extraction of locked up coal by strengthening of rib pillars with frp a comparative study through numerical modelling

Extraction of locked-up coal by strengthening of rib pillars with FRP – Acomparative study through numerical modelling CSIR-Central Institute of Mining and Fuel Research, Dhanbad 826001,

Extraction of locked-up coal by strengthening of rib pillars with FRP – A

comparative study through numerical modelling

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, accu-mulation of poisonous gases, severe stability issues leading to unsafe workings and environmental haz-ards 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 con-straints 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 stud-ied 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/)

1 Introduction

Coal being a finite and non-renewable natural resource, its

con-servation is an important issue The aspect of concon-servation 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

pri-mary energy requirement of the country, indigenous development

of suitable underground mining methods is of strategic

impor-tance 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

materi-als which leads to huge amounts of coal being locked-up

under-ground [1,2] Spontaneous heating and fire, accumulation of

poisonous gases, severe stability issues[3]leading to unsafe

work-ings 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

possi-ble 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-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 rem-nant 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 sta-bility in the conventional method of partial extraction where the percentage of extraction reduces drastically If the remnant/rib pil-lars 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 pil-lars by imposing lateral confinement[6], rib pillars may lose their strength gradually due to compaction of stowing material There-fore, it should be ensured that for extraction of coal under con-straints 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 mod-elling to ensure the increase of percentage of recovery of coal

2 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/ ).

⇑ Corresponding author.

E-mail address: arkajyoti19@gmail.com (A.J Das).

Contents lists available atScienceDirect

International Journal of Mining Science and Technology

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / i j m s t

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

numer-wrapping materials is required The numer-wrapping of a coal pillar by

high strength-low weight fibre provides passive confinement,

which increases both strength and ductility [7] It not only

pro-vides 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 1shows 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 2shows 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

eval-uate the increment of strength of the coal sample wrapped with

CFRP (Fig 3a) The properties of CFRP used for numerical modelling

are shown inTable 1.Fig 3b shows the increment of strength of a

coal sample using double layers of CFRP material

3 Concept of confinement

As a pillar is uniaxially compressed, Poisson’s effect induces

lat-eral 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

tri-axial state of stress in the coal pillar which thus exhibits superior

behaviour in both strength and ductility than a pillar under uniax-ial compression Since the FRP jacket acts to contain damaged sec-tions 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]

The above equation has been used by most researchers to esti-mate the ultiesti-mate strength of confined esti-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 stressrl can be computed according to the equation given below:

rl¼qcomEcomecom

FromFig 3b, it is found that the confinement provided by the CFRP is around 9 MPa During numerical modelling, the Young’s modulus of a coal sample wrapped with CFRP is estimated by Eq

(3) [11]

Wilson’s[13]confined core approach suggests that the strength

of a coal pillar increases with confinement of the core Due to finement, the width of the yield zone decreases and the elastic con-fined 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:

GFRP-1 layer

GFRP-2 layer

Fig 1 Wrapping of coal samples by GFRP single layer and double layers.

Fig 2 UCS testing of coal sample wrapped with double layers of CFRP.

Nomenclature

rcc strength of the confined concrete (MPa)

rc strength of the unconfined concrete (MPa)

rl lateral stress produced by the confinement (MPa)

k confinement effectiveness coefficient

qcom volumetric ratio of FRP composite qcom¼4A com

d c

Acom area of coal sample with FRP (m2)

dc diameter of coal sample (m)

Ecom modulus of elasticity of coal sample with FRP (GPa)

r1 major principle stress (MPa)

r3 minor principle stress (MPa)

rcm uniaixial rock mass compressive strength (MPa)

rtm rock mass tensile strength (MPa)

bm exponent in failure criterion for rock mass

rc uniaixial intact rock compressive strength (MPa)

rt intact rock tensile strength (MPa)

r1i induced major principle stress (MPa)

r3i induced minor principle stress (MPa)

ssm rock mass shear strength (MPa)

s0m rock mass coefficient of internal friction (MPa) /0m rock mass angle of internal friction (MPa)

H depth of working (m)

h height of working (m)

c unit weight of rock, (t/m3)

Xb width of yield zone (m) / internal friction angle (°)

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

numer-w¼ 2Xb¼h

cH

rcþrl

for rigid roof-floor

2

cH

rcþrl

k 1

 1

where K¼1þsin /

1sin /, F¼
k1 pffiffik

þ k1 k

 2

tan1 ffiffiffi k

p

4 Methodology of working

Different methodologies are studied through numerical

mod-elling 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

inTables 2 and 3 Dimensional details of the development of the

coal seam used for modelling are shown inTable 4 Two

method-ologies, i.e., single splitting and double splitting of a coal pillar

are simulated using FLAC3D (Fast Lagrangian Analysis of Continua

in 3 Dimensions)[15] A comparative study is made for the

extrac-tion of coal without stowing and strengthening of rib pillars for

their suitability of application and percentage extraction

5 Failure criterion of rock mass Sheorey[14]has adopted Balmer’s criterion for intact rock[16]

after applying it to 201 triaxial data sets for different rocks includ-ing coal This criterion reads as:

r1¼rc 1þr3

rt

ð5Þ

This equation is changed for rock masses as

r1¼rcm 1þr3

rtm

ð6Þ

These constants are related to RMR (1976 Rock Mass Rating of Bieniawski) as:

20

27

ð7Þ

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:

s¼ssm 1þ r

rtm

ð8Þ

where

ssm¼ rcmrtm

m

lom¼s2

tm

0mrtm

ssm

ð9Þ

Coal sample

CFRP

10 -5 m

U z=0

10 20 30 40 50 60

Coal sample UCS of coal

CFRP wrapped sample lab test

(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 3 Numerical modelling for uniaxial compressive strength test and comparison of strength of coal samples.

Table 1

Properties of CFRP.

Young’s modulus (GPa) 82

Tensile strength (MPa) 834 Ultimate tensile strain (%) 0.85 Unit weight (t/m 3

Table 2

Properties of strata (contd).

* 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

numer-The complete criterion is shown schematically inFig 4 This

cri-terion, 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 ofssmobtained from

the Sheorey criterion was increased by 10% and that of/0mwas

reduced by 5° to use them as Mohr–Coulomb parameters The post

failure values of cohesion and friction angle are given inTable 5

In this study, a local factor of safety is obtained with the

assump-tion that failure occurs by increasingr1and keepingr3unchanged

The maximum principal stress at the moment of failure (r1) can be

obtained from Eq.(6) The safety factor is estimated as:

r 1i  r 3i whenr3i<rtm

r 3 whenr3i>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

6 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

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 6shows 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 stow-ing, 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 con-finement, 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 pil-lars may be adapted.Fig 7shows 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 resin-grouted roof bolts, and the remnant pillars are then wrapped with double layers of CFRP A similar procedure is followed to extract

Table 3

Properties of strata.

) Cohesion (MPa) Friction angle (°) Uniaxial compressive strength (MPa) Uniaxial tensile strength (MPa)

* Medium grain sand stone.

** Fine grain sand stone.

*** Coarse grain sand stone.

Table 4

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

Mohr-coloumb criterion Sheorey criterion

tm

σ

0m

φ

τ

0m

0m

τ

0m

τ

1.1

Fig 4 Schematic showing the non-linear Sheorey criterion as against the linear

Mohr–Coulomb criterion adopted in FLAC3D.

Table 5 Change in cohesionssm and friction angle / 0m with shear strain.

Shear strain Cohesionssm (MPa) Friction angle / 0m

4.5

m 3.8 m

4.2 m 5.8 m

Rib pillar of 10.4 m×5.8 m

Split of 4.2 m width

Slice of 3.8 m width

Area of analysis

25 m

Fig 5 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

numer-another half of the coal pillar.Fig 8shows the safety factor

con-tours 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

per-centage of extraction goes up to 55%

6.1 Support design of 4.2 m split

TheFig 9shows that the rock load height of a safety factor up to

2 is 2.9 m Accordingly, the support system is designed considering

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 1 m Rock load is calculated as follows:

where h2.0= Height of rock of safety factor up to 2

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:

ð11Þ

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

6.2 Support design of 3.8 m slice FromFig 9, it is determined that the rock load height of safety factor up to 2 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

where h2.0= height of rock of safety factor up to 2

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):

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

7 Double splitting The double splitting method is also simulated to study the suit-ability of the method.Fig 10shows 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 11shows 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 non-availability of stowing materials

1.5241e-001 to 5.0000e-001 5.0000e-001 to 1.0000e+000

Interval = 5.0e-001

1.0000e+000 to 1.5000e+000 2.0000e+000 to 2.5000e+000 2.5000e+000 to 3.0000e+000 3.5000e+000 to 4.0000e+000 4.0000e+000 to 4.5000e+000 5.0000e+000 to 5.1500e+000

Fig 6 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.

4.5

m

Split gallery of 4.2 m width Slice of 3.8 m width Coal pillar of

25 m×25 m

Wrapping with double layer CFRP

Area of analysis

25 m

Rib pillar of 10.4 m×3.4 m

Fig 7 Single splitting method of extraction by wrapping the rib pillars with double

layers of CFRP.

4.8818e-001 to 5.0000e-001

Interval = 2.5e-001

7.5000e-001 to 1.0000e+000 1.0000e+000 to 1.2500e+000 1.5000e+000 to 1.7500e+000 2.0000e+000 to 2.2500e+000 2.2500e+000 to 2.5000e+000 2.7500e+000 to 2.8699e+000

Fig 8 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.

2.6 m at slice 2.9 m at split

4.7918e-001 to 6.0000e-001

Interval = 2.0e-001

8.0000e-001 to 1.0000e+000 1.0000e+000 to 1.2000e+000 1.4000e+000 to 1.6000e+000 1.8000e+000 to 2.0000e+000 2.0000e+000 to 2.0000e+000

Fig 9 Rock load height of safety factor 2 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.

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

numer-The double splitting method with strengthening the reduced

coal/rib pillars (shown inFig 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

dis-tance of 5.5 m from the corner of the pillar Then, the split is

sup-ported 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

support-ing the slice, the rib pillar is wrapped with a double layer of CFRP

Then, second and third slices are taken in similar fashion The

sec-ond 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

working It may also be possible to achieve 57% recovery of locked-up coal

7.1 Support design of 4.2 m split and slice The rock load height of a safety factor up to 2 is 3.0 m for 4.2 m exposure of split and slice as shown inFig 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 1 m the rock load is cal-culated as follows:

where h2.0= Height of rock of safety factor up to 2

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):

¼ ð14:28=6:6Þ ¼ 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

Split of 3.8 m width

Slice of 4.2 m width

Area of analysis

25 m

Coal pillar of

25 m×25 m

Rib pillar of 5.8 m×10.4 m

Fig 10 Conventional method of extraction by double splitting.

1.5241e-001 to 5.0000e-001 5.0000e-001 to 1.0000e+000

Interval = 5.0e-001

1.0000e+000 to 1.5000e+000 1.5000e+000 to 2.0000e+000 2.5000e+000 to 3.0000e+000 3.0000e+000 to 3.5000e+000 4.0000e+000 to 4.5000e+000 4.5000e+000 to 5.0000e+000

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.

Rib pillar of

Split of 4.2 m width

5.5 m 4.2 m

Area of analysis

25 m

Slice of 4.2 m width Coal pillar of

25 m×25 m

5.5 m×5.5 m

Wrapping with double layer CFRP

Fig 12 Double splitting method of extraction by wrapping the reduced coal/rib

pillars with double layers of CFRP.

4.8882e-001 to 5.0000e-001

Interval = 2.5e-001

7.5000e-001 to 1.0000e+000 1.0000e+000 to 1.2500e+000 1.5000e+000 to 1.7500e+000 2.0000e+000 to 2.2500e+000 2.2500e+000 to 2.5000e+000 2.7500e+000 to 2.9802e+000

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.

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 1.6000e+000 to 1.8000e+000 1.8000e+000 to 2.0000e+000 Interval = 2.0e-001

Fig 14 Rock load height for safety factor 2 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 6 Comparative study of extraction of locked up coal by the conventional splitting and slicing method and strengthening of rib pillars by CFRP.

Single splitting Double splitting Size of rib

pillar (m)

Percentage of recovery (%)

Size of rib pillar (m)

Percentage of recovery (%) Conventional

method

Wrapping of rib pillar

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

numer-8 Results

Table 6shows the comparative study of extraction of locked up

coal by the conventional splitting and slicing method and

strength-ening 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

9 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

test-ing 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

extrac-tion methodologies were studied to evaluate the effect of

strength-ening 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

insti-tutes/organisations to which they belong The testing of coal

sam-ples was carried out by CSIR-Central Building Research Institute,

Roorkee in CSIR-Central Institute of Mining and Fuel Research,

Dhanbad

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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