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A New Approach to Represent Impact of Discontinuity Spacing and Rock Mass Description on the Median Fragment Size of Blasted Rocks Using Image Analysis of Rock Mass

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Abstract

Several in-situ rock mass properties and blasthole parameters can affect the rock fragmentation. Because of the complexity of the variables affecting the fragmentation results of blasted rocks, to predict a proper value of the median fragment size has long been a difficult task. The blastability index (BI) represents the effect of five parameters of rock mass description (RMD), joint plane spacing (JPS), joint plane orientation, specific gravity and uniaxial compressive strength on the rock fragmentation. The median discontinuity spacing significantly varies with varying the scanline direction and an acceptable value of the median discontinuity spacing will not be expected in practice. The JPS rating also has a constant value of 20 for a wide range of joint spacing values between 0.1 and 1 m, whereas joint spacing can be in this range for most cases. A new method using image analysis of the in-situ rock mass was applied to represent JPS and RMD (belonging to one of the cases: friable, blocky and massive) as an alternative solution. The images contain the details of all individual discontinuities and interlocked in-situ small and large rock blocks. BI, rock strength factor, blasthole parameters, powder factor, fragment size distribution of blasted rock and in-situ block size distribution using image analysis technique were assessed in 15 zones of Sungun open pit copper mine, Angouran lead and zinc open pit mine, Bonab silica mine, Soufian limestone mine and Rashakan limestone mine. The results for rock mass properties and blasthole patterns cover a wide range using different mines. The fragment size distribution was assessed by Split Desktop program with proper delineating images using the Pixler software and blasting was carried out with electric delay detonators. The relations between fragment size and parameters such as in-situ block size (F50), σc, rating of joint plane orientation, powder factor (q), ϕh, Q and Lc were analyzed. The relations with high correlations were achieved by applying the new approach for the defined conditions. Not only the problem of assessing discontinuity spacing has been improved using this method but also the lower number of parameters that properly represent the factors affecting the rock fragmentation have been used. The results were also analyzed by the Sanchidrián and Ouchterlony model and modified Kuz–Ram models. The fragment size obtained by the new method in this study, Sanchidrián and Ouchterlony model and extended modified Kuz–Ram model by Cunningham (in: Proceedings of 3rd world conference on explosives and blasting, Brighton, 2005) after using correction factor [c(A)] significantly better fitted to the results than the modified Kuz–Ram models by Cunningham (in: Fourney, Dick (eds) Proceedings of 2nd international symposium on rock fragmentation by blasting, Keystone, 1987) and Gheibie et al. (Int J Rock Mech Min 46(6):967–973, 2009).

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Abbreviations

ϕ h :

Blasthole diameter in cm

Q :

Explosive weight per blasthole in m

S :

Spacing in m

H :

Blasthole length in m

L c :

(S × H)0.5 In m

RMD:

Rock mass description

JPS:

Joint plane spacing in cm

JPO:

Joint plane orientation

SGI:

Specific gravity influence

σ c :

Uniaxial compressive strength in MPa

H m :

Mohs hardness

BI:

Blastability index

JPA:

Joint plane angle

JF:

Joint factor (JPS + JPA)

HF:

Hardness factor (σci/5)

A :

Rock strength factor

X 50 :

Median fragment size in cm

F 50 :

Median in-situ block size in cm

q :

Powder factor in kg/m3

V :

Rock volume broken per blasthole in m3

Q e :

Explosive charge per hole, which is equivalent in in energy to TNT in kg.

S ANFO :

Relative weight strength to ANFO

P x :

The proportion of material passing the screen

P(%):

Percentage passing

X :

The screen size in cm

n :

The uniformity index

Hs:

Schmidt hammer rebound

X GMM :

Grand median measured discontinuity spacing for artificial discontinuity models in cm

X BS :

Block size of artificial discontinuity models in cm

V b :

Block volume of rock mass in m3

B :

Burden in m

U :

Subdrill in m

α :

Hole angle to vertical direction in degree

K :

Bench height in m

S t :

Stemming in m

L e :

Charge length in m

X P :

Fragment size for a percentage value P in cm

\(\overline{\sigma }\) :

The stored elastic energy at compressive failure

E :

Modulus of elasticity in MPa

e :

Specific explosive energy in MJ/kg

k 2 :

Represents the bench shape factor

θ 1 :

The inclination blasthole angle to the vertical in degree

k :

A coefficient that is determined by fitting function to the experimental data.

h :

A constant power that is determined by fitting function to the experimental data.

λ :

A constant power that is determined by fitting function to the experimental data.

κ :

A constant power that is determined by fitting function to the experimental data.

J F :

Joint correction factor

s j :

Median discontinuity spacing in m

L j :

A characteristic length, capped by a limiting value as for large joint spacing in m

a s :

A coefficient for influence of discontinuity spacing and is determined by fitting function to the experimental data.

j o :

Joints orientation index

a o :

A coefficient for influence of joints orientation index and is determined by fitting function to the experimental data.

C P :

P-wave velocity in m/ms

Δt :

The in-row delay in ms

Π t :

Delay factor

δ 1 :

A constant that is determined by the best fit to the experimental data.

δ 2 :

A constant that is determined by the best fit to the experimental data.

δ 3 :

A constant that is determined by the best fit to the experimental data

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Acknowledgements

The authors would like to thank Mr D. Taghizadeh, Mining Engineer of Rashakan Limestone Mine, Mr M. Baghernegad, Managing Director of Sungun Open Pit Copper Mine, Mr M. Shabany, Managing Director of Angouran Lead and Zinc Open Pit Mine and Mr Ahmadzadeh, Mining Engineer of Soufian Limestone Mine and Mr S. Habibi, Mining Engineer of Bonab silica mine, for their continuous support during carrying out the project.

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    Ali Azizi & Hassan Moomivand

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Azizi, A., Moomivand, H. A New Approach to Represent Impact of Discontinuity Spacing and Rock Mass Description on the Median Fragment Size of Blasted Rocks Using Image Analysis of Rock Mass. Rock Mech Rock Eng 54, 2013–2038 (2021). https://doi.org/10.1007/s00603-020-02360-4

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