Lilly - Use of Blast Ability Index

THE USE OF THE BLASTABILITY INDEX IN THE DESIGN OF BLASTS FOR OPEN PIT MINES

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Tbe BhtabiIity Index, f i t published by Lilly (1986). is re-examined and applied te a range of rock mass conditions which exist in open pit mines. A series of design charts based on the Blastabdity Index is provided which would assist mining engineers in estimating explosives consumption and blast hole dnIling requirements for a range of blast hole diameters and bench heights, The whnique is useful in feasibility studies, in the estimation of quantities and costs for mining contracts, and for minc planning Purposes.

ROCK MASS PARAMETERS SIGNIFICANTLY AFFECTING BLASTABILITY

structural mure of the rock mass is extremely important in blast design. If, for example, the rock mass i s very blocky, being made up of a multitude of joint-bunded bI& of m k material, hen the

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fragmentation characteristics of the m k mass will h strongly controlIed by rhese preexisting geological structures and lcss so by explosive-induced fractures. The opposite could I x said for massive rock masses. This initial, qualitative aswsrnenr of the rock mass to be blasted is therefore fundamenta1 to the success of the blast.

INTRODUCTION Among the most significant parameters which S e c t the outcome of a blast are those assclciated with the physical properiies of the rock mass. Therefore, it would be useful to have a method of describing the rock mass in a quantitative way for direct use in the blast design process. Lilly (1986) developed a rock mass classification system for use in blast engineering, called the Blastability Index, which is based on what the writer considers to bc the four main rock mass parameters which contribute! significmtly to the pmTomancc of a blast: o o

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h e sbuclural nature of the rock mass, eg. whether it is powdery, btmky or massive: the spacing and orientation of planes of w a e s s such as joints, bedding planes,

scbtosity and foIiation; Lhc s p i f i c gravity of the material; and the hardness/s&ngth

of the matwid.

Tk derivation of the Blastability Index is dixrusssd below. The use of the term *jointmin this discussion is meant to iracoqmate all geofogicd planes of wedmess which may pervade the rock mass, as well as mining-induced fractures which may in fllrence m k mass hgmenzatiun during blasting.

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Dames & Mmre Consulting Engineers, South Peah, Wesrern Auslralia.

The spacing of joints (and other pervasive planes of weakness in a rock mass) tends to conml the size and shape of the fragments which will EK found in a blasted muckpiIe and, all else k i n g equal. rock masses characterised by closely-spwd sets of joints will quire lower powder (or energy) factors for satisfactory fragmentation and displacement than those & masses which amtain wideIy-spaced joint sets,

In addition to spacing, joint plane orientation relative to the orientation of the free face (or, aItemalivcly, the movement direction of the muckpile) is important For example, the pmence of a signifimnt set of horizonkt1 joints in a mck mass during bench blasting opm'tions will, by virtue of a shallower suWil1 rcquiment, allow a lower powder factor to bc u x d than would othetwise be the case.

Rack mass dmsity is of lesser impthan structural characteristics but is d I l significant and, all else being equal, a rock mass having a higher density requires a higher powder factor to achieve adequate fragmentation and heave than one having a Iowcr density.

In the original formulation of the Blastability Index, the "hardness" of the rock matwial was used as the fourth parameter. The writer has found that an estimate of the compressive strength of the rock matcrid (that is, the strength excluding the influence of macro-scale planes of weakness, such as joints) is more easily obtained. Once again, a11 else king equal, suonger rocks require higher powder factors For fragmentation than weaker rocks.

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CALCULATION OF THE BLASTABILITY TNDEX

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LjIIy (1986) dcFcribes how h e rock mass parameters which significantly a f k t blasting arc raid and the rating combined to calcubtc the Rktabilily Index. The pmdm is similar u, that u s d in o~herrock maw classification systems, mad the wrious ratings are dcfined a? folIows: Parameter

an example, considcr a well-jointed. blacky banded iron Fonna~ion having a dcnsiry at 2.6Um' and a compressive strength of approximaaly ISOMPa, in which the Mding p1.m strike roughly

By way

Rating

Rock Mass Dxription @MD) Powdcry or friable Bldy Massive

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normal to thc bcnch face. Thc Rlxtability Indcx is calculaFed as follows:

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RMn=D. IPS = 15:

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JPO=30; RDS=15;

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S = 7.5:

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and

R I = 0 . 5 (20 + t 5 + 3 0 + 15 + T . 5 ) , w a p p x imael y 44.

USE OF THE INDEX IN

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IlLAST DESIGN CHARTS

Join1 Plane Spacing (JPS) Clare ( 4 . 1 m) Snmediatc (0. l m to l.Om) W idc (> l .Om)

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Joink Plane Orienlarian (m) Horiirmtal Dip out of f3ce Strike n m a t to facc Dip inlo fncc 'me rating for the Rcxk Dcnsify Influcncc (RDI) jr estimatd m follows:

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In ordcr lo 'mist mginecrs in thc estimation of blast holc h l l i n g requircmmu and explosives consumpdon in dilkrent mck maw t y p s , n scries of hcnch-hlarl design cham has Irtn dcvcloped far a range of RIxsl;~bili\yIndcx values. 'I'hc cham pipyres 1 m 6 ) show [he rclalionship htween tllc Index and powder factor or bbsl holc pnuem si7c (which is givcn by bunlcn distance mi~ltiplicd by blasi hoIe spacing) fur lhrcl: operating k n c h ticight. (5m,Ilm awl I h ) and four blmt holc diarnctcn (76mrn. 89mm, 102rnrn and Il4mm. or in morc convcnBonally-used Imperial u n i ~ 7*, , 3.5", 4" and 4.5").

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Thc ,?ssumptions u.wd in ~ h ccrcarion o l dicx charts arc as fullows:

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whcrc D is Lhe rock maqs dcnsily ~ i v e nin znnnes pct cubic metrc (t/mq. The rating for Lhc rock strcrigth influcncc (S) i s cstimared ac Follows:

95% of fragments should pa%$ an 80Ikna gtj~zly.and this fcagrncntalion rcq~~ircmcnl is dcrivcd riom thc K u z m fragmcntntion mdcl ( C u n n i n ~ t m ,2983; Lilly, 19XG); bla?~holes art vertical; him holcs arc driIIed on an cquila~eral triangulw pa~tcm in which h o b spcing is appmximatcIy 1. I6 limm rhc dnFlct1 burclcn dishnce; htast holcs arc drill& vcnical; sulxb~ll deplh is approximately 8 n i m ~thc Mast holc diamcler; drilling accuracy is g d ; b l m holes are chargcd with ANFO: stemming lcngth is approxhatcly qua1 lo nhe drilled burden distance.: and initialion sctlucnce is designed rn providc adquaic movcrncnl.

whcrc UCS is the uniaxial wmprcssive streng~hof the m k mafcrial given in m e g p h ~ l l (MPa). s

Thc R b s ~ ~ b i l i tIndex y (Rl) is calcuht& as fullows:

o+ P S + JEQ + RDI + 5 )

BI = 0.5

7hc Index was initially developed to assist w i b blast design in the m k m a w found in ihc iron ore mincs of thc Plbctra. In thcsc operations. Wlastability Indcx rangal from values of around 20 for some uT the soft. Friablc shales, to a maximum of 100 for Ihe exmmcly ssrong, massivc, iron-rich caprock, which has an Si; of aln~ul41/m'. In rhc cight ycars that Ihc writcr has uscd Ihc Index, it has b w n found to dcquatcly describe h e typical m k m= iypcs Q U L S ~ ~Lhe C Pilhara rcgion as well as lhose within it.

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Thc chart? shown an Fig~ucs I ro 6 arc a d as

follows: o

estimarc

the Blastabili~y Indcx from gcacchnical in~om!inn in rcpons, or frn~n observations in thc bcnch iacc;

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o o

o

establish rhe bench height and hoIe diamctcr prefemd (if these are not already mtabliskd); ensure that the assumptions made absve sn rhe m t i o n of the charts are valid for rhe problem

at hand, and read off the powder factor and blast hole pa(that is, burden times spacing) i r m the appr0pr;ate curves.

If &-iflingand explosives costs are known, the curves can be used to quickly assess the most cost-effective combina tim d bench height and hole diameter for a particular set of rmk mass conditions. 5.

CONCLUSIONS

The Blmbility Index is a rapidly-derived, quantitative rnethd of estimating a m k masses rcsponse lo blasting. It can be w i l y estimated from basic information which should be M y available in gmtechnid engineering reports, such as slope stability studies, Altcmatively, the parameters wed in the B W i l i t y Index can be readily estimated by observing the bench f x e . The design charts presented on Figutes 1 to 6 are applicabIe to h c h blasting using typical b c h heights, hole diarnams and od~er blasl design parameters, and shouId pmve to be useful to mining engineers who need blast design information for feasibility studies, for estimating quantities and msts for mining contraci renders, or for gencral minc planning p u r p x . Blast design cham incorpoming assumptions other than those listed above can be

rapidly derived.

The author wishes to thank Messrs Kent Bannister and Tony Abbs of Dames & Moore for rheir comments during the preparauon and review of this paperREFERENCES

Cunningham, C. (1983). "The Kuz-Ram Model for Prediction of Fragmentation from Blasting", F i I n t Symp. Rmk Fsag. by Blasting, LuIca, 439453. Lilly, PA. (1986). "An Empirical Method of Assessing Rock Mass Blastability", AwIhfM/JTAust Large Open Pit .Mining Cmfmce, Newman, 89-92

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Blastability lndex vs Burden x Spacing 5rn Bench Hcight

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Figure 1

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Blostabili lndex

Blostability lndex v s Powder Factor 5m Bench Height

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Figure 2

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Blastability Index vs Burden x S p a c i n g 8m h c h Height

Blastability Index vs Powder Factor Brn Bmch Height 1

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Blastability lndex vs B u r d e n x Spacing 10m Bench Height

Figure 5

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Blastability Index

Blastability lndex v s Powder Factor 10m Bench Height

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