact rock [16] after applying it to 201 triaxial data sets for different rocks including coal This criterion reads as: r1 ¼ rcm ỵ 4ị r3 rtm bm 6ị These constants are related to RMR (1976 Rock Mass Rating of Bieniawski) as: RMR À 100 20 RMR À 100 ¼ rt exp 27 rcm ¼ rc exp pffiffiffi tanÀ1 k rtm Methodology of working RMR bm ¼ b 100 Different methodologies are studied through numerical modelling by strengthening of rib pillars using the enhanced UCS value of coal as found from the laboratory testing of data The enhanced value of UCS is considered in the stability analysis of a rib pillar wrapped with CFRP In the laboratory, when the coal sample is wrapped with double layers of CFRP, the strength of coal is increased more than three times as found from tested data (Fig 3b) But, for analysis purposes, a threefold increase in the UCS value of coal is considered for calculating the safety factor [14] of the rib pillars Numerical modelling is carried out to analyse the stability of rib pillars by using the material properties as shown in Tables and Dimensional details of the development of the coal seam used for modelling are shown in Table Two methodologies, i.e., single splitting and double splitting of a coal pillar are simulated using FLAC3D (Fast Lagrangian Analysis of Continua in Dimensions) [15] A comparative study is made for the extraction of coal without stowing and strengthening of rib pillars for their suitability of application and percentage extraction ð7Þ bm < 0:95 Numerical modelling with strain-softening of coal mine pillars requires estimation of cohesive strength and friction angle The above criterion can be expressed as a Mohr envelope involving these parameters: s ¼ ssm ỵ r rtm cm 8ị where ssm ẳ rcm rtm !1=2 b bmm 1ỵbm ỵ bm ị s2sm ỵ bm ị r2tm 2ssm rtm ỵ bm ị 0:9 rtm cm ẳ l0m ssm l0m ¼ tan/0m lom ¼ ð9Þ Table Properties of strata (contd) * ** *** Strata Thickness (m) Young’s modulus (GPa) Shear modulus (GPa) Bulk modulus (GPa) Poisson’s ratio Floor: mgsst* Coal seam Roof layer 1: fgsst** Roof layer 2:cgsst*** Roof layer 3: shale 50.00 3.00 2.00 2.00 46.00 5.70 2.00 7.00 5.25 4.80 2.28 1.20 2.80 2.10 1.92 3.80 2.00 4.67 3.50 3.20 0.25 0.25 0.25 0.25 0.25 Medium grain sand stone Fine grain sand stone Coarse grain sand stone Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table Properties of strata Strata * Floor: mgsst Coal seam Roof layer 1: fgsst** Roof layer 2:cgsst*** Roof layer 3: shale * ** *** Density (kg/m3) Cohesion (MPa) Friction angle (°) Uniaxial compressive strength (MPa) Uniaxial tensile strength (MPa) 2310 1400 2500 2210 2310 2.17 0.78 2.43 1.38 0.85 37.44 36.50 39.23 34.88 42.73 55.80 32.00 60.60 53.50 38.20 3.72 2.10 4.83 3.50 2.54 Medium grain sand stone Fine grain sand stone Coarse grain sand stone Table Dimensional details of development of the coal seam used for modelling (m) Size of pillar (corner to corner) Depth of cover Width of gallery Height of extraction 25  25 100 4.2 Table Change in cohesion ssm and friction angle /0m with shear strain The complete criterion is shown schematically in Fig This criterion, when bm = 1, becomes the Mohr–coulomb criterion and the negative intercept on the r3 axis will cease to be the tensile strength To avoid this, an upper limit of bm = 0.95 has been placed It was, however, found that the values of rock mass shear strength, ssm, and friction angle, /0m, so determined had to be changed slightly to account for the fact that the MCSS Plasticity model in FLAC3D uses the linear Mohr–Coulomb criterion while the Sheorey criterion is non-linear The value of ssm obtained from the Sheorey criterion was increased by 10% and that of /0m was reduced by 5° to use them as Mohr–Coulomb parameters The post failure values of cohesion and friction angle are given in Table In this study, a local factor of safety is obtained with the assumption that failure occurs by increasing r1 and keeping r3 unchanged The maximum principal stress at the moment of failure (r1) can be obtained from Eq (6) The safety factor is estimated as: SF ¼ rr1i1 ÀÀrr3i3i SF ¼ À rrtm3 when when r3i < rtm r3i > rtm ð10Þ This approach considers failure of each element of the model In addition, weak zones around the excavation where supports are required can easily be found from this method Single splitting If the coal pillars are extracted with the single splitting method without stowing, it is found that large sizes of rib pillars are to be Cohesion ssm (MPa) Friction angle /0m 0.000 0.005 0.010 0.050 1.1ssm l.1ssm/5 0 /0m-5° /0m-7.5° /0m-10° /0m-10° left to take the load of overlying strata (Fig 5) In this method, a split of 4.2 m is driven at the middle of the pillars and two slices of 3.8 m width are taken by leaving 10.4 m  5.8 m remnant/rib pillars By this method, only 42% of the coal can be recovered Fig shows the safety factor contours of reduced coal pillars/rib pillars where the average safety factor is found to be 2.2 which ensures the long term stability of these remnant pillars The coal pillars may be extracted with stowing but availability of stowing material is itself a problem Moreover, even after stowing, strata movement may take place due to non-effective stowing or compaction of stowing material It is also a fact that for an inclined seam better stowing is possible which provides good confinement, but in the case of a flat seam, effective stowing it very difficult Considering the problems of stowing, the method of extraction of the coal pillars by wrapping the reduced coal/rib pillars may be adapted Fig shows the methodology of extraction of coal by wrapping reduced coal/rib pillars with double layers of CFRP At first, a split of 4.2 m width is driven at a distance of 10.4 m from the corner of the pillar Then, the split is supported with the resin-grouted roof bolts After supporting the split, a slice of 3.8 m is taken by leaving 10.4 m  3.4 m size pillars After that, support using resin-grouted roof bolts is installed in the slice and the remnant/rib pillar is then wrapped with double layers of CFRP Second and third slices are then taken, supported with resingrouted roof bolts, and the remnant pillars are then wrapped with double layers of CFRP A similar procedure is followed to extract Sh eo re y cr ite rio n τ Shear strain φ0m -5° Rib pillar of 10.4 m×5.8 m 5.8 m 1.1 τ 0m τ 0m 4.2 m 3.8 m 10.4 m φ 0m 3.8 m 4.5 m 25 m Mohr-coloumb criterion Split of 4.2 m width Area of analysis Slice of 3.8 m width 25 m σ tm σ Fig Schematic showing the non-linear Sheorey criterion as against the linear Mohr–Coulomb criterion adopted in FLAC3D Fig Conventional method of extraction by single splitting Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx 1.5241e-001 to 5.0000e-001 5.0000e-001 to 1.0000e+000 1.0000e+000 to 1.5000e+000 1.5000e+000 to 2.0000e+000 2.0000e+000 to 2.5000e+000 2.5000e+000 to 3.0000e+000 3.0000e+000 to 3.5000e+000 3.5000e+000 to 4.0000e+000 4.0000e+000 to 4.5000e+000 4.5000e+000 to 5.0000e+000 5.0000e+000 to 5.1500e+000 Interval = 5.0e-001 Fig Safety factor contours of reduced coal/rib pillar of size 10.4 m  5.8 m where single splitting conventional method is followed and stowing has not been done Rib pillar of 10.4 m×3.4 m 4.2 m 3.8 m 10.4 m 3.4 m 25 m Wrapping with double layer CFRP 4.5 m Split gallery of 4.2 m width Slice of 3.8 m width Coal pillar of 25 m×25 m Area of analysis a rock load height of 2.9 m The average rock density is taken as 2.2 t/m3 Therefore, the rock load coming on the support is 6.38 t/m2 As the width of the split is 4.2 m, three resin-grouted roof bolts are placed in a row at a spacing of 1.6 m in between two consecutive bolts in a row by leaving 0.5 m distance from the side of the gallery The distance between two consecutive rows is kept as m Rock load is calculated as follows: Rock load ¼ Density  h2:0 ¼ 2:2 2:9ị t=m2 ẳ 6:38 t=m2 where h2.0 = Height of rock of safety factor up to Three roof bolts in a row with a spacing of 1.6 m are installed with 1.0 m spacing between two consecutive rows Taking the anchorage strength of a resin-grouted bolt as 20 t, the support resistance and factor of safety are calculated as follows: Support resistance ¼ No: of bolt  Anchorage strength of roof bolt width of gallery  spacing between row 11ị Support resistance ẳ 20ị=4:2 1:0ị t=m2 ẳ 14:28 t=m2 Factor of safety ẳ Support resistance ẳ 14:28=6:38ị ẳ 2:2 Rock load 12ị The support resistance with this design is found to be 14.28 t/ m2 The safety factor obtained for this support system for the split is 2.2 25 m Fig Single splitting method of extraction by wrapping the rib pillars with double layers of CFRP 4.8818e-001 to 5.0000e-001 5.0000e-001 to 7.5000e-001 7.5000e-001 to 1.0000e+000 1.0000e+000 to 1.2500e+000 1.2500e+000 to 1.5000e+000 1.5000e+000 to 1.7500e+000 1.7500e+000 to 2.0000e+000 2.0000e+000 to 2.2500e+000 2.2500e+000 to 2.5000e+000 2.5000e+000 to 2.7500e+000 2.7500e+000 to 2.8699e+000 Interval = 2.5e-001 Fig Safety factor contours of reduced coal/rib pillar of size 10.4 m  3.4 m where single splitting method without stowing is followed but remnant pillars are wrapped with double layers of CFRP another half of the coal pillar Fig shows the safety factor contours of strengthened remnant/rib pillars With this methodology, it may be possible to achieve an average safety factor of 2.2 for the reduced coal/rib pillars which ensures long term stability The percentage of extraction goes up to 55% 6.1 Support design of 4.2 m split The Fig shows that the rock load height of a safety factor up to is 2.9 m Accordingly, the support system is designed considering 2.6 m at slice 2.9 m at split 4.7918e-001 to 6.0000e-001 6.0000e-001 to 8.0000e-001 8.0000e-001 to 1.0000e+000 1.0000e+000 to 1.2000e+000 1.2000e+000 to 1.4000e+000 1.4000e+000 to 1.6000e+000 1.6000e+000 to 1.8000e+000 1.8000e+000 to 2.0000e+000 2.0000e+000 to 2.0000e+000 Interval = 2.0e-001 Fig Rock load height of safety factor in split and slice where the reduced coal/ rib pillar of size 10.4 m  3.4 m is strengthened by double layers of CFRP 6.2 Support design of 3.8 m slice From Fig 9, it is determined that the rock load height of safety factor up to is 2.6 m The support is designed considering the rock load height The average rock density is considered as 2.2 t/m3 So, the rock load coming on support is 5.72 t/m2 As the width of a slice is 3.8 m, three resin-grouted roof bolts are placed in a row at a spacing of 1.4 m in between two consecutive bolts by leaving 0.5 m distance from the side of the gallery The spacing between two consecutive rows is kept as 1.2 m Rock load ¼ Density  h2:0 ¼ ð2:2  2:6Þ t=m2 ¼ 5:72 t=m2 where h2.0 = height of rock of safety factor up to Three roof bolts in a row with a spacing of 1.4 m are installed with 1.2 m spacing between two consecutive rows Taking the anchorage strength of a resin-grouted bolt as 20 t, the support resistance and factor of safety are calculated as per Eqs (11) and (12): Support resistance ¼ ð3  20Þ=ð3:8  1:2Þ t=m2 ¼ 13:15 t=m2 ¼ ð13:15=5:72Þ ¼ 2:3 The support resistance with this pattern of design is 13.15 t/m2 So, the safety factor obtained for this support system for the slice is 2.3 Double splitting The double splitting method is also simulated to study the suitability of the method Fig 10 shows the double splitting method without stowing The split of 3.8 m is driven at a distance of 5.8 m from the corner of the pillar and one slice of 4.2 m width is taken by leaving a 5.8 m  10.4 m size of remnant/rib pillar With this conventional method, only 42% of coal may be recovered Fig 11 shows the safety factor contours of remnant pillars where the average safety factor is found to be 2.2 which ensures the long term stability of these left-out pillars Therefore, it is found that only 42% coal may be possible to recover in case of nonavailability of stowing materials Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx 4.8882e-001 to 5.0000e-001 5.0000e-001 to 7.5000e-001 7.5000e-001 to 1.0000e+000 1.0000e+000 to 1.2500e+000 1.2500e+000 to 1.5000e+000 1.5000e+000 to 1.7500e+000 1.7500e+000 to 2.0000e+000 2.0000e+000 to 2.2500e+000 2.2500e+000 to 2.5000e+000 2.5000e+000 to 2.7500e+000 2.7500e+000 to 2.9802e+000 Interval = 2.5e-001 Rib pillar of 5.8 m×10.4 m 25 m Split of 3.8 m width 10.4 m 3.8 m Slice of 4.2 m width 4.2 m 5.8 m Area of analysis Coal pillar of 25 m×25 m 25 m working It may also be possible to achieve 57% recovery of locked-up coal Fig 10 Conventional method of extraction by double splitting 1.5241e-001 to 5.0000e-001 5.0000e-001 to 1.0000e+000 1.0000e+000 to 1.5000e+000 1.5000e+000 to 2.0000e+000 2.0000e+000 to 2.5000e+000 2.5000e+000 to 3.0000e+000 3.0000e+000 to 3.5000e+000 3.5000e+000 to 4.0000e+000 4.0000e+000 to 4.5000e+000 4.5000e+000 to 5.0000e+000 5.0000e+000 to 5.1500e+000 Interval = 5.0e-001 Fig 11 Safety factor contours of reduced coal/rib pillar of size 5.8 m  10.4 m where double splitting conventional method is followed and stowing has not been done Area of analysis 25 m Rib pillar of 5.5 m×5.5 m 4.2 m 4.2 m 5.5 m 5.5 m Fig 13 Safety factor contours of reduced coal/rib pillar of size 5.5 m  5.5 m where double splitting method without stowing is followed but rib pillars are wrapped with double layers of CFRP 25 m Split of 4.2 m width Wrapping with double layer CFRP Slice of 4.2 m width Coal pillar of 25 m×25 m 7.1 Support design of 4.2 m split and slice The rock load height of a safety factor up to is 3.0 m for 4.2 m exposure of split and slice as shown in Fig 14 So, the support is designed considering the rock load height as 3.0 m The average rock density is taken as 2.2 t/m3 Therefore, the load coming on support is calculated as 6.6 t/m2 As the width of split and slice both are 4.2 m, three resin-grouted roof bolts are placed in a row at a spacing of 1.6 m in between two consecutive bolts in a row by leaving 0.5 m distance from the side of the gallery The distance between two consecutive rows is kept as m the rock load is calculated as follows: Rock load ¼ Density h2:0 ẳ 2:2 3:0ị t=m2 ẳ 6:6 t=m2 where h2.0 = Height of rock of safety factor up to Three roof bolts in a row with spacing of 1.6 m are installed with 1.0 m spacing between two consecutive rows Taking the anchorage strength of a resin-grouted bolt as 20 t, the support resistance and factor of safety are calculated as per Eqs (11) and (12): Support resistance ẳ 20ị=4:2 1:0ị t=m2 ẳ 14:28 t=m2 ẳ 14:28=6:6ị ẳ 2:2 The support resistance with this design is found to be 14.28 t/ m2 The safety factor obtained for this support system both for splits and slices is 2.2 Fig 12 Double splitting method of extraction by wrapping the reduced coal/rib pillars with double layers of CFRP The double splitting method with strengthening the reduced coal/rib pillars (shown in Fig 12) is studied to evaluate the stability and percentage of extraction of coal in the case of non-availability of stowing material At first, a split of 4.2 m width is driven at a distance of 5.5 m from the corner of the pillar Then, the split is supported by resin-grouted roof bolts After supporting the split, a slice of 4.2 m is taken by leaving 5.5 m  5.5 m pillars Support with resin-grouted roof bolts is installed in the slice After supporting the slice, the rib pillar is wrapped with a double layer of CFRP Then, second and third slices are taken in similar fashion The second split of 4.2 m width is driven at a distance of 5.5 m from the first slice From the second split, slices are taken at both sides and similar procedures are followed to wrap the rib pillars Fig 13 shows the safety factor contours of strengthened pillars With this methodology, it may be possible to achieve an average safety factor of 2.2, which may ensure the long term stability of 3.0 m at slice 3.0 m at split 4.8119e-001 to 6.0000e-001 6.0000e-001 to 8.0000e-001 8.0000e-001 to 1.0000e+000 1.0000e+000 to 1.2000e+000 1.2000e+000 to 1.4000e+000 1.4000e+000 to 1.6000e+000 1.6000e+000 to 1.8000e+000 1.8000e+000 to 2.0000e+000 2.0000e+000 to 2.0000e+000 Interval = 2.0e-001 Fig 14 Rock load height for safety factor in split and slice where the reduced coal/rib pillar of size 5.5 m  5.5 m is strengthened by double layers of CFRP Table Comparative study of extraction of locked up coal by the conventional splitting and slicing method and strengthening of rib pillars by CFRP Conventional method Wrapping of rib pillar Single splitting Double splitting Size of rib pillar (m) Percentage of recovery (%) Size of rib pillar (m) Percentage of recovery (%) 10.4  5.8 42 5.8  10.4 42 10.4  3.4 55 5.5  5.5 57 Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 A.J Das et al / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Results References Table shows the comparative study of extraction of locked up coal by the conventional splitting and slicing method and strengthening of rib pillars by CFRP The comparison is done with the same safety factor of rib pillars i.e 2.2 It is found from the study that strengthening of the rib pillar gives sufficient amount of recovery with long term stability of rib pillars [1] Das AJ, Mandal PK, Ghosh CN, Sinha A An attempt to develop a technology for extraction of locked-up coal from underground mines using artificial pillars Indian Min Eng J 2014;53(5):18–25 [2] Singh TN, Dubey BK Scope of Bhuggatdih method of mining for thick coal seams In: Proceedings of workshop on wide stall mining of coal seams under constraints Dhanbad: Central Mining Fuel Research Institute; 1993 p 18 [3] Singh R Staggered development of a thick coal seam for full height working in single lift by blasting gallery method Int J Rock Mech Min Sci 2004;41 (5):745–59 [4] Das AJ Development of underground methodologies for extraction of lockedup coal from developed pillars using artificial pillars and/or strengthened reduced coal pillars based on numerical modelling study In: MTech Thesis; Academy of Scientific and Innovative Research, CSIR-Central Institute of Mining and Fuel Research, Dhanbad; 2014 [5] Mandal PK, Das AJ, Ghosh CN, Sinha A Extraction of locked-up coal by artificial pillars-an attempt for development of a technology In: Proc 5th Asian mining congress, Kolkata; 2014 p 13–5 [6] Gupta RN Thick seam extraction by partial stowing in bord and pillar workings-rock mechanics consideration and field trials In: Proc 2nd national conference on ground control in mining, India; 1996 p 377–84 [7] Nemcik JA, Baafi EY, Porter I, Lukey CA Computer modelling of polymer liner reinforcement in underground mines In: Proc application of computers and operations research in the mineral industry, Vancouver; 2009 p 537–45 [8] Nemcik JA, Porter I, Baafi EY Performance of polymer skin spray-on liner in coal mines In: Proc 12th ISRM international congress on rock mechanicsharmonising rock engineering, London; 2012 p 1565–8 [9] Bakis CE, Bank LC, Brown VL, Cosenza E, Davalos JF, Lesko JJ, et al Fibrereinforced polymer composites for construction-state-of-the-art review ASCE J Composit Constr 2002;6(2):73–87 [10] Bonacci JF, Maalej M Externally bonded FRP for service life extension of RC infrastructure ASCE J Infrastruct Syst 2000;41(1):41–51 [11] Toutanji HA Stress-strain characteristics of concrete columns externally confined with advanced fibre composite sheets ACI Mater J 1999;96 (3):397–404 [12] Richart FE, Brandtzaeg A, Brown RE The failure of plain and spirally reinforced concrete in compression In: Bulletin 190 Champaign: University of Illinois Engineering Experimental Station; 1929 [13] Wilson AH A hypothesis concerning pillar stability Min Engineer 1972;131 (6):409–17 [14] Sheorey PR Empirical rock failure criteria Netherlands: Balkema; 1997 p 176 [15] Itasca FLAC3D (Fast Lagrangian Analysis of Continua in Dimensions) Minnesota: Itasca Consulting Group; 2002 [16] Balmer G A general analytical solution for Mohr’s envelop In: Proc Am soc testing materials; 1952 p 1260–71 Conclusions This study was undertaken for extraction of locked-up coal in pillars under different constraints considering the huge amount of coal locked-up in different coalfields in India In the absence of any suitable existing technological option with reasonably high recovery, the locked-up coal is proposed to be extracted by strengthening the rib pillars by wrapping CFRP In this study, the increase in percentage recovery of locked-up coal is discussed if the strengthening of rib pillars method is adopted Laboratory testing was carried out on a large number of coal samples of NX size From the results of testing, it was observed that the UCS can be used to increase more than three times the original UCS of a coal sample Stability analysis was carried out by considering the increase of UCS value of coal by three times Two types of extraction methodologies were studied to evaluate the effect of strengthening of rib pillars and increase of percentage of extraction It is found that the double splitting method with strengthening the rib pillar may be suitable for extraction of the locked-up coal Acknowledgments This work is a part of the 12th Five Year Plan Project (No ESC 0105), acronymed as ‘‘DeCoalArt” The views expressed in this paper are those of the authors and not necessarily of the institutes/organisations to which they belong The testing of coal samples was carried out by CSIR-Central Building Research Institute, Roorkee in CSIR-Central Institute of Mining and Fuel Research, Dhanbad Please cite this article in press as: Das AJ et al Extraction of locked-up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling Int J Min Sci Technol (2017), http://dx.doi.org/10.1016/j.ijmst.2017.01.024 ... Mining and Fuel Research, Dhanbad Please cite this article in press as: Das AJ et al Extraction of locked- up coal by strengthening of rib pillars with FRP – A comparative study through numerical modelling. .. grain sand stone Coarse grain sand stone Please cite this article in press as: Das AJ et al Extraction of locked- up coal by strengthening of rib pillars with FRP – A comparative study through numerical. .. Conventional method of extraction by single splitting Please cite this article in press as: Das AJ et al Extraction of locked- up coal by strengthening of rib pillars with FRP – A comparative study through