an experimental investigation into the trapping model core pillars with reinforced fly ash...

8/2/2019 An Experimental Investigation Into the Trapping Model Core Pillars With Reinforced Fly Ash Composites

1/5June/July 2008 1

Paper 11 Coal and Oil Sands

INTRODUCTION

Pillar design is an important aspect of room-and-pillar methods of extraction because it allows formaximum recovery while maintaining the stability ofthe mine. The popular ultimate strength approachfor pillar design dictates that failure occurs after theload reaches its ultimate strength (i.e. the load-bear-ing capacity of a pillar reduces to zero the moment itsultimate strength is exceeded). Stresses in a pillar arerelated to (a) overburden pressure, (b) developmentof entries, (c) extraction of coal pillar in adjoiningarea, including the load due to over-riding of pillarsand (d) dynamic loading due to blasting, roof falls,tectonic stresses, etc. However, ever-increasing min-ing depths pose some serious challenges to the con-ventional pillar design methodology. With thechallenge of discovering new economic deposits,efforts have been made to extract as much

mineral/coal as possible. At times these measures callfor completely knocking out some support pillars orreducing their dimension (Tesarik, Seyman, Yanske, &McKibbin, 1985).

Pillar failure affects the efficiency of mining oper-ations, the economic consequence of which can beserious, especially for capital-intensive longwall min-ing or highly mechanized room-and-pillar mining. Toincrease production and consequently improve mineeconomics, many attempts have been made toreduce the pillar sizes by increasing pillar strength arti-ficially without compromising mine stability. Sandstowing in coal mines and backfilling of stopes in

hardrock mines have been practiced for a long timeby many mines around the world to address ground

An experimental investigationinto the trapping model core pillars

with reinforced fly ash compositesM.K. Mishra, National Institute of Technology, Rourkela, India, andU.M.R. Karanam, Indian Institute of Technology, Kharagpur, India

ABSTRACT Relative to slender pillars, larger size pillars exhibit improved strata control. Fly ash,

though traditionally treated as a waste product, is emerging as an alternate engineering material.

This paper deals with the development of a strong fly ash composite material with additives. The

geotechnical properties of this fly ash composite were determined and used to trap model core pil-

lars, which resemble squat pillars. The trapped samples were tested under uni-axial loading in the

laboratory and the results showed higher load-bearing capacity of the model core pillars. The rein-

forced fly ash composites provide lateral confinement and exhibit improved post- failure behaviour

of the model cores.

control problems, increase mine system stability andincrease productivity. Fly ash is emerging as an alter-nate material for many engineering applications, asdescribed elsewhere (Kumar, Ahuja, Dattatryulu,Bhaskar Rao, Ghosh, & Sharma, 2003; Polariski,1993).

The current study experimentally investigates thedevelopment of a stronger fly ash composite and itsreinforcement with wire mesh. The potential of the flyash composite for altering the behaviour of modelcore pillars is examined.

LABORATORY EXPERIMENTATION

Model core pillars (57 mm in diameter and 200mm in length) were prepared from a sand andcement mixture with adequate care to maintain uni-formity in all samples; the cement-to-sand ratio wasmaintained as shown in Table 1. Fly ash composite

from untreated fly ash, obtained from a local thermalpower plant furnace, lime and gypsum were also pre-

KEYWORDS Fly ash composite materials, Model core pillar trapping, Lateralconfinement, Rock mechanics

Table 1. Different properties of model core pillars

Cement:Sand 1:1 1:1.5 1:1.75 1:2 1:2.5

Cement:Water 1:0.45 1:0.45 1:0.45 1:0.45 1:0.45

Youngs Modulus 3,227 MPa 2,869 MPa 2,678 MPa 2,536 MPa 2,322 MPaof Elasticity

Poissons Ratio 0.22 0.21 0.20 0.19 0.18

Uniaxial 38.79 MPa 33.5 MPa 29.58 MPa 26.65 MPa 23.12 MPacompressive strength

Angle of internal 41 40.6 40.1 39.5 38.9friction ,

Cohesion, MPa 7.42 6.09 5.23 4.63 3.94

Density, kg/m3 2109 2131 2145 2154 2162


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pared; the engineering properties of the compositeare shown in Table 2. Model core pillars (100 mm indiameter and 200 mm in height) were then trapped

with the prepared fly ash composite. Commerciallyavailable 0.9 mm thick GI-type wire mesh (2 mm/cm2,122 Kgf/cm2 tensile strength and 7.17*105 Kgf/cm2

elasticity) was used to provide reinforcement. Thefinal trapped model core pillars were cured for 28 and56 days before they were tested under uni-axial com-pressive loading.

RESULTS AND DISCUSSION

The physico-chemical characteristics of fly ashare shown in Tables 3 and 4, and the grain size dis-tribution is shown in Figure 1. The geotechnical prop-erties of the composite materials are shown in Table2. The cement and sand ratios were maintained asshown in Table 1, along with their respective engi-neering properties.

EFFECT OF CURING PERIOD

Model core pillars were trapped with reinforced flyash composites and tested at 28 and then at 56 daysof curing. From the experimental results it was

observed that each type of fly ash composite influ-enced the failure strength of the model core pillar. Thepercentage of increase in failure strength of trapped

model core pillars varied with the type of composite,the curing period and the ratio of the annular thicknessof fill area to model core pillar radius (Tf = the thicknessof annular fill area and Tc = the radius of the modelcore pillar). The percentage increase in the ultimatebearing capacity for the model core pillar of ratio 1:1(cement-to-sand) varied between 4% and 15% at 28days of curing for different fly ash composites. The cor-responding variation in gain strength for similar coresamples of the same cement-to-water ratio rangedfrom 8 to 27% at 56 days of curing. It was observedthat the load-bearing capacities of trapped model corepillars improved with a longer curing period. Relative tothe 28-day curing period, the increase in strength at 56days was 3.8% for Tf / Tc = 0.75 when trapped withreinforced fly ash composite containing only 15% lime.However, when 5% gypsum was added, the strengthincrease was 7%. The maximum increase wasobserved for the ratio of Tf / Tc = 1.62 in the fly ashcomposite containing 20% lime and 5% gypsum withmodel core pillars of type 1:2.5. The increased per-centages ranged from 10 to 12% for various Tf/ Tcratios and different fly ash composites. It was con-cluded that a sufficient curing period is required forthe fly ash composites to achieve maximum uniaxialcompressive strength (UCS).

2 CIM Bulletin I Vol. 101, N 1109

Coal and Oil Sands

Table 2. Engineering properties of fly ash composite material (L= Lime %; G =Gypsum %)

Parameters 15% L - O% G 15% L - 5% G 20% L - O% G 20% L - 5% G

28 Days 56 Days 28 Days 56 Days 28 Days 56 Days 28 Days 56 Days

Compressive strength 5.4579 8.5144 8.0778 11.7892 6.8770 10.0426 8.5144 12.225MPa MPa MPa MPa MPa MPa MPa MPa

Youngs Modulus, E 293.2 320.3 316.9 349.2 304.5 331.2 321.2 348.7MPa MPa MPa MPa MPa MPa MPa MPa

Poissons ratio, 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35

Cohesion, C 1.09 1.16 1.17 1.26 1.40 1.54 1.43 1.59MPa MPa MPa MPa MPa MPa MPa MPa

Angle of internal friction, 26 27.7 29.6 30.2 30.0 31.2 30.2 31.6

Slake durability 1st cycle 96.15 96.9 97.1 97.8 96.5 97.4 98.16 99Index 2nd cycle 82.3 84.2 86.2 87.1 85.3 86.5 92.1 93(%)

Normalized dry density 1,812.8 1,779.8 1,832.5 1790.3

Normalized water content 20.99% 21.98% 21.64% 22.09%

Fig. 1. Grain size analysis of the untreated fly ash.

Table 3. Chemical characteristics of fly ash

Constituents Percentage Constituents Percentage

Carbon 2.10% P2

O5

0.17%

Volatile matter 0.147 SO3 0.24%

Fe2O3 8.83% K2O 0.79%

MgO 0.84% CaO 1.11%

Al2O3 27.73% Na2 0.14%

SiO2 5.89% TiO2 2.09%

Table 4. Physical characteristics of fly ash

Parameters

Colour Light grey

Dry density (kg/m3) 1,380

Optimum moisture content (%) 38.7

Permeability (m/sec.) (3.53.7) * 10-6

Liquid limit (%) 40.89Plastic limit (%) Non-plastic


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The addition of 5% gypsum improved the per-centage gain in the failure strength at 28 days of cur-ing. In the case of the model core pillar with acement-to-sand ratio of 1:1, with Tf / Tc = 0.75 andwith 15% lime, the strength increased by 2.9% at 28days of curing. In contrast, when the lime percentagewas increased 20%, the overall increase in the

strength was 1.4% for the same curing time. Thestrength gain with gypsum was higher at 28 days ofcuring compared to the strength values at 56 dayscuring. A maximum strength gain of about 14% wasachieved with a model core pillar cement-to-sandratio of 1:2.5 for fly ash composite containing 15%lime and 5% gypsum, as well as for fly ash compos-ite material with only 20% lime content. It wasobserved that, beyond an optimum percentage, theaddition of gypsum as well as lime did not show a lin-ear increase in the strength gain percentage of thetrapped pillar, though the strength increased inabsolute terms.

FAILURE AND POST-FAILURE PROFILE

OF TRAPPED MODEL CORE PILLARS

The trapped model core pillars continued to takeload even after the core pillar reached its peakstrength and failed. It was observed that compared tothe unconfined model core pillar, there was a sub-stantial increase in the failure load as well as in thepost-failure load-bearing capacity of each model corepillar. Compared to the unconfined core, the trappedmodel core pillar showed some marginal axial defor-mation change due to the confinement provided byfly ash composites. Failure of the core samplesoccurred predominantly along a shear plane.

The unconfined model core pillar under uni-axialcompressive loading split vertically due to tensileforces. This splitting was due to the brittle nature ofthe unconfined cores, which exhibited no post failureprofile as they lost all their strength soon after fail-ure. When the same sample was trapped with rein-forced fly ash composite material, the core failedalong a shear plane. The moment the wire ropesnapped, the developed radial cracks in the fly ashcomposite material widened and the trapped pillarshowed punch shear type behaviour. When the samemodel core pillar was trapped with an even higher

thickness of annular filling area (i.e. Tf/ Tc > 7.0), thefailure mode resembled a typical tri-axial testing(Figs. 3 to 6). This finding indicates that fly ash com-posite material offers radial confinement stresseswhich induce a model core pillar to change from abrittle failure pattern to a ductile pattern within theelastic range. However, because the elasticity of thefly ash composite material was lower than that of thecore, it only improved the bearing strength of thecore sample.

It was observed that the model core pillars exhib-ited a tendency to strain soften (i.e. to increase indeformation while decreasing in load bearing capac-

ity). With continued axial loading after the specimenreached its peak strength, the core pillar further

slipped along the fracture plane which, in turn,pushed the fly ash composite out. The adhesionbetween the wire mesh and the fly ash compositewas very strong. There were no instances of wiremesh cutting into the composites. The fragmented flyash composites were resisted by the tensile strengthof the wire mesh and provided local confinement to

the model core pillars. Thus, in line with Wilsons(1972) and Prices (1979) writings, the strength of thetrapped pillar was reduced to a residual level equiva-lent to the frictional resistance of the interlocking bro-ken fly ash composites. The trapped model corepillars exhibited substantial strength, followed by aninitial sharp and moderate drop in load-bearingcapacity after failure. The residual strength for modelcore pillars of type 1:1 varied between 10 and 20%of their respective peak load bearing capacities.

In contrast, the unconfined model core crushedimmediately after its peak failure strength wasreached and showed no post failure path of the core.

Figures 3 through 6 show the failure as well as postfailure profile of the load-bearing capacity of sometrapped model core pillars and the correspondingunconfined model core pillars. Similar results werealso observed for other types of model core pillartypes, though with different strength magnitudes andpost failure profiles.

CONCLUSION

Loose sand as a backfill material merely occupiesthe underground space created by mining operationand past studies have indicated that no lateralstresses develop because sand filling assists the stabil-ity of the opening (Srivastava, 1995). In light of thisobservation, one of the objectives of the currentinvestigation was to study both the physical attributesas well as the engineering properties of the devel-oped fly ash composite material as an alternative tosand as a backfilling material. With the promise ofpaste backfill technology, the transportation of fly ashis not a serious economic handicap.

Fly ash composite developed with the additionof lime and gypsum and reinforced with wire meshsignificantly contributes to strength characteristics.

Coal and Oil Sands

Fig. 2. Tri-axial failure pattern observed for the core inside.


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Fly ash composite also improves the load-bearingcapacity and the post failure behaviour of modelcore pillars. A maximum gain in strength isobserved for the weakest core types. Unlike thebrittle failure exhibited by unconfined core pillars,confined fly ash composites make the core modelpillar ductile and show signs of residual load evenafter failure. The trapped pillars fail gradually,rather than collapsing suddenly, soon after thepeak load (Figs. 3 to 6). In the literature, this resid-ual load has been referred to as a post-critical load(Pytel, 2003), which is only a portion of the maxi-

mum load. Because the post-critical load dependsupon the geometry of the pillar, trapping the pillarincreases the magnitude of the pillars post-criticalstrength.

ACKNOWLEDGMENTS

The authors acknowledge the financial assistancereceived from the Central Scientific Research Station(CSIR), New-Delhi, under the Extra Mural Research

(EMR) fund (reference TMP 22(0341)/02/EMR-II)dated March 28, 2002.

Paper reviewed and approved for publication by theCoal and Oil Sands Society of CIM.

Manoj Ku Mishra graduated with a B.Sc. degree in miningengineering from Regional Engineering College, Rourkela, in1985. He obtained his M.Sc. and PhD degrees from SouthernIllinois University Carbondale, Illinois, and the Indian Institute ofTechnology, Khar,agpur, in 1992 and 2004, respectively. He hasbeen teaching and researching since 1993 and, currently, is anassistant professor in the Department of Mining Engineering atthe National Institute of Technology, Rourkela. His researchinterests are rock mechanics, ground control and gainfulapplication of fly ash.

U.M. Rao Karanam is a professor at the Indian Institute ofTechnology, Kharagpur. He obtained his B.Sc. in miningengineering in 1983 and worked for three years in opencast andunderground hard rock mines. He obtained his M.Sc. in mineplanning and designing in 1987, specializing in rock mechanics,and has a PhD in 1994. He has been teaching and researchingsince 1987, working at various levels in the Department of MiningEngineering at the Indian Institute of Technology, Kharagpur. Heis also a visiting professor at CNU, South Korea.

4 CIM Bulletin I Vol. 101, N 1109

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At Tf/Tc = 0.75at28 Days curin

Fig. 3. Failure and post-failure behaviour of trapped model core pillars:Model core 1:1 type with fly ash composite material of 15 %L-0 %G.

At Tf/Tc = 0.75at

56 Days curing


At Tf/Tc = 1.34at28 Days curin

Fig. 5. Failure and post-failure behaviour of trapped model core pillars:Model core 1:1 type with fly ash composite material of 15%L-0%G.

At Tf/Tc = 1.34at56 Days curing



5/5Copyright 2008. All rights reserved. ISSN 1718-4169 5

Coal and Oil Sands

REFERENCES

Chugh, Y.P. (1984). Workshop on the design of mine open-ings in room-and-pillar mining, SIUC, IL, USA

Bieniawski, Z.T. and W.L. Van Heerden. (1975). The Signifi-cance of In Situ Tests on large Rock Specimens. IJRM and Min.Sci., v 12, 101-113

Chugh, Y.P., M. Phillips, K. Chandrasekhar, A. Atri and S. Haq.(1990). Identification of mine characteristics, conditions andprocedures for design of stable partial extraction room-and-pillar mines in the Herrin (No. 6) coal seam in Illinois. FinalReport submitted to IMSRP, ISGS, Champaign, IL, USA

Galvin, J.M. and Wagner, H. (1982). Use of ash to improvestrata control in bord and pillar working. Proceed. Symposiumon Strata Mechanics. Univ. of Newcastle upon Tyne, 264-269

Grice, T. (1998). Underground Mining with Backfill. 2nd

Annual Summit. Mine Tailings Disposal System, Brisbane, Nov24-25, 1-13

Horino, F.G., W.I. Duval and B.T. Brady. (1971). The use ofRock Bolts or wire Rope to increase the strength of fracturedmodel pillars. RI 7568, USDI, Bureau of Mines, 1-24

Kumar, V., B.P. Ahuja, J.V. Dattatryulu, B. Bhaskar Rao, C.N.Ghosh and A.K. Sharma. (2003). Hydraulic Stowing of PondAsh in underground mines of Manuguru, India. Proced. 3rd

Int. Conf. On Fly ash utilization and Disposal, Feb 19-21, NewDelhi, India, VI-1-7

Lunder, PJ and Palakins, R, (1997). Determination of theStrength of Hard Rock Mine Pillars. Bulletin of Canadian Insti-tution of Mining and Metallurgy, 90, 51-55

Martin, C.D. and Maybee, W.C. (2000). The Strength of HardRock Pillars. Int. Jour Rock Mech. & Min Sc., 37, 1239-46

Palariski, J. (1993). The use of fly ash tailings, rock and bind-ing agents as consolidated backfill for coal mines. In Proceed.of Mine fill-1993, Edi. H.W. Gelen, SAIMM, 403 408

Price, A.M. (1979). The effect of Confining pressure on thepost-yield characteristics of rocks. Unpublished Ph.D. Thesis,University of New-Castle upon Tyne, UK

Pytel, Y. (2003). Rock massmine workings interaction modelfor Polish copper mine conditions. IJRM, Volume 40, Issue 4,June, 497-526

Ringwald, J.P. and Brawness, C.O. (1989). Reinforcing Con-crete Model Pillars with Grouted Rock Bolts. Mining Scienceand Technology, 8, 31-47

Srivastava, S.D. (1995), .An Investigation into the design ofpost pillars in cut-and-fill method of mining., UnpublishedPh.D. Dissertation, I.I.T. Kharagpur, India

Tannant, D.D and V. Kumar. (2000). Properties of Fly ash sta-

bilized haul road construction material. Int. Jou. of SurfaceMining Reclamation and Environment, 14, 121-135

Tesarik, D.R, J.B. Seyman, T.R. Yanske and R.W. McKibbin.(1995). Stability Analysis of A Backfilled Room-and-Pillar mine.USDI, Bureau of Mines, RI 9565

Wilson, A.H. (1972). Research into determination of pillarssize- A hypothesis concerning pillar stability. The Mining Engi-neer, Vol. 131, No. 141, 409-417

Yu, T.R. and Counter, D.B. (1988). Use of Fly ash as Backfill atKidd Greek Mines. CIM Bulletin, January, 44-50

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