designofbackfillassupportin polishcoalmines - saimm · pipelinep(lp,hp),theequationforpressurecan...

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Synopsis

Longwalling is used inmost Polish coal seams,with bacJdilling tosupport the roof, tocreate an artj/icialhangingwall to preventmovement if therockmass, and toeliminate thefire hazardin abandoned mineworkings.

This paper discussesthe results if testsconducted on 7 differentbacJdil1 mixtures, andgives equations for theestimation ifthejlow ifbacJdi11slurries throughgravitational pipelines.

* TU Gliwice, Poland.@ The South 4frican

Institute if Mining and

Metallurgy, 1994. SA

ISSN 0038-223X13.00

+ 0.00. Paper received,Sept. 1993: revisedApr. 1994.

~ 218

Design of backfill as support inPolish Coal Minesby J. Palarski *

Introduction

Tailings backfill was introduced into Polish coalmines in 1893. Over the past hundred years,considerable work has been done on the assess-ment of optimum mixtures, the development offlexible delivery system, and the adjustment offill technology to a wide variety of mining andgeological conditions. Sand or industrial wastematerial for the backfilling of undergroundworkings is used as a support mechanism tocontrol ground deformations and surfacesubsidence.

Underground Mining Methods

The mining methods commonly used in Polishcoal mines are illustrated in Figure 1. The long-wall method is employed in most coal seams,where coal is extracted from advancing orretreating faces, the faces usually being morethan 150 m wide and 500 m long. The shapeand advance of the winning face are defined inFigure 2. The fills used in coal-mining operationshave evolved from loosely dumped rock (pillarsconstructed of rock and roll fill) and hydraulicallyplaced sand fills, through pneumatic and thowingstowing, to today's hydraulically transported,densified paste fills containing fly ash.

Nowadays, fly ash with a binder is injected,through a hole into cavity zones of the roof andsmall voids, to support the roof, to create anartificial hangingwall, to eliminate the movementof the rockmass, or to eliminate the fire hazardin abandoned mine workings.

Backfill Materials

The backfills currently used in Polish coal minesare produced from four raw materials:

~ sand~ crushed and milled development waste

rock~ processing-plant tailings (flotation waste

and slime)~ power-station ash, fly ash, and slag.

AUGUST 1994

Different combinations of these materials arealso used In 1992, about 86 per cent of the back-fill placed underground in longwalls was sandwith crushed waste rock,S per cent was crushedwaste, 8 per cent was a mixture of ash, fly ash,slag, and tailings or crushed waste rock, and 1per cent was drainage-free hydraulic fill.

Various combinations of materials arecharacterized to their particle-size distributionand optimum pipe-transport and placementproperties.

The required properties of a backfill materialdepend on the geological and mining conditions.It is fundamentally important that cognizance istaken of the following in situ conditions:

~ the confined compression and stress-strainbehaviour, and closure in the stopes

~ the percolation ratio~ the spreading of a backfill mixture and

filling voids and the fill stability along thelongwall

~ the fill pressure on a backfill fence.

Stress-strain Behaviour and CompressiveStrength of Backfill

The amount of strata deformation is reduced inproportion to the amount of backfill materialadded to the voids. The packing density and fillratio of the material can vary significantly forany given particle-size distribution, and dependon the amount of water in the backfill mixture,the drainage technique, and the manner ofplacement.

It is generally agreed that the main factorsdetermining the deformation of the overburdenstrata in longwall mining with backfill are asfollows:

~ the stress acting on the overburden strata,resulting from the redistribution of geo-static stresses during the mining of coal

~ the seam height~ the compressibility of the backfill and the

fill ratio~ the layout of the longwalls.

The Joumal of The South African Institute of Mining and Metallurgy

Design of backfill as support in Polish coal mines

Room and pillar

Longwall with back fill Ascending slicing of soamby longwall with back fill

Caving long wall with fillingDescending slicing of seam through boreholes from'by long wall with back fill surface

IFigure 1-Methods used in the mining of coal seams

+dip

Up-dip Up-dip

,J,lllInclined

IFigure 2-Stoping directions of longwalls

+dip

{I'll~,~:coal:(~.~

II~IDown-dip

~dip

The Journal of The South African Institute of Mining and Metallurgy

Seven different mixtures were tested to showthe influence of backfiIl composition on itscompressibility, From Figure 3 it is apparent thatbackfiIl materials containing a binder behavedifferently from the stress-strain behaviour ofcohesionless backfiIl,

The maximum compressive strength ofbackfiIl containing binders depends on

>- the type of material, Le, its particle-sizedistribution

>- the type of content of the binder>- the water-to-solid ratio,

The test results showed that a mixture withIow porosity, a minimum water content, andadequate active binder produces stronger backfiIlfor a given amount of binder (Figures 4-6).Mixes stabilized with cement displayed higherearly strength than those stabilized withmaterials containing CaO. However, the 5 to 6day strength values were higher for the mixescontaining binders other than cement.

Based on the results summarized in Figure 7,the highest strength for the backfiIl was obtainedwhen alpha semi-hydrate (CaSO4,l/Z'HzO)wasused as a binder. Mixes with 50 per cent alphasemi-hydrate and 50 per cent crushed waste rockor 50 per cent fly ash achieved compressivestrengths of 8 MPa and 16 MPa respectively inonehour.

Rapid setting of the backfiIl eliminates theneed for drainage by locking water within thebackfiIl mass, An addition of fly ash to the alphasemi-hydrate improved the setting time(hardening time), as shown in Figure 8,

~diP ~dip~~t¥~~f1~~~~~lf~~

~:,':?;:~:coal "5:;;:"-.'"'S"t

.~1~F~~i~~~1~!

Along strike

~dip

Inclined

AUGUST 1994 219 <Ill!

35

3<'

25

"20

cI'~0; 15

10

00

.. 1-S6a.

::E

~c;,1

'~1c:!!';;0

1> -\\6-:;::a.E lI,a,0

u

\\'S6

100

80

'".cC,

~60

~~0.40

E0

'-'

20

00

._.._,-~-----

Mass, %Component of the slurry

1 2 3

Fly ash 60,0 69,8 59,6Flotation tailings 0,0 20,0 0,0Sand 30,0 10,0 30,0Phosphogypsum 2,0 2,0 2,0Furnace slag 4,0 4,0 4,0Portland cement 1,5 1,5 2,0Activator 2,5 2,7 2,4

Design for backfill as support in Polish coal mines

,,_.,,-' ":'C, coc' .aste

,-"..- ',/",

,/'

/ ------------- -:~ ""0" .'C'. '0;:. .",e

50% fly ash + 3% ta;lings + 7% cement30% sand + 70% cock waste

40% sand + 60% rock waste50% d--- semi hydrate + 50% fly ash

100% sand

to 15Stress MPa

20 25

Figure 3-Stress-strain curves for various backfills

117.~~%

. efo4e

tf'\'.,. ~Qf'~ef'\.

C.~

Figure 4-28-day strengths of a backfill containing: 5 per cent fly ash, 95per cent tailings, cement, and water

:I;1""III"'sIm1l111111J1n poroSl1V)

,40

,',{)10 20 3D

Mass % t,ne.

Specimens 10,50mm rock wasteIcoar"'1

O,OI-2mm sand lfines)5% portland cement, concontr"""n t,v mass

G""

Figure 5-Strength of a cemented rock-waste fill versus the fines content

~ 220 AUGUST 1994

0,85

0,80

.,3.//.

../. /?/'

,/ I. ,// /". .,/

./ ././//

//././/

//./

~

O,75

'"a.::E:5 0,70DJc:~Vi 0 65QJ ,

>';;;

~ O,60a.E0U O,55

0,50

O,450 4 7 11

Time days

14 18 ?I

Figure 6-Compressive strength of stabilized backfill

30

o'--semi hydrate + rock waste

"',J.Jo,'c---sem"nva'ate

25

:::; 20

'"§:§u; 15m>;;;:g~ to0u

,~m'-"ia'a'~ + "i as"

"'-serm-hydrate + anhydrite

o'--semi-hydrate + 25% fly ash+ 25% rock waste

Ih 5h 24h 7d 28dTime

Specimens, 50% d-semi-hydrate 50% other materials Concentration by mass 66%

Figure 7-Compressive strength of stabilized backfill


300

250

200

~150

" ~100'"~i'i

"50

<0

~" 0(;Il.

-50

-100

-1500 10


'" 302:Jc::

'E 25Q)

.§

0> 20c::-a;

Cl) 15

45

40

35 //

100% rJ..-semi.hYdrate/

//

//

~/--~10

5

00) 1 OJ2 0,3 0,4

Water /solid ratio

0,5

Figure 8-Setting time as a function of water-to-solids ratio

Drainage of the Fill in Mine Stopes

Hydraulic fill is transported to longwalls throughpipelines in the form of a slurry typically con-taining 50 per cent water (by volume). It is notdesirable for the water introduced with the slurryto be retained within the fill void. Small particlesand binders added to increase the strength of thefill are leached away as the water percolatesdownwards. Variations in material properties,such as porosity and hydraulic conductivity, maysignificantly effect the drainage process.

In longwall mining, backfill is placed innarrow, extensive paddocks built of timber andgeotextiles, where high stope compression and,consequently, high stresses in the fill airgenerated. The backfill paddocks are built andfilled behind mechanized support as the faceadvances every 1,8 to 3,6 m. The longwall movesup dip. It is known that, as the porosity of abackfill under pressure is reduced, the fillmaterials display stiffer response.


O,G

At the beginning, the fill is usually in asaturated state, During the fill placement waterdrains continuously through the fill and thehighly permeable fences (barricades). Water isremoved from the placed fill by two mechanisms.After the fill materials have settled and consol-idated soon after their placement, excess wateraccumulates on the fill surface. The 'drainagewindows', (timber raises), allow the surfacewater to drain away rapidly. It is known thatdrainage reduces the pore-water pressureswithin the settled fill. The distribution of pore-water pressure is a function of the fill composi-tion, fill capacity, geometry of the paddock, andthe location of drainage gates or 'drainagewindows'.

The tests showed that the pore-water nearthe high permeable fence were Iow. The maxi-mum pore-water pressures were recorded at thecentre of the backfill area, about 25 to 40 mbehind the fill paddock (Figure 9). Further away,the pore-water pressure dropped sharply andbecame negative.

Backfill in a coal mine frequently consists ofcrushed or milled waste and sand, Le. cohesion-less material. The shear strength of a granularmaterial is determined directly by the pore-waterpressure according to the effective stress law.Therefore, in fill design and mine planning, it isimportant to ensure that significant pore-watercannot develop in a backfill area. High pore-water pressure leads to a complete loss of shearresistance and to subsequent liquefaction of a fillmaterial.

The permeability of a fill determines thedrainage condition. Investigations indicate thatsuccessful fill materials have a permeabilitycoefficient in the range (2 to 10) x 10-5m/so

60% sand + 30% rocl< was!<'+ 10% flV ".11

20 30 40 50 61.1

Distance to face rn

11.1 91.1 !JlI

Figure 9-Distribution of pore water pressure in filledvoidLongwall: width 180 m, dip 18°, heigth 2,8 m

AUGUST 1994 221 ...


The investigated fill materials were producedfrom fly ash, slag, and processing waste, or amixture of these components. Hydraulic place-ment of these materials results in a loose fillstructure with a porosity of these materialsresults in a loose fill structure with a porosity of62 per cent. Figure 10 shows the percolationrates for different backfill materials.

20

16

1 Flotation tailings 0-3mm 75%, slag 250/0

2 Sand 100%3 Slag 100%4 Flotation tailings 1-3mm 100%5 Flotation tailings 3.6mm 100%6 Flotation tailings 3-6mm 50%, fly ash 50%7 Flotation tailings 0-6mm 33%, slag 330/0,

fly ash 33%

\18 \

\\\\\\\\

, \\ \

Q)10 \

\~ \ \~ 8 \ ""0' ,~ \ \ "a. \\ "'-6 , "

". \, """"'.. .....". " -"'-""::'-.." -'::::'--::::--54

""" \""'. -- 4""" """"""'.".,.,'~ -'-.

""'..." 7"'... ..................."-"'-"-""'.w~~"""""""" 1-- 2

(/) 14--E0

';' 120

x

23

00 302010 40 50 60 70 BD

Time minutes

Figure 1G-Percolation rates for various backfill materials

tOO

#90

!!!! 80

u:

70

600 12

Length of pipe m

204 16 24

,I: discharge I

.JII-I:I

,

h pipe in longwall

Figure 11-Distribution of fill density around measuring pipe

~ 222 AUGUST 1994

Underground, a backfill of low water contentdoes not display any apparent cohesion, which isnecessary to allow free-standing vertical walls offill. The slurry is discharged into the paddockevery 8 to 12 m to fill up the void homogeneouslycompletely.

Various processes occur with hydraulic fill,and variations in the backfill material can lead toa non-homogeneous fill. During the placement ofcrushed waste rock mixed with fine materials,segregation occurs, with the coarser particlessettling close to the discharge point. Furtherheterogeneity of the fill arises from differentlocal settling rates and from the differences inthe impact and compaction of coarse and fineparticles (Figure11).

Fill Pressure

The drainage gates are fenced from the fill areathrough side barricades or packs, which allowthe fill to drain and are designed to give resis-tance to the backfill pressure. The packs are builtto isolate the gates from the fill area, wheredangerous materials are disposed. In that case,water is prevented from draining out of the fillarea. The packs are designed to give sufficientresistance to the roof and to the fill pressure.

The total pressure after drainage in thehydraulic backfill acting normal to the fence is asum of the silo effect and the convergencecomponents. Pressure loads that were measuredat the barricades during the placement of fill arereported in Figure 12.

The measurements show that, for a smallheight of fill, the pressure increases rapidly(hydrostatic head) when the face is within 25 to40 m away (at a seam dip 18 degrees), andreaches its peak values from 0,75 P mgto rhoP mg(Pm= density of the mixture). Further, thepressure decrease with the placement of fill.After reaching a minimum at a fill distance ofabout 50 to 60 m behind the face, the pressurerises slowly again. This increase of pressure atthe barricade is caused by the weight of the over-laying strata, which is incompletely transferred tothe backfill in the fenced void. However, the loca-tion of the peak pressure varies from area toarea owing to differences in backfill composi-tions and strata characteristics.

Distribution of Vertical Stresses andClosure around a Longwall

Backfill was first used to control surface subsi-dence. The modern structural function of backfillis to facilitate full mining of coal without loss ofcontrol of the rockmass.


Measure point

300

250

200('tJ

a...:s:.Q)...

150::JI/)I/)Q)...a..

100

50


Coal face

Floor

00 20 40 60 80 100 120

Figure 12-Distribution of pressure at the barricade in a filled area

Distance to face m

-50

- Caving-- Backfill

--------,,------"-30 10 30 50 70 90 Distance to face rn

Immediate roof

EE"

30 ~c:.,0>iD>c:0

50 U

,"""H'~.,."-",, ..,.".,..".

-" """ """',' """",,"'" ," "","" '" "Seam

Main roof

Gob/backfill

Figure 13-Distribution of vertical stresses and closure around a longwall coal face withcaving and with backfill (depth 480 m, seam height 2,5 rn, dip 15°, sand backfill 0,1602,0 mm)


As local support, backfill can prevent spatiallyprogressive disintegration of the fractured rock-mass, reduce rockburst damage, and providebetter convergence control. If backfill is properlyplaced and confined, it can act as a global supportelement in the mine structure.

Figure 13 shows the distribution of verticalstresses and closure around longwall coal faceswith caving and with backfill. In an infilled long-wall, the vertical stress rapidly increases with thedistance into the yield zone in the unmined coal,and reaches its peak 1 to 5 m ahead of the face.The peak pressure ranges from 3 to 6 times theoverburden stress. In fIlled stopes, the stressreaches its peak 8 to 20 m ahead of the face. Thepeak stress so produced is two to four times theoverburden stress, and depends on the stiffnessof the backfill. In both cases, with increasingdistance into the unmined coal, the vertical stressdecreases towards the overburden stress.

In the mined-out area in unfilled and filledstopes, the vertical stress increases with distancefrom the face and rib side. The pressure increasesmore slowly in an unfilled area than in a filledarea.

The closure rates of the immediate roof stratain a filled longwall are significantly lower thanthe closure of the main roof in an unfilledlongwall.

Fill Technology

Preparation of the Fill

Various types of fill and placement methods areused in Polish coal mines. Details of the fill types,the preparation, and the required properties aregiven in the literature!.2 and only a brief descrip-tion of the fill technology is given here.

A backfill mixture that contains sand orcrushed waste is prepared within a simple plant,where the material is washed out of a tank bywater jets or is fed mechanically onto screens towhich water is added. The mixture thus obtainedflows through screens to a hopper, where wateris supplied in the quantity needed to give therequired concentration.

A multi-component slurry containing bindersis prepared in mixers to which all the componentsare supplied in the required quantities. From themixer, the slurry is fed into a hopper and thenflows by gravity through pipelines to the mineworkings.

AUGUST 1994 223 ...


Gravitational Transportation of BackfillSlurry

In coal mines, pipelines connect the backfill planton the surface to the underground workings.These pipelines form a network to the shaftswhich are equipped with at least two pipelin~s,one of them being a reserve. Horizontal pipelinescan branch but, during backfilling, the streammust not divide into two or more parts. In work-ings such as a longwall, the pipeline branchesout every 8 to 16 m, and the backfill mixture isdirected through these branches to the void thatis to be filled (Figure 14).

The flow of slurry through a gravitationalpipeline can be described by the modifiedBernoulli equation on the assumption that theconcentration of slurry does not change alongand across the pipeline.

v2p + hp mg + 2 Pm + !!..pI= cons!,

where: p = pressureh =depthPm= density of mixture

v = velocity of flowI:1p= elementary resistance of flow

I =lengthg =gravity

When the formulafor the conservationofenergy, equation [1], is appliedto the input andoutput of a pipeline,the equation for the flowofbackfillslurry changes to

nHp mg = I.!!..pk

i=l

i.

~

"

F~gur~ 14-Schematic diagram of a network of hydraulic backfill pipelines (1 hopper, 2pipeS In shaft, 3 branch point, 4 fill areas)

~ 224 AUGUST 1994

It was established by the derivation of thisformula that the difference in atmospltericpressures between the input and output sectionof the (Pin'f'.,ut)and the value of the pressurevo~~ is extremely small. Based on the sameprinciple for an input at a certain point in thepipeline P(lp, hp), the equation for pressure canbe presented as

v 2Hp mg = Pp +hpp mg+-Lpm

2k

+I.t1.pJii=1

[3]

[1]

where: H = total depth of the pipelinehp = depth of point P in the pipelinelp = length of pipeline from an input

to point Pvp = velocity of low in point P

n = total number of parts into whichthe pipeline is divided

k =number of parts of pipeline froman input to point P

Pp =pressure at point P.

To solve these equations, the equationdescribing the elementary resistance of flow in apipeline, I:1Pi'must be known. In the gravita-tional transportation of backfill material, the flowis turbulent. The models for particular mixtures,which were established on the basis oflabora-~ory me~surements at industrial plants, are givenIn equation [4] to [6], the parameters beingdefined after equation [6].

(a) For a slurry containing and and up to 50per cent crushed waste rock with a maxi-mum particle size of 50 mm.

2v

I:1P=A -pw2D

w

c~+kasws '- Pwg(Pa/m).

v

[2]

[4]

(b) For a slurry containing sand, crushed

waste rock, furnace slag, and fly ash in avertical pipeline;

V2 v2!:;,p= Aw

2DPw + Afa

2DPwxfaCv

a,c,x,w,~

~ ds+k Pwg (Pa/m)v

[Sa]

(c) For slurry (b) but in a horizontal pipeline;

The Joumaf of The South African Institute of Mining and Metallurgy


.r:::

E

E 3000>-uc:!!!~0; 200

3:~u..

2 2v v

J1p = Aw - Pw + Afa - PwXfa Cv2D 2D

a,cvx,w,~

~ ds+k Pwg

v

+ !rarXrCvPwg + fslasixsiCvPwg (Pa/m)

(d) For a slurry containing flotation tailings,fly ash, and different binders;

2v

J1p = Aw - Pw (1- Cv)2D

V2+Af - PfCv (Pa/m)

2D

The parameters used in equations [4] to [6]are as follows:

Pr -Pwa=-r

Pw'

Ps -Pwas=-'

Pw

Psi -Pwas! =

Pw

c =Pm - Pw

- concentration by volumevPs - Pw

500

400

Flow efficiency am

\ ~...--.,........

"-~""200

"n

'.150

",1>~

"<00

100 100 3

:r

0 00,80 0,2 0,70,1 0)3 0,4 0,5

Concentration, Cv

0,6

L = 3000m, H = 1000m, D = 0" 150m

- Crushedwaste rocks 90% + sand 10%--- Crushed waste rocks 30% + sand 70%-.- Fly ashes 50% + sand 50%

Figure 1~fficiency of filling in relation to the concentration of various slurries


[5b]

ds = diameter of particles

D = diameter of pipelinef, =coefficient of friction for waste rock

(0,34 to 0,38)

.!sI = coefficient of friction for furnace slag(0,40 to 0,43)

k = coefficient (0,45 to 0,50)

xr. = quantity of fly ash

Xr = quantity of waste rock in dry mixture

Xs = quantity of sand in dry mixture

Xsl = quantity of furnace slag in dry mixture

ws = sedimentation velocity

Ar = coefficient of resistance for the mixtureof flotation tailings, fly ash, andadditives (0,022 to 0,026)

Ara= coefficient of resistance for fly ash(0,020 to 0,023)

A.w= coefficient of resistance for water(0,012 to 0,017)

Pr = density of the mixture of flotationtailings, fly ash, and additives

Pia= density of fly ash

Pr = density of crushed waste rock

Ps = density of sand

Psi= density of furnace slag

Pw = density of water.

From these equations, the velocity of flowcan be calculated. The intensity of flow and othereffective parameters are calculated as follows:

Intensity of a flow of slurry, Qm:

Qm= ITif v (m3/s) [7]

Efficiency of transportation of solid bodies, Qs:

Qs =CvQm (m3/s) [8]

Efficiency of filling the voids, Qp:

Qp = PeQm (m3/s)

[6]

[9]

where: Pe (the empirical efficiency coefficient)

= 0,839 (Pm - 1).

The following two conditions are the deciding

criteria for the design parameters of the mixture

and the pipeline.

Mv,r s v< vba andPvp <P<Pts,

where: M = flow index (1,1 to 1,3)

ver = critical velocity of flow

vba = maximum velocity (8 to 10 m/s),

Le. maximum acceptable velocityon account of wear of the pipewall and reliability of the flow

Pvp= saturated vapour pressure

PtB = tear strength for establishedthickness of pipe wall.

AUGUST 1994 225 ....


References

1. PALARSKI J. The experi-

mental and practicalresults of applying

backfill. Proceedings

Symposium 'Mining with

Backfill', Montreal,(Canada), 1989.

2. PALARSKI J. Assessment ofbackfill materials and

roof strata behaviour indeep coal mining.

Technical Challenges in

Deep Level Mining,

Johannesburg, South

Africa Institute of Mining

and Metallurgy, 1990.

Figure 15 shows the efficiency of filling froma gravitational pipeline in relation to the concen-tration of slurry and type of fill material. It isapparent that the filling efficiency increases withan increase in the concentration of fine-grainedmaterials. For coarse-grained materials, theefficiency increases only up to a certain concen-tration. Figure 15 shows curves for both thecapacity of the mixture flow and for criticalvalues of the mixture flow. Flow is impossiblefor concentrations greater than the point ofintersection of the flow curves.

The distribution of pressure in a pipelinedepends mainly on the spatial arrangement ofthe pipeline, on its diameter, and on the para-meters of the mixture being transported.

Flushing of a Pipeline

The gravitational transportation of mixturesbegins and ends with the flushing of the pipeline.Flushing is unnecessary only when the mixturebeing transported does not settle in the pipelineafter the flow ceases. Flushing means that wateris delivered to a fill area, and it must be drainedback to the surface. .

~ 226 AUGUST 1994 The Jouma/ of The South African Institute of Mining and Metallurgy

designofbackfillassupportin polishcoalmines - saimm · pipelinep(lp,hp),theequationforpressurecan...

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